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Understanding how individual differences arise and how their effects propagate through groups are fundamental issues in biology. Individual differences can arise from indirect genetic effects (IGE): genetically based variation in the conspecifics with which an individual interacts. Using a clonal species, the Amazon molly ( Poecilia formosa ), we test the hypothesis that IGE can propagate to influence phenotypes of the individuals that do not experience them firsthand. We tested this by exposing genetically identical Amazon mollies to conspecific social partners of different clonal lineages, and then moving these focal individuals to new social groups in which they were the only member to have experienced the IGE. We found that genetically different social environments resulted in the focal animals experiencing different levels of aggression, and that these IGE carried over into new social groups to influence the behaviour of naive individuals. These data reveal that IGE can cascade beyond the individuals that experience them. Opportunity for cascading IGE is ubiquitous, especially in species with long-distance dispersal or fission–fusion group dynamics. Cascades could amplify (or mitigate) the effects of IGE on trait variation and on evolutionary trajectories. Expansion of the IGE framework to include cascading and other types of carry-over effects will therefore improve understanding of individual variation and social evolution and allow more accurate prediction of population response to changing environments.

The Amazon molly is a unique clonal fish species that originated from an interspecies hybrid between Poecilia species P. mexicana and P. latipinna. It reproduces by gynogenesis, which eliminates paternal genomic contribution to offspring. An earlier study showed that Amazon molly shows biallelic expression for a large portion of the genome, leading to two main questions: (1) Are the allelic expression patterns from the initial hybridization event stabilized or changed during establishment of the asexual species and its further evolution? (2) Is allelic expression biased toward one parental allele a stochastic or adaptive process? To answer these questions, the allelic expression of P. formosa siblings was assessed to investigate intra- and inter-cohort allelic expression variability. For comparison, interspecies hybrids between P. mexicana and P. latipinna were produced in the laboratory to represent the P. formosa ancestor. We have identified inter-cohort and intra-cohort variation in parental allelic expression. The existence of inter-cohort divergence suggests functional P. formosa allelic expression patterns do not simply reflect the atavistic situation of the first interspecies hybrid but potentially result from long-term selection of transcriptional fitness. In addition, clonal fish show a transcriptional trend representing minimal intra-clonal variability in allelic expression patterns compared to the corresponding hybrids. The intra-clonal similarity in gene expression translates to sophisticated genetic functional regulation at the individuum level. These findings suggest the parental alleles inherited by P. formosa form tightly regulated genetic networks that lead to a stable transcriptomic landscape within clonal individuals.

Asexual hybrids are important model organisms for addressing questions in evolution and ecology, especially for understanding the role of hybridization in speciation. They are rare in nature and several hypotheses have been suggested to explain this. We use an asexual fish, the Amazon molly (Poecilia formosa), to establish the area in which it was formed via hybridization 125 000 years ago. Using species distribution models and climate models for the Last Interglacial (LIG) we found that model projections to the LIG show a similar map to the present climate model and parental species potentially overlapped in a relatively small area near Tampico, Mexico. This makes P. formosa one of a few hybrid species for which we know the parental species, the time of hybridization, and likely the place of hybridization. Based on the small area of overlap, our data is in agreement with the idea that asexual hybrids may be rare not because they are evolutionary dead ends but are formed rarely.

Simple Summary In many species, including humans, individuals in a population have personalities: collections of correlated behaviors that are consistent across different environments (i.e., mating, eating). Personalities are affected by competitors for food or mates and the hormones produced by individuals). Competitors can include other individuals of the same species or closely related species. The all-female, Amazon molly is a hybrid species, and needs to coexist with one of its bisexual (males and females), parent species, to reproduce. One parent species of the Amazon molly is the sailfin molly. Female sailfin and Amazon mollies compete for access to males for mating and food which could affect the personalities of individuals of each species. We found that both species have similar personalities consisting of a correlation between exploration and activity. We did not detect a relationship between a stress response hormone, cortisol, and individual personality. However, the all-female Amazons had higher cortisol release rates than sailfins. Personalities may be similar due to genetic constraints that link these behaviors, and might benefit Amazons if this causes male sailfin mollies to mismate with them. However, the differences in cortisol release rates may be a useful mate identification cue for males to offset such mating mistakes. Abstract Studies of suites of correlated behavioral traits (i.e., behavioral syndromes) aid in understanding the adaptive importance of behavioral evolution. Behavioral syndromes may be evolutionarily constrained, preventing behaviors from evolving independently, or they may be an adaptive result of selection on the correlation itself. We tested these hypotheses by characterizing the behavioral syndromes in two sympatric, closely related species and testing for differences between the species. We studied the unisexual Amazon molly (Poecilia formosa) and one of its bisexual, parent species, the sailfin molly (P. latipinna). Sympatric female sailfin and Amazon mollies compete for mating which could affect the behavioral syndromes found in each species. We identified a behavioral syndrome between exploration and activity in both species that did not differ between species. Additionally, we explored the relationship between a stress response hormone, cortisol, and behavioral type, and did not detect a relationship. However, P. formosa differed from P. latipinna in their cortisol release rates. Behavioral syndromes may be constrained in this complex, aiding in mate acquisition for P. formosa by virtue of having a similar behavioral type to P. latipinna. The difference between the females in cortisol release rates may be a useful mate identification cue for males to offset higher mating mistakes associated with the similar behavioral types.

