Deleterious mutations can accumulate in the germline with age, decreasing the genetic quality of sperm and imposing a cost on female fitness. If these mutations also affect sperm competition ability or sperm production, then females will benefit from polyandry as it incites sperm competition and, consequently, minimizes the mutational load in the offspring. We tested this hypothesis in the guppy (Poecilia reticulata), a species characterized by polyandry and intense sperm competition, by investigating whether age affects post-copulatory male traits and sperm competition success. Females did not discriminate between old and young males in a mate choice experiment. While old males produced longer and slower sperm with larger reserves of strippable sperm, compared to young males, artificial insemination did not reveal any effect of age on sperm competition success. Altogether, these results do not support the hypothesis that polyandry evolved in response to costs associated with mating with old males in the guppy.

Keywords:
male age; post-copulatory sexual selection; sperm competition; sperm morphology; sperm production; sperm traits; sperm velocity; sperm viability.

Our understanding of the evolution of genes of the major histocompatibility complex (MHC) is rapidly increasing, but there are still enigmatic questions remaining, particularly regarding the maintenance of high levels of MHC polymorphisms in small, isolated populations. Here, we analyze the genetic variation at eight microsatellite loci and sequence variation at exon 2 of the MHC class IIB (DAB) genes in two wild populations of the Trinidadian guppy, Poecilia reticulata. We compare the genetic variation of a small (Ne ≈ 100) and relatively isolated upland population to that of its much larger (Ne ≈ 2400) downstream counterpart. As predicted, microsatellite diversity in the upland population is significantly lower and highly differentiated from the population further downstream. Surprisingly, however, these guppy populations are not differentiated by MHC genetic variation and show very similar levels of allelic richness. Computer simulations indicate that the observed level of genetic variation can be maintained with overdominant selection acting at three DAB loci. The selection coefficients differ dramatically between the upland (s ≥ 0.2) and lowland (s ≤ 0.01) populations. Parasitological analysis on wild-caught fish shows that parasite load is significantly higher on upland than on lowland fish, which suggests that large differences in selection intensity may indeed exist between populations. Based on the infection intensity, a substantial proportion of the upland fish would have suffered direct or indirect fitness consequences as a result of their high parasite loads. Selection by parasites plays a particularly important role in the evolution of guppies in the upland habitat, which has resulted in high levels of MHC diversity being maintained in this population despite considerable genetic drift.
Key words
Balancing selection, computer simulation, guppy, Gyrodactylus, major histocompatibility complex, parasite, random genetic drift.

The highly diverse genes of the major histocompatibility complex (MHC) are important in the adaptive immune system and are expected to be under selection from pathogens. Thus, the MHC genes provide an exceptional opportunity to investigate patterns of selection within and across populations. In this study, we analyzed genetic variation at the MHC class IIB gene and six microsatellite loci across 10 populations of guppies (Poecilia reticulata) in the northern range of Trinidad. We found a high level of diversity at the MHC, with a total of 43 alleles in 142 individuals. At the population level, we found that neutral evolution could not fully account for the variability found at the MHC. Instead, we found that MHC F_ST statistics were lower than F_ST derived from the microsatellite loci; 33 of 45 population pairwise estimates for the MHC were significantly lower than those for the microsatellite loci, and MHC F_ST estimates were consistently lower than those predicted by a coalescent model of neutral evolution. These results suggest a similar selection acting across populations, and we discuss the potential roles of directional and balancing selection. At the sequence level, we found evidence for both positive and purifying selection. Furthermore, positive selection was detected within and adjacent to the putative peptide-binding region (PBR) of the MHC. Surprisingly, we also found a purifying selection at two sites within the putative PBR. Overall, our data provide evidence for selection for functional diversity at the MHC class IIB gene at both the population and nucleotide levels of guppy populations.

Keywords:
major histocompatibility complex, Poecilia reticulata, natural selection, population differentiation

Sexual selection is thought to be opposed by natural selection such that ornamental traits express a balance between these two antagonistic influences. Phenotypic variation among populations may indicate local shifts in this balance, or that different stable ‘solutions’ are possible, but testing these alternatives presents a major challenge. In the guppy (Poecilia reticulata), a small freshwater fish with male-limited ornamental coloration, these issues can be addressed by transplanting fish among sites of varying predation pressure, thus effectively manipulating the strength and nature of natural selection. Here, we contrast the evolutionary outcome of two such introductions conducted in the Trinidadian El Cedro and Aripo Rivers. We use sophisticated colour appraisal methods that account for full spectrum colour variation and which incorporate the very latest visual sensitivity data for guppies and their predators. Our data indicate that ornamentation evolved along different trajectories: whereas Aripo males evolved more numerous and/or larger orange, black and iridescent markings, El Cedro males only evolved more extensive and brighter iridescence. Examination of the El Cedro experiment also revealed little or no ornamental evolution at the control site over 29 years, which contrasts markedly with the rapid (approx. 2–3 years) changes reported for introduction populations. Finally, whole colour-pattern analysis suggested that the greatest visual difference between El Cedro introduction and control fish would be perceived by the two most salient viewers: guppies and the putatively dangerous predator Crenicichla alta. We discuss whether and how these evolutionary trajectories may result from founder effects, population-specific mate preferences and/or sensory drive.