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Placental reproduction is widespread across vertebrate taxa, but little is known about its life‐history correlates and putative adaptive value. We studied variation in life‐history traits in two populations of the placental poeciliid fish Poeciliopsis prolifica to determine whether differences in post‐fertilization maternal provisioning to embryos have a genetic basis and how food availability affects reproduction. Life histories were characterized for wild‐caught females and for second‐generation lab‐born females raised under two levels of food availability. We found that the two populations did not differ significantly in the wild for any life‐history traits except for the lipid dry weight in females and in embryos at an advanced stage of development. When environmental effects were experimentally controlled, however, populations exhibited significant differences in several traits, including the degree of maternal provisioning to embryos. Food availability significantly affected female size at first parturition, brood size and offspring dry weight at birth. Altogether, these results demonstrate that population differences in maternal provisioning and other life‐history traits have a genetic basis and show a plastic response to food availability. We infer that phenotypic plasticity may mask population differences in the field. In addition, when comparing life‐history patterns in these two populations with known patterns in placental and non‐placental poeciliids, our results support the hypotheses that placentation is an adaptive reproductive strategy under high‐resource conditions but that it may represent a cost under low‐food conditions. Finally, our results highlight that age at maturity and reproductive allotment may be key life‐history traits accompanying placental evolution. J. Exp. Zool. 307A, 2007. © 2007 Wiley‐Liss, Inc.

Natural populations of guppies that co‐occur with the pike cichlid Crenicichla altaand associated predators mature at smaller body sizes, produce more and smaller offspring per litter reproduce more frequently, and have higher reproductive allotments (weight of developing embryos/total body weight) than guppies that co‐occur with just the killifish Rivulus harti(Reznick and Endler, 1982). I here consider three forms of repeatability in these life‐history patterns: i) among replicate samples collected on the same day from the same locality, ii) between Crenicichlaand Rivuluscommunities among a new series of localities, and iii) among a smaller series of Crenicichlaand Rivuluslocalities sampled in two wet and two dry seasons. In the analysis of replicate collections from two localities, seven of eight statistical comparisons revealed no significant difference. The usual methodology for estimating these variables therefore accurately represents guppy life‐history patterns at a given locality. Differences among guppies from Rivulusand Crenicichlalocalities, covering a wider geographical area than considered by Reznick and Endler (1982), were virtually identical to the previous comparison. Wet‐season samples were associated with significant decreases in reproductive allotment and fecundity and significant increases in the size of mature males and the minimum size of reproducing females. Differences between guppies from Rivulusand Crenicichlalocalities persisted across all samples and were consistent with all other observations, although they tended to be smaller during the wet season. Discriminant analyses on female reproductive traits showed that fecundity and offspring size made strong, independent contributions to discriminating between guppies from the two types of localities. The contribution from reproductive allotment was considerably smaller. There was more overlap between predator treatments during the wet season. Only 8.5% of the individuals were misclassified during the dry season, but 19.5% were misclassified during the wet season.

I present two models for the genesis of phenotypic variation in age and size at maturity in male guppies. Both are qualitatively consistent with the observation that age at maturity increases and size at maturity decreases as food availability (and growth rate) decline. The first model (“fixed development”) has a constant size which signals the initiation of the process of maturation and a constant time interval between the initiation and completion of maturation. There is thus no response in the pattern of development to differences in growth rate. Variation in age at maturity results from the different amounts of time required to reach the size at initiation. Variation in size results from different amounts of growth between the initiation and completion of maturation. Phenotypic variation is therefore an epiphenomenon of expressing a fixed pattern of development in different environments. In the second model (“flexible development”), fast growers initiate maturation at a larger size than slow growers; this results in a later age and larger size at maturation than their fast growing counterparts in the fixed development model. Slow growers mature at an earlier age and smaller size than their counterparts in the fixed development model. In the second model, the pattern of development changes with the environment. Two experiments with guppies demonstrate that the second model accurately describes their pattern of development. I conclude that phenotypic plasticity is in part the product of plastic development and may represent an adaptation. Testing whether or not the response is adaptive will require further investigations. A general message of this investigation is that characterizing the reaction norm alone is not sufficient to evaluate the adaptive significance of plasticity; one must also consider the processes that generate this plasticity.