Attention, although limited, is a mechanism used to filter large amounts of information and determine what stimuli are most relevant at a particular moment. In dynamic social environments, multiple individuals may play a pivotal role in any given interaction where a male’s attention may be divided between a rival, a current mate, and/or future potential mates. Here, we investigated impacts of the social environment on attention allocation in male sailfin mollies, Poecilia latipinna, which are a part of a sexual-unisexual mating system with the Amazon molly, Poecilia formosa. We asked: 1) Does the species of female influence the amount of attention a male allocates to her? And 2) Is a male’s attention towards his mate influenced by different social partners? Males direct more attention toward a stimulus female when she was a conspecific. We also show that males perceive a larger male as a more relevant stimulus to pay attention to than a smaller male, and a conspecific female as a more relevant stimulus compared to a heterospecific female. Our results show differential allocation of attention is dependent upon multiple components of the social environment in which an individual interacts.SignificanceThis study investigates how attention is allocated in males when presented with social distractions. Assuming that attentional capacity is finite, males may face a tradeoff between different cognitive-demanding stimuli, such as rival males and potential future mates, when mating. Here, we show that male attention allocation in both intra- and intersexual interactions is multifaceted and context dependent. This suggests that individuals within the social environment vary in how meaningful (i.e., able to capture attention) they are to males during mating encounters. Understanding how social partners can cause a shift of attention away from a mating opportunity is essential to understanding the influence of the social context on sexual selection.

Sexual reproduction is nearly ubiquitous in the vertebrate world, yet its evolution and maintenance remain a conundrum due to the cost of males. Conversely, asexually reproducing species should enjoy a twofold population increase and thus replace sexual species all else being equal, but the prevalence of asexual species is rare. However, stable coexistence between asexuals and sexuals does occur and can shed light on the mechanisms asexuals may use in order to persist in this sex-dominated world. The asexual Amazon molly (Poecilia formosa) is required to live in sympatry with one of its sexual sperm hosts –sailfin molly (Poecilia latipinna) and Atlantic molly (Poecilia mexicana)—and are ecological equivalents to their host species in nearly every way except for reproductive method. Here, we compare the visual lateralization between Amazon mollies and sailfin mollies from San Marcos, Texas. Neither Amazon mollies nor sailfin mollies exhibited a significant eye bias. Additionally, Amazon mollies exhibited similar levels of variation in visual lateralization compared to the sailfin molly. Further investigation into the source of this variation –clonal lineages or plasticity—is needed to better understand the coexistence of this asexual-sexual system.

The capacity to compensate for environmental change determines population persistence and biogeography. In ectothermic organisms, performance at different temperatures can be strongly affected by temperatures experienced during early development. Such developmental plasticity is mediated through epigenetic mechanisms that induce phenotypic changes within the animal’s lifetime. However, epigenetic modifiers themselves are encoded by DNA so that developmental plasticity could itself be contingent on genetic diversity. In this study, we test the hypothesis that the capacity for developmental plasticity depends on a species’ among-individual genetic diversity. To test this, we exploited a unique species complex that contains both the clonal, genetically identical Amazon molly (Poecilia formosa), and the sexual, genetically diverse Atlantic molly (Poecilia mexicana). We predicted that the greater among-individual genetic diversity in the Atlantic molly may increase their capacity for developmental plasticity. We raised both clonal and sexual mollies at either warm (28°C) or cool (22°C) temperatures and then measured locomotor capacity (critical sustained swimming performance) and unforced movement in an open field across a temperature gradient that simulated environmental conditions often experienced by these species in the wild. In the clonal Amazon molly, differences in the developmental environment led to a shift in the thermal performance curve of unforced movement patterns, but much less so in maximal locomotor capacity. In contrast, the sexual Atlantic mollies exhibited the opposite pattern: developmental plasticity was present in maximal locomotor capacity, but not in unforced movement. Thus our data show that developmental plasticity in clones and their sexual, genetically more diverse sister species is trait dependent. This points toward mechanistic differences in how genetic diversity mediates plastic responses exhibited in different traits.

Although the majority of studies on mate choice focus on female mate choice, there is growing recognition of the role of male mate choice too. Male mate choice is tightly linked to 2 other phenomena: female competition for males and ornamentation in females. In the current article, I review the existing literature on this in a group of fishes, Poeciliidae. In this group, male mate choice appears to be based on differences in female quality, especially female size, which is a proxy for fecundity. Some males also have to choose between heterospecific and conspecific females in the unusual mating system of the Amazon molly. In this case, they typically show a preference for conspecific females. Whereas male mate choice is relatively well documented for this family, female ornamentation and female competition are not.

Kin selection explains conditions under which closely related individuals should be less antagonistic towards one another. One benefit of kin selection is a reduction in aggression towards kin in various social contexts, such as foraging. In the gynogenetic Amazon molly, females have been shown to differentiate between clone types, preferring to associate with clonal sisters to non-sisters, regulating their aggressive behaviours accordingly. We ask if Amazon mollies in resource-limited environments retain the ability to regulate aggressive behaviours according to relatedness. We found that focal females regulated their aggressive behaviours depending on partner type. Females spent more time behaving aggressively towards the heterospecific females than either of the clonal lineages, and towards non-sister clones compared to clonal sisters. We are able to confirm that kin discrimination is maintained, resulting in females showing more aggression towards heterospecific females and non-sister clones in a food-limited environment, and that this aggression scales with relatedness.

