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Sea star wasting-marked in a variety of sea star species as varying degrees of skin lesions followed by disintegration-recently caused one of the largest marine die-offs ever recorded on the west coast of North America, killing billions of sea stars. Despite the important ramifications this mortality had for coastal benthic ecosystems, such as increased abundance of prey, little is known about the causes of the disease or the mechanisms of its progression. Although there have been studies indicating a range of causal mechanisms, including viruses and environmental effects, the broad spatial and depth range of affected populations leaves many questions remaining about either infectious or non-infectious mechanisms. Wasting appears to start with degradation of mutable connective tissue in the body wall, leading to disintegration of the epidermis. Here, we briefly review basic sea star biology in the context of sea star wasting and present our current knowledge and hypotheses related to the symptoms, the microbiome, the viruses, and the associated environmental stressors. We also highlight throughout the article knowledge gaps and the data needed to better understand sea star wasting mechanistically, its causes, and potential management.

We report the first verified DNA barcode data for the tropical chthamalid barnacle Rehderella belyaevi (Zevina & Kurshakova, 1973) and place it in phylogenetic and biogeographic context among other lineages of Pacific barnacles. Hitherto found on a few remote but widely separated islands across the South Pacific, R. belyaevi is the sole described species of its genus. Phylogenetic analyses indicate that Nesochthamalus intertextus (Darwin, 1854) is its sister taxon and that this taxon pair is nested in a cluster of South Pacific chthamalid taxa. We found that the Pitcairn Is. population includes an admixture of diversity from French Polynesia and Rapa Nui (Easter I.), with those populations being quite divergent. In light of the long history of ocean-going vessel movements in this region, beginning with Polynesian voyagers and the propensity of chthamalid barnacles to occur in biofouling communities, the sequence diversity on Pitcairn is suggestive of ship-borne influence, the more so since the tremendous distances between these islands may preclude natural larval dispersal among them. The true distributional range of Rehderella in the South Pacific is unknown, as shown by our report herein of an undescribed lineage of this genus in French Polynesia based on specimens collected 20 years ago. Furthermore, our discovery that this species occurs on floating plastic debris, a vast amount of which is now present throughout the South Pacific, suggests that the ocean voyages of Rehderella may continue.

Genomic diversity and relatedness among sample sites are often used to explore landscape-level processes of how and where organisms are limited in movement. In many cases, these patterns of diversity and relatedness can be useful for understanding larger ecological patterns. A prior study has suggested that larval input – inferred from landscape genomic data – of the ribbed mussel Geukensia demissa, a species with important roles in stabilizing salt marsh ecosystems, could be indicative of longer-term recruitment patterns of high marsh plant species. Here we use new observations of mitochondrial diversity in the same region but with more sites sampled to show that this prior study was wrong in suggesting that relationship. The same mitochondrial data are useful for monitoring cryptic patterns of climate response in these mussels relative to a subtropical congener.

In the past 40 years, the ability to distinguish phenotypically similar species by using molecular methods has rapidly changed the study of taxonomy, biogeography, and community diversity. A cosmopolitan family of acorn barnacles, Chthamalidae Darwin, 1854, can be found on almost every non-boreal coast, with very similar habitat requirements, larval life histories, and external (test) morphologies among its species. Here I review how molecular methods have aided the description of new species as well as the characterization of phylogenetic diversity within species, and also broadened our understanding of the biogeography of this family. Multiple regional analyses have shown that the coastal habitat appears to drive diversity and diversification in generally similar ways regardless of location, while a global perspective allows us to identify regions and questions that merit further study. Variation in the spatial patterning of genomic diversity among species sometimes provides evidence as to how key life history parameters determine responses of species to ocean currents and forcing, thermal environments, and competition.

The barnacle Balanus glandula is a broadly distributed species in the temperate northeastern Pacific that is notable for a robust genetic cline between about 36° and 40° N latitude. Prior work established the evolutionary origins of this pattern and proposed that it is maintained by environmental selection. In recent years, "climate velocity" studies in marine habitats have shown dramatic distributional shifts for many species as they track their preferred temperature range in a warming ocean. We re-sampled B. glandula across its entire geographic range to determine whether there has been any shift in this genetic distribution, a development signaling that temperature or other climate factors are maintaining this genetic cline. Additionally, we asked whether the spatially distributed mitochondrial lineages also vary in reproductive output with latitude, using location as a proxy for temperature and other coastal environmental factors. Here we show that although the distribution of the genetic cline has not appreciably changed, there is a notable association of decreased reproductive output at lower latitudes of the distribution in the "northern" lineage of B. glandula.