The guppy, Poecilia reticulata, a teleostean fish of the order Cyprinodontiformes, has been used extensively in studies of host-parasite interactions, courtship behavior, and mating preference, as well as in ecological and evolutionary genetics. A related species was among the first poikilotherm vertebrates to be used in the study of histocompatibility genes. All these studies could benefit from the identification and characterization of the guppy major histocompatibility complex (Mhc) genes. Here, both class I and class II genes of the guppy are described. The number of expressed loci, as determined by representation of clones in a cDNA library, sequencing, and Southern blot analysis, may be low in both Mhc classes: combined evidence suggests that there may be one expressed class II locus only and one or two expressed class I loci. The variability of aquaristic guppy stocks is very low: only three and two genes have been detected at the class I and class II loci, respectively, in the stocks examined. This genetic paucity is most likely the consequence of breeding practices employed by aquarists and commercial establishments. Limited sampling of wild guppy populations revealed extensive Mhc polymorphism at loci of both classes in nature. Comparison of guppy Mhc sequences with those of other vertebrates has revealed the existence of a set of insertions/deletions which can be used as characters in cladistic analysis to infer phylogenetic relationships among vertebrate taxa and the Mhc genes themselves. These indels are particularly frequent in the regions coding for the loops of a1 and a2 domains of class I proteins.

Previous investigations (Reznick and Endler, 1982; Reznick, 1982a, 1982b) demonstrated that genetic differences in guppy life histories were associated with differences in predation. Guppies from localities with the pike cichlid Crenicichla altaand associated predators matured earlier, had greater reproductive efforts, and produced more and smaller offspring than did guppies from localities with only Rivulus hartias a potential predator. Crenicichlapreys primarily on large, sexually mature size‐classes of guppies, while Rivuluspreys primarily on small, immature size‐classes. These patterns of predation are hypothesized to alter mean age‐specific survival. Theoretical treatments of such differences in survival predict the observed trends in age at maturity and reproductive effort. We are using introduction experiments to evaluate the role of predators in selecting for these life‐history patterns. The experiment whose results are presented here was conducted in a tributary to the El Cedro River (Trinidad), where a waterfall was the upstream limit to the distribution of all fish except Rivulus.Guppies collected from the Crenicichlalocality immediately below the waterfall (the downstream control) were introduced over the waterfall in 1981. This introduction released the guppies from Crenicichlapredation, exposed them instead to Rivuluspredation only, and also introduced them to a different environment, since the introduction site has greater canopy cover than the site of origin. Changes in guppy life‐history patterns can be attributed to predation and/or the environment. Evidence from fish collected and preserved in the field demonstrated that, by mid‐1983, guppies from the introduction site above the waterfall matured at larger sizes and produced fewer, larger offspring. There were no consistent differences in reproductive allotment (weight of offspring/total weight). With the exception of reproductive allotment, these patterns are identical to previous comparisons between Rivulusand Crenicichlalocalities. A laboratory genetics experiment demonstrated that males from the introduction site matured at a later age and at a larger size than did males from the control site downstream, as predicted from the “age‐specific predation” hypothesis. No differences between localities were observed for female age and size at maturity or for reproductive effort. The trends for fecundity and offspring size were the reverse of those observed in the field. Because only the males changed in the predicted fashion, it is not possible either to reject or to accept the hypothesis of age‐specific predation at this time. We discuss the possible causes for these patterns and the high degree of plasticity in the life history, as evidenced by the differences in fecundity and offspring size between the field and laboratory results.

This corrects the article DOI: 10.1038/nature15256

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