Species recognition is an important process prior to copulation that is thought to be continuous with mate and individual recognition. Amazon mollies—a sperm-parasitic fish—show recognition at the clonal level; here, we investigate if the sexual host species have the same capabilities. We show that both sexual hosts do not exhibit a detectable preference for different clonal females. This raises the possibility that clonal recognition evolved in the Amazon molly, not in its ancestors.

All known vertebrate clones have originated from hybridization events and some have produced distinct evolutionary lineages via hybrid speciation. Amazon mollies (Poecilia formosa) present an excellent study system to investigate how clonal species have adapted to heterogeneous environments because they are the product of a single hybridization event between male sailfin mollies (Poecilia latipinna) and female Atlantic mollies (Poecilia mexicana). Here, we ask whether the hybrid species differs from the combination of its parental species' genes in its plastic response to different environments. Using a three-way factorial design, we exposed neonates produced by Amazon mollies and reciprocal F1 hybrid crosses to different thermal (24°C and 29°C) and salinity (0/2, 12, and 20 ppt) regimes. We measured various ontogenetic and life history characteristics across the life span of females. Our major results were as follows: (1) Reaction norms of growth and maturation to temperature and salinity are quite similar between the two hybrid crosses; (2) Amazon molly reaction norms were qualitatively different than the P. latipinna male and P. mexicana female (L×M) hybrids for the ontogenetic variables; (3) Amazon molly reaction norms in reproductive traits were also quite different from L×M hybrids; and (4) The reaction norms of net fertility were very different between Amazon mollies and L×M hybrids. We conclude that best locale for Amazon mollies is not the best locale for hybrids, which suggests that Amazon mollies are not just an unmodified mix of parental genes but instead have adapted to the variable environments in which they are found. Hybridization resulting in asexuality may represent an underappreciated mechanism of speciation because the unlikely events required to produce such hybrids rarely occur and is dependent upon the genetic distance between parental species.

espanolFrente al cambio ambiental inducido por el hombre, se requieren datos biologicos basicos para las especies amenazadas por los impactos de las actividades humanas para disenar y ejecutar estrategias adecuadas de conservacion. Aqui reportamos datos basicos de algunos aspectos de la historia de vida de 2 poblaciones del topote del Tamesi Poecilia latipunctata (Poeciliidae), nativa de los manantiales del rio Tamesi en el noreste de Mexico y catalogada en peligro critico. Ademas, comparamos su historia de vida con la de 2 especies sintopicas, el topote del Atlantico (Poecilia mexicana) y el topote del Amazonas (Poecilia formosa). Las historias de vida de P. latipunctata son, en gran parte, similares a las de otros topotes: el tamano del macho se distribuyo normalmente en ambas poblaciones y las hembras produjeron huevos de forma discreta, predominantemente con vitelo para el aprovisionamiento de embriones. Sin embargo, en nuestros 2 sitios de muestreo, los machos de P. latipunctata fueron mas pequenos que los machos de P. mexicana y las hembras de P. latipunctata produjeron crias significativamente mas grandes que P. formosa o P. mexicana. Basandonos en los patrones de tamano de los machos y de los descendientes, sugerimos que, ademas de la perturbacion antropogenica, P. latipunctata tambien este sufriendo una fuerte competencia interespecifica, por lo que recomendamos mas investigaciones sobre la biologia basica de esta especie (incluida su capacidad competitiva). EnglishIn the face of human-induced environmental change, basic biological data for species threatened by the impacts of human activities are sorely needed to devise and execute proper conservation strategies. Here we provide aspects of basic life-history data for 2 populations of the critically endangered Tamesi molly Poecilia latipunctata (Poeciliidae), which is native to the headwaters of the Rio Tamesi in northeastern Mexico. Furthermore, we compare their life history to that of 2 syntopic species, the Atlantic molly (Poecilia mexicana) and the Amazon molly (Poecilia formosa). Life histories of P. latipunctata are largely similar to that of other mollies: male size was normally distributed in both populations, and females produced 1 clutch at a time, while relying predominantly on yolk for embryo provisioning. However, at our 2 sample sites, P. latipunctata males were smaller than P. mexicana males, and P. latipunctata females produced significantly larger offspring than either P. formosa or P. mexicana. Based on patterns of male and offspring size, we cautiously suggest that, besides anthropogenic disturbance, P. latipunctata might also be suffering from strong interspecific competition, and we call on more research into the basic biology of this species (including its competitive ability).

Male mate discrimination may be affected by the social environment (presence or absence of rival males or mates), which can also affect stress and sex hormones (e.g., cortisol and 11-ketotestosterone (11-KT)). The Amazon molly, Poecilia formosa, is an all-female fish species dependent on sperm from mating with male P. latipinna. We investigated male mate choice in P. latipinna between conspecific females and P. formosa with a rival male present and no rival male present. We measured cortisol and 11-KT release rates from all fish. The presence of a rival male had no effect on male mate choice for conspecific females nor overall mating effort. Male 11-KT decreased on the second day after exposure to a rival male on the first day. Focal male 11-KT is positively correlated with the size of the rival male. Both conspecific and heterospecific females released more 11-KT when in the rival male treatment than when not. Neither male nor female cortisol was affected by the presence or absence of the rival male. We did not find an effect of rival males on male mate choice in contrast to our prediction. Instead, our findings may indicate a hormonal response to social competition.