Knowledge of species ages and their distribution enhance our understanding of processes that create and maintain species diversity at both local and regional levels. The largest family of freshwater mussels (Unionidae), reach their highest species diversity in drainages of the southeastern united states. By sequencing multiple loci from mussel species distributed throughout the drainages in this region, we attempt to uncover historical patterns of divergence and determine the role of vicariance events on the species formation in mussels and extend our hypothesis to freshwater animals in general. We analyzed 346 sequences from five genera encompassing 37 species. Species were sampled across 12 distinct drainages ending either in the Atlantic Ocean or the Gulf of Mexico. Overall the topologies of the different genera returned phylogenetic trees that were congruent with geographically contiguous drainages. The most common pattern was the grouping between the Atlantic slope and gulf coast drainages, however the Tennessee drainage was often the exception to this pattern grouping with the Atlantic slope. Most mussel species find a most recent common ancestor within a drainage before finding an ancestor between drainages. This supports the hypothesis of allopatric divergence followed by later burst of speciation within a drainage. Our estimated divergence times for the Atlantic-Gulf split agree with other studies estimating vicariance in fish species of the Atlantic and gulf coast.

Sea star wasting (SSW) disease describes a condition affecting asteroids that resulted in significant Northeastern Pacific population decline following a mass mortality event in 2013. The etiology of SSW is unresolved. We hypothesized that SSW is a sequela of microbial organic matter remineralization near respiratory surfaces, one consequence of which may be limited O2 availability at the animal-water interface. Microbial assemblages inhabiting tissues and at the asteroid-water interface bore signatures of copiotroph proliferation before SSW onset, followed by the appearance of putatively facultative and strictly anaerobic taxa at the time of lesion genesis and as animals died. SSW lesions were induced in Pisaster ochraceus by enrichment with a variety of organic matter (OM) sources. These results together illustrate that depleted O2 conditions at the animal-water interface may be established by heterotrophic microbial activity in response to organic matter loading. SSW was also induced by modestly (∼39%) depleted O2 conditions in aquaria, suggesting that small perturbations in dissolved O2 may exacerbate the condition. SSW susceptibility between species was significantly and positively correlated with surface rugosity, a key determinant of diffusive boundary layer thickness. Tissues of SSW-affected individuals collected in 2013–2014 bore δ15N signatures reflecting anaerobic processes, which suggests that this phenomenon may have affected asteroids during mass mortality at the time. The impacts of enhanced microbial activity and subsequent O2 diffusion limitation may be more pronounced under higher temperatures due to lower O2 solubility, in more rugose asteroid species due to restricted hydrodynamic flow, and in larger specimens due to their lower surface area to volume ratios which affects diffusive respiratory potential.

Crustacea display diverse life histories and occur in all marine habitats. This makes them particularly useful when thinking about how we can predict geographical distribution from life history and ecology. As would be expected from such diversity, crustaceans exhibit various population connectivity patterns, from panmictic, well-connected populations to small and isolated populations. Here we ask first what can be learned from exploring crustacean phylogeography and connectivity around well-understood vicariance events with known ages. We find that vicariance events are generally useful in calibrating molecular rates of evolution, but that there is substantial variation between taxa. This variation can be linked, on the one hand, to habitat differences (which determine when gene flow between populations actually ceased) and, on the other hand, to population size differences (which determine how fast genetic differences accumulate). In a few instances, populations must have diverged much earlier or later than the hypothesized vicariance event, providing evidence of earlier or later dispersal or more ancient separation. Second, we ask when comparative studies of multiple taxa show consistent results, predictable from their similar life history, and when not. For example, species that disperse little, such as brooding peracarids, have smaller, more isolated populations than species with planktonic larvae, such as decapods. Less consistent are the patterns across biogeographic breaks. While gene flow is clearly limited across such breaks in some species, other species do not seem to perceive them. This to-date-unexplained variation challenges our understanding of marine phylogeography.

Sea star wasting disease describes a condition affecting asteroids that resulted in significant Northeastern Pacific population decline following a mass mortality event in 2013. The etiology of sea star wasting is unresolved. We hypothesized that asteroid wasting is a sequela of microbial organic matter remineralization near respiratory surfaces which leads to boundary layer oxygen diffusion limitation (BLODL). Wasting lesions were induced in Pisaster ochraceus by enrichment with a variety of organic matter (OM) sources and by experimentally reduced oxygen conditions. Microbial assemblages inhabiting tissues and at the asteroid-water interface bore signatures of copiotroph proliferation before wasting onset, followed by the proliferation of putatively facultative and strictly anaerobic taxa. These results together illustrate that suboxic conditions at the animal-water interface may be established by heterotrophic bacterial activity in response to organic matter loading. Wasting susceptibility was significantly and positively correlated with rugosity, a key determinant of boundary layer thickness. At a semi-continuously monitored field site (Langley Harbor), wasting predictably occurred at annual peak or decline in phytoplankton biomass over 5 years, suggesting that primary production-derived OM may contribute to BLODL. Finally, wasting individuals from 2013 – 2014 bore stable isotopic signatures reflecting anaerobic processes which suggests that this phenomenon may have affected asteroids during mass mortality. The impacts of BLODL may be more pronounced under higher temperatures due to lower O2 solubility, in more rugose asteroid species due to restricted hydrodynamic flow, and in larger specimens due to their lower surface area to volume ratios which affects diffusive respiratory potential. Moreover, our results demonstrate that marine invertebrate disease may result from heterotrophic microbial activity that occurs adjacent to respiratory tissues, which raises important questions about the etiology of marine diseases in other benthic taxa.

Phenotypic variation can lead to variation in the strength and outcome of species interactions. Variation in phenotypic traits can arise due to plastic responses to environmental stimuli, underlying genetic variation, or both, and may reflect differences in the focal organism or aspects of the extended phenotype (e.g., associated microbes). We used a reciprocal transplant experiment of Porites corals to evaluate the role of plasticity vs. heritable diversity on phenotypic traits and performance of corals that varied in their prior exposure to vermetid gastropods, an organism known to reduce coral growth and survival. We measured a suite of phenotypic traits associated with coral performance, many of which showed a plastic response to vermetid exposure. Vermetids decreased calcification of corals, increased microbial diversity, and shifted microbial composition. Most traits also showed a signature of previous exposure environment that persisted even when exposure was reversed: i.e., under the same conditions, corals naive to vermetids had slower calcification rates, thicker tissues, higher Symbiodiniaceae densities, and different microbiomes than corals previously exposed to vermetids. We suggest the phenotypic differences are heritable, as reefs with and without vermetids were comprised of two different mitotypes, that revealed high, consistent genetic variation. Vermetids were only found on the fast-growing coral mitotype that was characterized by thin tissue, and that likely had a history of disturbance. As extended phenotypes can have community impacts, we suggest vermetid, in addition to microbes, are part of the extended community phenotype of these corals. Coral genotypes can establish different reef trajectories, with thin-tissue types more prone to disturbance and subsequent colonization by other species, like vermetids, which can further facilitate the degradation of coral reefs. The effects of the extended phenotype of species likely influence heterogeneity across landscapes as well as temporal differences in community composition.

As organisms are faced with intense rapidly changing selective pressures, new genetic material is required to facilitate adaptation. Among sources of genetic novelty, gene duplications and transposable elements (TEs) offer new genes or new regulatory patterns that can facilitate evolutionary change. With advances in genome sequencing it is possible to gain a broader view of how gene family proliferation and TE content evolve in non-model species when populations become threatened. Freshwater bivalves (Unionidae) currently face severe anthropogenic challenges. Over 70% of species in the United States are threatened, endangered or extinct due to pollution, damming of waterways, and overfishing. We have created a reference genome for M. nervosa to determine how genome content has evolved in the face of these widespread environmental challenges. We observe a burst of recent transposable element proliferation causing a 382 Mb expansion in genome content. Gene family expansion is common, with a duplication rate of 1.16 x 10^-8 per gene per generation. Cytochrome P450, ABC transporters, Hsp70 genes, von Willebrand proteins, chitin metabolism genes, mitochondria eating proteins, and opsin gene families have experienced significantly greater amplification and show signatures of selection. We use evolutionary theory to assess the relative contribution of SNPs and duplications in evolutionary change. Estimates suggest that gene family evolution may offer an exceptional substrate of genetic variation in M. nervosa, with Psgv=0.185 compared with Psgv=0.067 for single nucleotide changes. Hence, we suggest that gene family evolution is a source of "hopeful monsters" within the genome that facilitate adaptation., Comment: Main text 42 pages, 1 table 8 figures; SI 12 pages, 8 tables, 2 figures; Gene tree phylogenies added to directly address incomplete lineage sorting