Understanding how animal groups form and function is a major goal in behavioural ecology. Both genetic relatedness and familiarity among group mates have been shown to be key mediators of group composition. However, disentangling the two in most species is challenging as the most familiar individuals are often the most related, and vice versa. In order to gain a complete understanding of how individual interactions shape group behaviour it is crucial to understand the role each of this social relationship factors plays individually. To this end, we manipulated the level of familiarity among groups of the naturally clonal, and genetically identical fish, the Amazon molly (Poecilia formosa) and monitored group behaviour in an open-field and when given the opportunity to forage. Contrary to our predictions, fish that were the most familiar with each other showed the highest levels of aggression. Additionally, fish that were less familiar with each other exhibited the highest group cohesion and took the longest to begin feeding, compared to the more familiar fish. These results suggest that familiarity may socially buffer individuals from the perception of risk in novel environments, such as is common in most behavioural tests designed to test group behaviour. Increases in aggression that are associated with increasing familiarity as shown here might be a mechanism by which fish maintain a fission-fusion society with important consequences for the patterns of associations in group living animals.

Why are asexual vertebrates so rare? One seldom explored avenue to understanding the evolutionary persistence of extant asexual species is their sensory ecology—how they perceive and respond to the environment. Asexual species formed by hybridization have been hypothesized to have an expanded sensory repertoire because they carry 1 allele from each of their parental species, including alleles that impact sensory function. The ability to detect odorants in the environment is a likely candidate for this expansion but has never been explored in this context. Here, we explore the olfactory abilities of the asexual Amazon molly, a gynogenetic fish formed by hybridization 100000 years ago. We test whether Amazon mollies can use only olfactory cues to detect conspecifics, detect heterospecific males, and discriminate between males infected with a common parasite. We further explore whether a female’s size, a proxy for age, explains any variation in her behavior. We find strong evidence that Amazon mollies use olfactory cues to detect conspecifics but surprisingly may avoid heterospecific males based on olfactory cues alone. We find no evidence that females use chemical cues to discriminate between infected and noninfected males. We also find that smaller Amazon mollies are more likely to use chemical cues. This study highlights the potential importance of sensory systems in asexual vertebrates.

Pre-existing female biases are female preferences for a particular trait that evolved prior to the evolution of that trait. Phylogenies are needed to show when the preference and trait have originated. In several live-bearing fishes (Poeciliidae), females show pre-existing biases for male swords, a colorful extension of the caudal fin. Here, we investigated the pre-existing bias hypothesis by predicting preferences for a sword in several molly species, including 2 unusual species in the monophyletic subclade Mollienesia: the Amazon molly, Poecilia formosa, a sperm-dependent hybrid form, and the Tamesi molly, Poecilia latipunctata, a species in the long-fin molly clade, that has a short-fin morphology. Using published sequence data available for this family, behavioral approaches, robust phylogenetic analyses, and Bayesian ancestral state reconstructions, we tested whether the hybrid P. formosa has a preference similar to the maternal ancestor, Poecilia mexicana, or the paternal ancestor, Poecilia latipinna. Surprisingly, the preference shown by P. formosa was variable between populations and matched the preference found in the co-occurring host species. In P. latipunctata, we found a pre-existing bias for sworded males, suggesting that this represents an ancestral trait for the long-fin molly clade. On the basis of the combined evidence from multiple studies, it seems as if pre-existing biases for sworded males are relatively basal to poeciliids and that existing phylogenetic relationships allow us to predict sensory biases.

The extreme rarity of asexual vertebrates in nature is generally explained by genomic decay due to absence of meiotic recombination, thus leading to extinction of such lineages. We explore features of a vertebrate asexual genome, the Amazon molly, Poecilia formosa, and find few signs of genetic degeneration but unique genetic variability and ongoing evolution. We uncovered a substantial clonal polymorphism and, as a conserved feature from its interspecific hybrid origin, a 10-fold higher heterozygosity than in the sexual parental species. These characteristics seem to be a principal reason for the unpredicted fitness of this asexual vertebrate. Our data suggest that asexual vertebrate lineages are scarce not because they are at a disadvantage, but because the genomic combinations required to bypass meiosis and to make up a functioning hybrid genome are rarely met in nature. This work was supported by grants to W.C.W. (NIH: 2R24OD011198-04A1), M.W.H. (NSF DBI-1564611), M.S. (German Research Foundation DFG projects Scha408/10-1 and Scha408/12-1), M.St. (Heisenberg-Fellowship STO 493/2-2 of the German Science Foundation/DFG), T.M.B.(MINECO BFU2014-55090-P (FEDER), U01 MH106874 grant, Howard Hughes International Early Career, Obra Social ‘La Caixa’ and Secretaria d’Universitats i Recerca del Departament d’Economia i Coneixement de la Generalitat de Catalunya) and R.B.W. (NIH: R24OD011120). The genome annotation work carried out by NCBI was supported by the Intramural Research Program of the NIH, National Library of Medicine. The genome annotation work by Ensembl was supported by funding from the Wellcome Trust (WT108749/Z/15/Z and WT098051), the National Institutes of Health (R24 RR032658-01) and the European Molecular Biology Laboratory