Symbiotic relationships are often species specific, allowing symbionts to adapt to their host environments. Host generalists, on the other hand, have to cope with diverse environments. One coping strategy is phenotypic plasticity, defined by the presence of host-specific phenotypes in the absence of genetic differentiation. Recent work indicates that such host-specific phenotypic plasticity is present in the West Pacific lineage of the commensal barnacle Chelonibia testudinaria (Linnaeus, 1758). We investigated genetic and morphological host-specific structure in the genetically distinct Atlantic sister lineage of C. testudinaria. We collected adult C. testudinaria from loggerhead sea turtles, horseshoe crabs, and blue crabs along the eastern U.S. coast between Delaware and Florida and in the Gulf of Mexico off Mississippi. We find that shell morphology, especially shell thickness, is host specific and comparable in similar host species between the Atlantic and West Pacific lineages. We did not detect significant genetic differentiation related to host species when analyzing data from 11 nuclear microsatellite loci and mitochondrial sequence data, which is comparable to findings for the Pacific lineage. The most parsimonious explanation for these parallel patterns between distinct lineages of C. testudinaria is that C. testudinaria maintained phenotypic plasticity since the lineages diverged 4-5 mya.

Male genitalia are extraordinarily diverse, and this diversity makes them valuable taxonomic and phylogenetic markers. Genitalia of true crabs (Brachyura) consist of two pairs of modified appendages, or gonopods. The first pair is tubular, and holds the second pair during copulation. Whether the second pair is shorter or longer than the first pair has important implications for copulation. Its length is often similar in closely related species, e.g. within genera and families, but is highly variable in the Brachyura at large. The genus Calappa Weber, 1795 contains species with long, short, and intermediate-length second gonopods, representing an unusually variable pattern. We investigated the evolution of the second gonopods in Calappa and other members of Calappidae. We assembled a molecular multi-locus phylogeny (mitochondrial marker: COI and 16S, nuclear marker: Enolase, H3 and 28S). Based on this phylogeny, long second gonopods are the ancestral state for Calappa as well as Calappidae in general. Short second gonopods evolved once, while intermediate-length second gonopods evolved independently at least four times. Scanning electron microscopic images of second gonopods of 22 species of Calappidae reveal that shortening of the second gonopod is always achieved by a shortening of the terminal segment. Congruently, taxon-specific structures at the tip of the terminal segment of the second gonopod, such as spines and pistil-like protrusions, are limited to long second gonopods. Based on current knowledge, a shortening of the second gonopod should be accompanied by extensive changes in the copulatory mechanism of males and females, including changes to the copulatory mechanism, mechanism of species recognition, targets of sexual selection, and the potential for sperm removal. Calappa therefore provides a vital example for the study of changes in copulatory mechanisms.

Beginning in 2013, sea stars throughout the Eastern North Pacific were decimated by wasting disease, also known as "asteroid idiopathic wasting syndrome" (AIWS) due to its elusive aetiology. The geographic extent and taxonomic scale of AIWS meant events leading up to the outbreak were heterogeneous, multifaceted, and oftentimes unobserved; progression from morbidity to death was rapid, leaving few tell-tale symptoms. Here, we take a forensic genomic approach to discover candidate genes that may help explain sea star wasting syndrome. We report the first genome and annotation for Pisaster ochraceus, along with differential gene expression (DGE) analyses in four size classes, three tissue types, and in symptomatic and asymptomatic individuals. We integrate nucleotide polymorphisms associated with survivors of the wasting disease outbreak, DGE associated with temperature treatments in P. ochraceus, and DGE associated with wasting in another asteroid Pycnopodia helianthoides. In P. ochraceus, we found DGE across all tissues, among size classes, and between asymptomatic and symptomatic individuals; the strongest wasting-associated DGE signal was in pyloric caecum. We also found previously identified outlier loci co-occur with differentially expressed genes. In cross-species comparisons of symptomatic and asymptomatic individuals, consistent responses distinguish genes associated with invertebrate innate immunity and chemical defence, consistent with context-dependent stress responses, defensive apoptosis, and tissue degradation. Our analyses thus highlight genomic constituents that may link suspected environmental drivers (elevated temperature) with intrinsic differences among individuals (age/size, alleles associated with susceptibility) that elicit organismal responses (e.g., coelomocyte proliferation) and manifest as sea star wasting mass mortality.