Amazon molly (Poecilia formosa) females reproduce asexually, but they need sperm to initiate the process. Such gynogenetic reproduction can be called sperm parasitism since the DNA in the sperm is not used. Since all offspring of asexually reproducing females are females, they can locally outcompete sexually reproducing ones, but their persistence is threatened by the lack of males. Therefore, the existence of Amazon mollies is puzzling. A metapopulation structure has been suggested to enable the coexistence of gynogenetic and sexual species. Previously only Levins-type metapopulation models have been used to investigate this question, but thy are not defined on the individual level. Therefore we investigate the evolution of sperm parasitism in a structured metapopulation model, which incorporates both realistic local population dynamics and individual-level dispersal. If the reproduction strategy is freely evolving in a large well-mixed population or in the structured metapopulation model, strong discrimination of asexually reproducing females by males results in evolution to full sexuality, whereas mild discrimination leads to too small probability of sexual reproduction, so that the lack of males causes the extinction of the evolving population, resulting in evolutionary suicide. This classification remains the same also when both sexual reproduction and dispersal are freely evolving. Sexual and asexual behaviour can be observed at the same time in this model in the presence of a trade-off between the reproduction and dispersal traits. However, we do not observe disruptive selection resulting in the evolutionarily stable coexisence of fully sexual and fully asexual females. Instead, the presence of sexual and asexual behaviour is due to females with a mixed reproduction trait.

Kin selection theory predicts that closely related individuals should be more altruistic and less antagonistic towards one another. In addition, it would predict that the higher the relatedness value (R) between individuals, such as in clonal organisms, the more likely kin selection is to evolve. One benefit of kin selection is a reduction in aggression towards kininvarious social contexts, such as foraging. In the gynogenetic Amazon molly, Poecilia formosa, females have been shown to differentiate between clone types, preferring to associate with clonal sisters to non-sisters, and regulate their aggressive behaviors accordingly. We ask ifAmazon mollies in resource-limited environments: 1) still maintain the ability to regulate aggressive behaviors according to relatedness (heterospecific females, clonal sisters or non-sisters), and 2) how their aggressivebehaviors change relative to a female’s social partner? Using a repeated-measures design, we found that focal females regulated their aggressive behaviors depending on partner type (i.e., a heterospecific female, clonal sister, or non-sister). Heterospecific females and the non-sister clones spent more time behaving aggressively towards the focal females, and these females also received significantly more bites from heterospecific females. Interestingly, the clonal sisters, when compared to the other two female types, performed significantly more tail beats towards focal females.We are able to confirm that females do show more aggression towards heterospecific females and non-sister clones in a food-limited environment, andthat their aggression scales with relatedness (R).Lay SummaryKin recognition allows individuals to adjust costly behaviors, such as aggression, according to the degree of genetic relatedness. We show that in a food-limited environment, a clonal species of fish, the Amazon molly, females regulate aggressive behaviors depending on how closely related they are to the recipient female, behaving more aggressively to both heterospecific females and non-sister clones. The ability to regulate costly behaviors under variable socialconditions is advantageous, especially when resources are limited.

Across a wide range of animal taxa, winners of previous fights are more likely to keep winning future contests, just as losers are more likely to keep losing. At present, such winner and loser effects are considered to be fairly transient. However, repeated experiences with winning and/or losing might increase the persistence of these effects, generating long-lasting consequences for social structure. To test this, we exposed genetically identical individuals of a clonal fish, the Amazon molly ( Poecilia formosa ), to repeated winning and/or losing dominance interactions during the first two months of their life. We subsequently investigated whether these experiences affected the fish's ability to achieve dominance in a hierarchy five months later after sexual maturity, a major life-history transition. Individuals that had only winning interactions early in life consistently ranked at the top of the hierarchy. Interestingly, individuals with only losing experience tended to achieve the middle dominance rank, whereas individuals with both winning and losing experiences generally ended up at the bottom of the hierarchy. In addition to demonstrating that early social interactions can have dramatic and long-lasting consequences for adult social behaviour and social structure, our work also shows that higher cumulative winning experience early in life can counterintuitively give rise to lower social rank later in life.

Expansions in sensory systems usually require processes such as gene duplication and divergence, and thus evolve slowly. We evaluate a novel mechanism leading to rapid sensory repertoire expansion: hybrid-sensory expansion (HSE). HSE occurs when two species with differently tuned sensory systems form a hybrid, bringing together alleles from each of the parental species. In one generation, a sensory repertoire is created that is the sum of the variance between parental species. The Amazon molly presents a unique opportunity to test the HSE hypothesis in a “frozen” hybrid. We compared opsin sequences of the Amazon molly, Poecilia formosa, to those of the parental species. Both parental species are homozygous at the RH2–1 locus and each of the four long wavelength sensitive loci, while P. formosa possess two different alleles at these loci; one matching each parental allele. Gene expression analysis showed P. formosa use the expanded opsin repertoire that was the result of HSE. Additionally, behavioral tests revealed P. formosa respond to colored stimuli in a manner similar or intermediate to the parental species P. mexicana and P. latipinna. Together these results strongly support the HSE hypothesis. Hybrid-sensory repertoire expansion is likely important in other hybrid species and in other sensory systems.

Gynogenesis is a peculiar mode of clonal reproduction in which eggs need to be pseudo-fertilized by sperm, but the male genes are not passed on to the offspring. One mating system in which gynogenesis is found involves a unisexual hybrid, the Amazon molly (Poecilia formosa), which typically uses males of its two parental species as sperm donors. Most gynogenetic lineages do not sexually parasitize males that were not involved in their hybrid origin and although some gynogens have the ability to utilize males from additional species, they rarely occur in sympatry with more than one sperm host. A few populations of the Amazon molly, however, do occur syntopically with more than one host species, raising the question of whether specific preferences have evolved in P. formosa and whether Amazon mollies can now act like Red Queens, driving the evolution of discrimination abilities in the host species. Near Ciudad Mante, Mexico, the critically endangered Tamesi molly (P. latipunctata) occurs in exclusive sympatry with Amazon mollies and one of P. formosa’s parental species, the Atlantic molly (P. mexicana). In this study we tested the initial and post-exposure preference of allopatric and sympatric P. formosa (with regards to P. latipunctata) between P. latipunctata and P. mexicana males. We predicted that P. formosa should favour P. mexicana because this is a parental species to P. formosa and the asexual shares half of the P. mexicana genome. Contrary to our predictions, we found no significant differences between preference scores in allopatric and sympatric populations of P. formosa in initial or post-exposure trials. Our findings may indicate that males of both species are perceived as equally effective in triggering embryogenesis by P. formosa. This study, along with limited previous work in the system, calls to attention the exploration of the adaptive ability of gynogenetic species relationships, and their implications for the long-term persistence of not only asexual lineages, but also the parasitized sexual species.

In the asexual all-female fish species Poecilia formosa, the Amazon molly, supernumerary chromosomes have frequently been found in both laboratory-reared and wild-caught individuals. While wild-caught individuals with B chromosomes are phenotypically indifferent from conspecifics, individuals carrying B chromosomes from recent introgression events in the laboratory show phenotypic changes. Former analyses showed that the expression of a pigment cell locus is associated with the presence of these B chromosomes. In addition, they contain a so far unidentified locus that confers a higher susceptibility to tumor formation in the presence of pigmentation pattern. Isolation by microdissection and hybridization to metaphase chromosomes revealed that they contain one or several sequences with similarity to a highly repetitive pericentromeric and subtelomeric sequence in A chromosomes. Isolation of one particular sequence by AFLP showed that the B chromosomes contain at least 1 copy of an A-chromosomal region which is highly conserved in the whole genus Poecilia, i.e. more than 5 million years old. We propose it to be a single copy sequence.

All-female 'species' of fish have been shown to be great models in ecological and evolutionary studies because of the insights they can provide into the origin and evolution of asexuality, the ecology of hybrids, associations between genotype and environment, and the maintenance of sex. Gynogenetic organisms that evolved from sexual ancestors, and combine the disadvantageous traits from sexuality and asexuality, have long baffled evolutionary biologists trying to understand their origin and persistence with their sympatric sexual counterparts. In this issue, a new study using an integrated molecular phylogenetic and classical genetic approach has uncovered compelling evidence regarding the obscure asexual origin of the Amazon molly, Poecilia formosa. By performing an extensive phylogeographic analysis, Stock et al. (2010) provide evidence that the Amazon molly arose only once within its history, with monophyly being strongly supported by mitochondrial DNA and microsatellite analyses. This result, combined with an elaborate failed attempt to resynthesize the lineage, suggests that vertebrate gynogens such as the Amazon molly are not rare because they are at a disadvantage to their sexual counterparts, but because the genomic conditions under which they arise are rare. Organisms that apparently combine the disadvantages of both sexuality and asexuality remain difficult to understand from both an ecological and an evolutionary perspective, and Stock et al. (2010) highlight several outstanding important questions. Nonetheless, given that we now have a better knowledge of the origin and history of this unique 'species', this should allow researchers to better understand how these frozen F1's can persist amidst the masterpiece of nature.

Considering its immediate costs of producing dispensable males, the maintenance of sexual reproduction is a major paradox in evolutionary biology. Asexual lineages that do not face such costs theoretically should replace sexuals over time. Nonetheless, several systems are known in which closely related sexual and asexual lineages stably coexist. In the present study, we studied a sexual/asexual mating complex of a sperm-dependent parthenogenetic fish (amazon molly, Poecilia formosa) and its sexual congeners, the sailfin molly P. latipinna and the Atlantic molly P. mexicana. We asked whether differences in feeding behavior could contribute to their stable coexistence. We conducted a laboratory experiment to compare feeding efficiencies and also measured the competitive abilities between the two reproductive forms. Additionally, we measured gut fullness of fishes caught in natural habitats. Contrary to our predictions, we could not find P. formosa to be less efficient in feeding. We argue that food competition in mollies plays a minor role in mediating coexistence between closely related asexual and sexual mollies.

In a microchromosome-carrying laboratory stock of the normally all-female Amazon molly Poecilia formosa triploid individuals were obtained, all of which spontaneously developed into males. A comparison of morphology of the external and internal insemination apparatus and the gonads, sperm ploidy and behaviour, to laboratory-bred F(1) hybrids revealed that the triploid P. formosa males, though producing mostly aneuploid sperm, are partly functional males that differ mainly in sperm maturation and sexual motivation from gonochoristic P. formosa males.