A genotypic polymorphism in the sea starPisaster ochraceushas been associated with possible overdominant maintenance of diversity, and subsequent studies of this polymorphism suggested that intermittent disease outbreaks could be a driving factor in this system. However, comparative transcriptomic studies of individuals carrying distinct genotypes at the elongation factor 1-alpha (EF1A) region indicated that the marker was not accurately describing the constitutive differences among individuals. Here we more thoroughly assess this EF1A intron region to better understand how polymorphic diversity could be associated with differential disease outcomes and physiological responses, and find that the underlying genetic model is incorrect. In fact, rather than an instance of homozygous lethality, it is clear that previous genotyping efforts were misled by a PCR artefact. We reanalyze results from two previous studies to show that the effects are not as clear as believed.

Dispersal and adaptation are the two primary mechanisms that set the range distributions for a population or species. As such, understanding how these mechanisms interact in marine organisms in particular – with capacity for long‐range dispersal and a poor understanding of what selective environments species are responding to – can provide useful insights for the exploration of biogeographic patterns. Previously, the barnacle Notochthamalus scabrosus has revealed two evolutionarily distinct lineages with a joint distribution that suggests an association with one of the two major biogeographic boundaries (~30°S) along the coast of Chile. However, spatial and genomic sampling of this system has been limited until now. We hypothesized that given the strong oceanographic and environmental shifts associated with the other major biogeographic boundary (~42°S) for Chilean coastal invertebrates, the southern mitochondrial lineage would dominate or go to fixation in locations further to the south. We also evaluated nuclear polymorphism data from 130 single nucleotide polymorphisms to evaluate the concordance of the signal from the nuclear genome with that of the mitochondrial sample. Through the application of standard population genetic approaches along with a Lagrangian ocean connectivity model, we describe the codistribution of these lineages through a simultaneous evaluation of coastal lineage frequencies, an approximation of larval behavior, and current‐driven dispersal. Our results show that this pattern could not persist without the two lineages having distinct environmental optima. We suggest that a more thorough integration of larval dynamics, explicit dispersal models, and near‐shore environmental analysis can explain much of the coastal biogeography of Chile.

We analyzed the population structure of the edible barnacle Austromegabalanus psittacus (Molina, 1782) along most of the coast of Chile. The analysis of population structure was based on nucleotide sequences of the mitochondrial cytochrome oxidase I (COI) gene region. We also tested for differences between the regions to the north and south of 30-33°S, as these latitudes represent a recognized biogeographic break and important oceanographic transitions occur in that area. No geographic differentiation was evident when using Hudson’s nearest-neighbor ( S nn ) statistic to analyze genetic differences between all populations. F st values nevertheless showed overall genetic structure among sites. Significant geographic structure was found using S nn and analysis of molecular variance (AMOVA) when locations were separated into northern and southern regions, with a stronger signal when the geographic division is set at 33°S. Our results support the idea that oceanographic transitions can affect the genetic structure in species with pelagic larvae. We also discuss observations on size structure differences within the natural range of A. psittacus and this barnacle’s sympatric occurrence with another barnacle, Megabalanus concinnus (Darwin, 1854) in its northern range.

Androdioecy is a sexual system characterized by the coexistence of hermaphrodites and males. It has evolved several times independently in ancestrally hermaphroditic barnacles. Life history and sex allocation theory suggest that dwarf males can occur in hermaphroditic populations with very small mating groups, low growth rates and high mortality rates. We tested these predictions in the androdioecious barnacle Chelonibia testudinaria (Linnaeus, 1758), an epibiont on loggerhead sea turtles, blue crabs, and horseshoe crabs. The potential number of mates is indeed very small on invertebrate hosts but larger on sea turtles. Growth rates are host-specific but comparable to those of other purely hermaphroditic barnacle species. The maximum age of hermaphrodites was less than three years, which is lower than in most purely hermaphroditic species. Our data suggest that small mating groups on some hosts and high mortality on all hosts contribute to the evolutionary origin and persistence of dwarf males.

Classic biogeographic studies emphasized diff erences in species composition between regions to defi ne biogeographic provinces and delimit biogeographic boundaries. Here we analyze the permeability of biogeographic boundaries to diff erent species to gain mechanistic insight into the processes that maintain species boundaries in the coastal ocean. We identify sites with high frequencies of range boundaries using almost 1800 benthic marine invertebrates along the northwestern Atlantic coast and address whether their magnitude and location vary as a function of species ’ taxonomy, pelagic larval duration and depth distribution. We observed clusters of species boundaries at Cape Hatteras, Cape Cod and the Bay of Fundy that are largely independent of taxonomic group. However, the boundaries were permeable and asymmetric, with a higher percentage of species shared across boundaries in the equatorward direction (82%) than in the reverse direction (59%). Th is pattern was particularly strong for shallow species (median occurrence depth � 20 m). Pelagic larval duration was more important to explain distributions of boundaries for deep species (median occurrence depth � 20 m), where species with long larval dispersal had signifi cantly higher occurrence of boundaries than species with short larval dispersal. When they do exist, species boundaries seem to be set by the interaction of currents, depth distribution and pelagic larval duration. Importantly, species boundaries tend to be pinned to regions of reduced water transport, which might explain why species boundaries are concentrated in narrow geographical areas.

An overdominant mutation in an intron of the elongation factor 1-α (EF1A) gene in the sea star Pisaster ochraceus has shown itself to mediate tolerance to “sea star wasting disease”, a pandemic that has significantly reduced sea star populations on the Pacific coast of North America. Here we use RNA sequencing of healthy individuals to identify differences in constitutive expression of gene regions that may help explain this tolerance phenotype. Our results show that individuals carrying this mutation have lower expression at a large contingent of gene regions. Individuals without this mutation also appear to have a greater cellular response to temperature stress, which has been implicated in the outbreak of sea star wasting disease. Given the ecological significance of P. ochraceus, these results may be useful in predicting the evolutionary and demographic future for Pacific intertidal communities.

Determining whether a population is introduced or native to a region can be challenging due to inadequate taxonomy, the presence of cryptic lineages, and poor historical documentation. For taxa with resting stages that bloom episodically, determining origin can be especially challenging as an environmentally-triggered abrupt appearance of the taxa may be confused with an anthropogenic introduction. Here, we assess diversity in mitochondrial cytochrome oxidase I sequences obtained from multiple Atlantic and Pacific locations, and discuss the implications of our findings for understanding the origin of clinging jellyfishGonionemusin the Northwest Atlantic. Clinging jellyfish are known for clinging to seagrasses and seaweeds, and have complex life cycles that include resting stages. They are especially notorious as some, although not all, populations are associated with severe sting reactions. The worldwide distribution ofGonionemushas been aptly called a “zoogeographic puzzle” and our results refine rather than resolve the puzzle. We find a relatively deep divergence that may indicate cryptic speciation betweenGonionemusfrom the Northeast Pacific and Northwest Pacific/Northwest Atlantic. Within the Northwest Pacific/Northwest Atlantic clade, we find haplotypes unique to each region. We also find one haplotype that is shared between highly toxic Vladivostok-area populations and some Northwest Atlantic populations. Our results are consistent with multiple scenarios that involve both native and anthropogenic processes. We evaluate each scenario and discuss critical directions for future research, including improving the resolution of population genetic structure, identifying possible lineage admixture, and better characterizing and quantifying the toxicity phenotype.