Despite the advantage of avoiding the costs of sexual reproduction, asexual vertebrates are very rare and often considered evolutionarily disadvantaged when compared to sexual species. Asexual species, however, may have advantages when colonizing (new) habitats or competing with sexual counterparts. They are also evolutionary older than expected, leaving the question whether asexual vertebrates are not only rare because of their 'inferior' mode of reproduction but also because of other reasons. A paradigmatic model system is the unisexual Amazon molly, Poecilia formosa, that arose by hybridization of the Atlantic molly, Poecilia mexicana, as the maternal ancestor, and the sailfin molly, Poecilia latipinna, as the paternal ancestor. Our extensive crossing experiments failed to resynthesize asexually reproducing (gynogenetic) hybrids confirming results of previous studies. However, by producing diploid eggs, female F(1) -hybrids showed apparent preadaptation to gynogenesis. In a range-wide analysis of mitochondrial sequences, we examined the origin of P. formosa. Our analyses point to very few or even a single origin(s) of its lineage, which is estimated to be approximately 120,000 years old. A monophyletic origin was supported from nuclear microsatellite data. Furthermore, a considerable degree of genetic variation, apparent by high levels of clonal microsatellite diversity, was found. Our molecular phylogenetic evidence and the failure to resynthesize the gynogenetic P. formosa together with the old age of the species indicate that some unisexual vertebrates might be rare not because they suffer the long-term consequences of clonal reproduction but because they are only very rarely formed as a result of complex genetic preconditions necessary to produce viable and fertile clonal genomes and phenotypes ('rare formation hypothesis').

The livebearing all-female fish Poecilia formosa reproduces by gynogenesis, a modified form of parthenogenesis. P. formosa forms at least two breeding complexes: in its northern range it exists sympatrically with Poecilia latipinna and in its southern range with Poecilia mexicana. Differences between these complexes and their possible origin are discussed. Embryogenesis is triggered by sperm of males of these closely related sympatric species. Because inheritance is strictly maternal, from the male point of view energy and time invested are totally lost. In this study we wanted to elucidate whether males are able to distinguish between conspecific and parasitic females. It could be shown that males are able to distinguish females optically, but that this ability was obscured as soon as chemical and/or tactile contact was possible. Furthermore, we found that females in an attractive phase of their sexual cycle are always preferred, regardless of species. This is possibly the mechanism by which parasitic females obtain the matings they need to reproduce.

One of the most complex and important problems in modern evolutionary biology remains to explain the evolution and persistence of sexual reproduction given the costs of sexuality. One way to study the relative strengths and weaknesses of sexuality and asexuality is to study the dynamics of the coexistence of sexual and asexual organisms. An excellent model system for such an approach is the unisexual Amazon molly, Poecilia formosa. This livebearing fish is clonal, but requires sperm to trigger embryogenesis. Amazon mollies need to obtain sperm from males of closely related species. In an effort to understand the apparently paradoxical coexistence of the Amazon molly with its sexual hosts, many theories were tested, but most seem to have little relevance to understanding stability, whereas male mating behavior is potentially very important.

The evolution and maintenance of sexual reproduction is still one of the major unresolved problems in evolutionary biology. Sexual reproduction is fraught with a number of costs as compared to asexual reproduction. For example, sexuals have to produce males, which–given a 1:1 sex ratio—results in a two-fold advantage for asexuals that do not produce males. Consequently, asexuals will outperform and replace sexuals over time assuming everything else is equal. Nonetheless, a few cases of closely related asexuals and sexuals have been documented to coexist stably in natural systems. We investigated the presence of a two-fold cost in a unique system of three closely related fish species: the asexual Amazon Molly (Poecilia formosa), and two sexual species, Sailfin Molly (P. latipinna) and Atlantic Molly (P. mexicana). Amazon Molly reproduce gynogenetically (by sperm dependent parthenogenesis) and always coexist with one of the sexual species, which serves as sperm donor. In the laboratory, we compared reproductive output between P. formosa and P. mexicana as well as P. formosa and P. latipinna. We found no differences in the fecundity in either comparison of a sexual and the asexual species. Under the assumption of a 1:1 sex ratio, the asexual Amazon Molly should consequently have a full two-fold advantage and be able to outcompete sexuals over time. Hence, the coexistence of the species pairs in nature presents a paradox still to be solved.

Selection acting on individuals is not predicted to maximize population persistence, yet examples that explicitly quantify conflicts between individual and population level benefits are scarce. One such conflict occurs over sexual reproduction because of the cost of sex: sexual populations that suffer the cost of producing males have only half the growth rate compared to asexuals. Male behaviour can additionally impact population dynamics in a variety of ways, and here we study an example where the impact is unusually clear: the riddle of persistence of sperm-dependent sexualasexual species complexes. Here, a sexually reproducing host species coexists with an ameiotically reproducing all-female sperm parasite. Sexualasexual coexistence should not be stable because the proportion of asexually reproducing females will rapidly increase and the relative abundance of the sexually reproducing host species will decline. A severe shortage of males will lead to sperm limitation for sexual and asexual females and the system collapses. Male mate choice could reduce the reproductive potential of the asexual species and thus potentially prevent the collapse. In the gynogenetic (spermdependent parthenogenetic) Amazon molly Poecilia formosa and its host (P. latipinna or P. mexicana), males discriminate against asexual females to some extent. Using a population-dynamical model, we examine the population dynamics of this species complex with varying strengths of male discrimination ability and efficiency with which they locate females and produce sperm. The sexual species would benefit from stronger discrimination, thus preventing being displaced by the asexual females. However, males would be required to evolve preferences that are probably too strong to be purely based upon selection acting on individuals. We conclude that male behaviour does not fully prevent but delays extinction, yet this is highly relevant because low local extinction rates strongly promote coexistence as a metapopulation.