Reproductive compatibility proteins have been shown to evolve rapidly under positive selection leading to reproductive isolation, despite the potential homogenizing effects of gene flow. This process has been implicated in both primary divergence among conspecific populations and reinforcement during secondary contact; however, these two selective regimes can be difficult to discriminate from each other. Here, we describe the gene that encodes the gamete compatibility protein bindin for three sea star species in the genus Pisaster. First, we compare the full-length bindin-coding sequence among all three species and analyze the evolutionary relationships between the repetitive domains of the variable second bindin exon. The comparison suggests that concerted evolution of repetitive domains has an effect on bindin divergence among species and bindin variation within species. Second, we characterize population variation in the second bindin exon of two species: We show that positive selection acts on bindin variation in Pisaster ochraceus but not in Pisaster brevispinus, which is consistent with higher polyspermy risk in P. ochraceus. Third, we show that there is no significant genetic differentiation among populations and no apparent effect of sympatry with congeners that would suggest selection based on reinforcement. Fourth, we combine bindin and cytochrome c oxidase 1 data in isolation-with-migration models to estimate gene flow parameter values and explore the historical demographic context of our positive selection results. Our findings suggest that positive selection on bindin divergence among P. ochraceus alleles can be accounted for in part by relatively recent northward population expansions that may be coupled with the potential homogenizing effects of concerted evolution.

A mitochondrial cline along the Chilean coast in the barnacle Notochthamalus scabrosus suggests a species history of transient allopatry and secondary contact. However, previous studies of nuclear sequence divergence suggested population genetic homogeneity across northern and central Chile. Here, we collect single-nucleotide polymorphism data from pooled population samples sequenced with restriction site−associated DNA sequencing procedures, confirm these data with the use of a GoldenGate array, and identify a discordance between population genetic patterns in the nuclear and mitochondrial genomes. This discordance was noted in previous work on this species, but here it is confirmed that the nuclear genome exhibits only slight phylogeographic variation across 3000 km of coastline, in the presence of a strong and statistically significant mitochondrial cline. There are nevertheless markers (approximately 5% of nuclear single-nucleotide polymorphisms) exhibiting cytonuclear disequilibrium relative to mitotype. Although these data confirm our previous explorations of this species, it is likely that some of the nuclear genomic diversity of this species has yet to be explored, as comparison with other barnacle phylogeography studies suggest that a divergence of similar magnitude should be found in the nuclear genome somewhere else in the species range.

Eight whole mitochondrial genomes from the barnacle Notochthamalus scabrosus, with one from the northern lineage and seven from the divergent southern lineage, are presented. The annotated and aligned data were analyzed for signals of non-neutral evolution. Overall, these data are consistent with purifying selection operating on the protein-coding regions of the mitochondrion. However, a notable region of nonsynonymous substitution at the 3′ end of the ND2 gene region, along with unusual site frequency spectra in two other gene regions, was identified.

North American freshwater mussels are critically imperiled organisms that generally require fish hosts in order to complete their life cycle. Although numerous studies have focused on the parasitic relationship between mussels and fishes, few have examined the benefits that mussels provide to other organisms. During sampling of Altamaha River, Georgia, we observed foreign eggs occurring within body cavities of native mussels across a 253-km reach of the river basin. Eggs were recovered from 6% of the 757 mussels examined among seven sites. Foreign eggs were present in 17% and 18% of examined mussels at two sites. Using molecular techniques, eggs were identified as American Shad Alosa sapidissima. This discovery appears to be the first documented occurrence of native fish eggs in live North American mussels. Further research into the nature and mechanism of this symbiosis is warranted to assess whether this relationship is amensalistic, mutualistic, or commensalistic as American Shad and many fres...

Genetic variation at the major histocompatibility complex (MHC) is vitally important for wildlife populations to respond to pathogen threats. As natural populations can fluctuate greatly in size, a key issue concerns how population cycles and bottlenecks that could reduce genetic diversity will influence MHC genes. Using 454 sequencing, we characterized genetic diversity at the DRB Class II locus in montane voles (Microtus montanus), a North American rodent that regularly undergoes high-amplitude fluctuations in population size. We tested for evidence of historic balancing selection, recombination, and gene duplication to identify mechanisms maintaining allelic diversity. Counter to our expectations, we found strong evidence of purifying selection acting on the DRB locus in montane voles. We speculate that the interplay between population fluctuations and gene duplication might be responsible for the weak evidence of historic balancing selection and strong evidence of purifying selection detected. To further explore this idea, we conducted a phylogenetically controlled comparative analysis across 16 rodent species with varying demographic histories and MHC duplication events (based on the maximum number of alleles detected per individual). On the basis of phylogenetic generalized linear model-averaging, we found evidence that the estimated number of duplicated loci was positively related to allelic diversity and, surprisingly, to the strength of purifying selection at the DRB locus. Our analyses also revealed that species that had undergone population bottlenecks had lower allelic richness than stable species. This study highlights the need to consider demographic history and genetic structure alongside patterns of natural selection to understand resulting patterns of genetic variation at the MHC.