In asexual all-female species, such as the Amazon molly, Poecilia formosa, that depend on sperm from “host males” only to trigger embryogenesis, mate choice does not provide any indirect, genetic benefits to the choosing female, although direct benefits are possible. Asexual species are thought to have a low evolutionary potential or evolvability due to the absence of meiotic recombination. Hence, theory predicts that mating preferences in P. formosa for male ornaments—if existent—should resemble those of females of the two parental species (Poecilia latipinna and Poecilia mexicana) involved in the natural hybridization that gave rise to the asexual P. formosa. When examining the female preference for dummy males with or without black vertical bars in the two parental species and in two lineages of P. formosa, a preference was detected in P. latipinna, but not in P. mexicana females. Interestingly, P. formosa living syntopic with P. latipinna also preferred striped males, while others living syntopic with P. mexicana preferred non-striped males. The evolutionary significance of this phenomenon remains largely unexplained, but it might indicate the evolution of mating preferences in a species with low evolutionary potential. Possible mechanisms include introgression and mitotic gene conversion. Females might use male coloration as indicator mechanisms for male traits that matter in terms of direct benefits.

The evolution of sex is still a major unsolved puzzle in biology. One of the most promising theoretical models to answer this question is the Red Queen hypothesis. The Red Queen hypothesis proposes a fast adaptation of pathogens to common genotypes and therefore a negative frequency-dependent selection against common genotypes. Clonal organisms should be especially endangered when co-occurring with closely related sexual species. In this context, major histocompatibility (MHC) genes have been discussed to be auspicious candidates that could provide the genetic basis on which selection for immune competence could act. In this study, we investigated MHC variability in a clonal teleost fish: the Amazon molly, Poecilia formosa. The Amazon molly is an ideal candidate to test the Red Queen hypothesis as it is a clonal species but co-occurs with a closely related sexual species and should therefore be especially susceptible to pathogen infection. We found that allele numbers did in general not differ between sexual and clonal 'species' but that genotypic variability is reduced in the clonally reproducing fish, especially in the polyploids. We conclude that in clonal organisms, genotype frequency might be more important for immune competence than MHC allele number. Amazon mollies and their co-occurring parental species clearly fulfil a prerequisite of the Red Queen hypothesis and should therefore provide an ideal system to experimentally test this basic principle probably underlying the evolution of sex.

Most females exhibit preferences for certain males and females based on mating and social decisions. Unisexual females that reproduce by gynogenesis are also expected to express association preferences, which may have been inherited from parental species, or may have evolved in response to selection pressures associated with their unisexual mating system. The unisexual, gynogenetic Amazon molly (Poecilia formosa) is a hybrid of the Atlantic molly (P. mexicana) and sailfin molly (P. latipinna). Although none of these three species have sword tails, the two parental species differ in preference for swords on conspecifics. We further examined the variation in pre-existing bias within this species complex by testing Amazon molly preference to associate with male and female sailfin mollies with artificially attached swords. Amazon mollies significantly avoided females with colored swords, but were indiscriminate towards swords on males. Thus, this hybrid species shares its lack of preference for swords on males with female Atlantic mollies and females from one population of sailfin mollies. Avoidance of females with swords by Amazon mollies is unique in this species complex. We discuss the implications and functions of association preferences expressed by Amazon mollies and propose hypotheses for the differential response towards swords based on sex.

When making mating decisions, individuals may rely on multiple cues from either the same or multiple sensory modalities. Although the use of visual cues in sexual selection is well studied, fewer studies have examined the role of chemical cues in mate choice. In addition, few studies have examined how visual and/or chemical cues affect male mating decisions. Male mate choice is important in systems where males must avoid mating with heterospecific females, as is found in a mating complex of Poecilia. Male sailfin mollies, Poecilia latipinna, are sexually parasitized by gynogenetic Amazon mollies, P. formosa. Little is known about the mechanism by which male sailfin mollies base their mating decisions. Here we tested the hypothesis that male sailfin mollies from an allopatric and a sympatric population with Amazon mollies use multiple cues to distinguish between conspecific and heterospecific females. We found that male sailfin mollies recognized the chemical cues of conspecific females, but we found no support for the hypothesis that chemical cues are by themselves sufficient for species discrimination. Lack of discrimination based on chemical cues alone may be due to the close evolutionary history between P. latipinna and P. formosa. Males from populations sympatric with Amazon mollies did not differentially associate with females of either of the two species when given access to both visual and chemical cues of the females, yet males from the allopatric population did associate more with conspecific females than with heterospecific females in the presence of both chemical and visual cues. The lack of discrimination by males from the sympatric population between conspecific and heterospecific females based on both chemical and visual cues suggests that these males require more complex combinations of cues to distinguish species, possibly due to the close relatedness of these species.

The maintenance of sex is still an evolutionary puzzle given its immediate costs. Stably coexisting complexes of asexually and sexually reproducing forms allow to study mechanisms that balance the costs and benefits of both asexual and sexual reproduction. Here, we tested whether coexisting asexual and sexual fish of the genus Poecilia differed in neonate mortality when exposed to environmental stress in the form of fluctuating temperatures and food deprivation. We find that asexual Amazon mollies, Poecilia formosa, are significantly more sensitive to food stress than their sexual relative Poecilia latipinna, but both are equally unaffected by variable temperatures. Differences in the susceptibility to environmental stress may contribute to diminishing the asexuals’ benefits of a higher intrinsic population growth rate and thus mediate stable coexistence of the two reproductive forms.