Approximate Bayesian computation (ABC) is useful for parameterizing complex models in population genetics. In this study, ABC was applied to simultaneously estimate parameter values for a model of metapopulation coalescence and test two alternatives to a strict metapopulation model in the well-studied network of Daphnia magna populations in Finland. The models shared four free parameters: the subpopulation genetic diversity (θS), the rate of gene flow among patches (4Nm), the founding population size (N0) and the metapopulation extinction rate (e) but differed in the distribution of extinction rates across habitat patches in the system. The three models had either a constant extinction rate in all populations (strict metapopulation), one population that was protected from local extinction (i.e. a persistent source), or habitat-specific extinction rates drawn from a distribution with specified mean and variance. Our model selection analysis favoured the model including a persistent source population over the two alternative models. Of the closest 750 000 data sets in Euclidean space, 78% were simulated under the persistent source model (estimated posterior probability = 0.769). This fraction increased to more than 85% when only the closest 150 000 data sets were considered (estimated posterior probability = 0.774). Approximate Bayesian computation was then used to estimate parameter values that might produce the observed set of summary statistics. Our analysis provided posterior distributions for e that included the point estimate obtained from previous data from the Finnish D. magna metapopulation. Our results support the use of ABC and population genetic data for testing the strict metapopulation model and parameterizing complex models of demography.

In many species, alternative developmental pathways lead to the production of two distinct phenotypes, promoting the evolution of morphological novelty and diversification. Offspring type in marine invertebrates influences transport time by ocean currents, which dictate dispersal potential and gene flow, and thus has sweeping evolutionary effects on the potential for local adaptation and on rates of speciation, extinction and molecular evolution. Here, we use the polychaete Streblospio benedicti to investigate the effects of dimorphic offspring type on gene flow and genetic structure in coastal populations. We use 84 single nucleotide polymorphism (SNP) markers for this species to assay populations on the East and West Coasts of the United States. Using these markers, we found that in their native East Coast distribution, populations of S. benedicti have high-population genetic structure, but this structure is associated primarily with geographic separation rather than developmental differences. Interestingly, very little genetic differentiation is recovered between individuals of different development types when they occur in the same or nearby populations, further supporting that this is a true case of poecilogony. In addition, we were able to demonstrate that the recently introduced (~100 ya) West Coast populations probably originated from a lecithotrophic population near Delaware.

We evaluate the population genetic structure of the intertidal barnacleJehlius cirratusacross a broad portion of its geographic distribution using data from the mitochondrial cytochrome oxidase I (COI) gene region. Despite sampling diversity from over 3,000 km of the linear range of this species, there is only slight regional structure indicated, with overall ΦCTof 0.036 (p J. cirratushad indicated, the pattern of diversity is still far more subtle than in other similarly-distributed species with similar larval and life history traits. We compare these data and results with recent findings in four other intertidal species that have planktotrophic larvae. There are no clear patterns among these taxa that can be associated with intertidal depth or other known life history traits.

Microsatellite markers remain an important tool for ecological and evolutionary research, but are unavailable for many non-model organisms. One such organism with rare ecological and evolutionary features is the epizoic barnacleChelonibia testudinaria(Linnaeus, 1758).Chelonibia testudinariaappears to be a host generalist, and has an unusual sexual system, androdioecy. Genetic studies on host specificity and mating behavior are impeded by the lack of fine-scale, highly variable markers, such as microsatellite markers. In the present study, we discovered thousands of new microsatellite loci from next-generation sequencing data, and characterized 12 loci thoroughly. We conclude that 11 of these loci will be useful markers in future ecological and evolutionary studies onC. testudinaria.