Ultrastructural changes in the urophysis of Mollienesia sphenops following adaptation to seawater

Summary The urophysis or neurohemal contact site of the caudal neurosecretory system of Mollienesia sphenops, the black molly, was studied in animals adapted to an artificial seawater environment. This species of fish was chosen for these studies because of its known ability to osmoregulate and its adaptability to the laboratory aquarium. The urophysis of freshwater acclimated mollys contained an abundance of neurosecretory granules. However, in fish subjected to a seawater environment for one week the number of neurosecretory granules was significantly decreased. In addition, there was an increase in blood cell infiltration of the urophysis.Supported by PHS 5429-16-19 (5-23311)The Author wishes to thank Drs. W. Young and J. Wells for their careful reading of this report and W. Boldosser for technical assistance     

Size and morphological variation in males of the sailfin molly,Poecilia latipinna

Synopsis Mature males of the sailfin molly, Poecilia latipinna (Poeciliidae), display extensive variation in size and morphology. Small males, less than about 30 mm SL, usually exhibit no distinctive color or morphological development. Large males, greater than about 45 mm SL, display full expression of the sailfin traits, including bright blue and gold life colors, distinctive pigment markings on the fins and body, and a greatly enlarged dorsal fin. Intermediate-sized males, 30–45 mm SL, display intermediate and variable development of secondary sexual characteristics. For convenience, small, intermediate, and large males are respectively referred to as Types 1, II, and III. A multi-year analysis of four study populations in east-central Florida revealed that males did not occur in discrete classes but, instead, occurred in a size and morphological continuum. Coloration and morphological development were highly correlated with size, and the mensural traits examined displayed strong allometry. Size distributions of mature males were relatively stable at any study site over time, but there were large differences between sites. At one extreme, males at the VABI site were always relatively small (16–40 mm SL) and Type III males were absent. At the other extreme, the population at the BDGST site contained a full array of male sizes (18–60 mm SL) and Type III males were present in small numbers throughout the summer months. Regression analysis revealed only minor differences between these extreme populations in the relationship between morphological development and body size. It appears that environmental, genetic, and social factors interact in a complex and poorly understood way to control variation in male size and morphology.     

The predictability and magnitude of life‐history divergence to ecological agents of selection: a meta‐analysis in livebearing fishes

Environments causing variation in age‐specific mortality – ecological agents of selection – mediate the evolution of reproductive life‐history traits. However, the relative magnitude of life‐history divergence across selective agents, whether divergence in response to specific selective agents is consistent across taxa and whether it occurs as predicted by theory, remains largely unexplored. We evaluated divergence in offspring size, offspring number, and the trade‐off between these traits using a meta‐analysis in livebearing fishes (Poeciliidae). Life‐history divergence was consistent and predictable to some (predation, hydrogen sulphide) but not all (density, food limitation, salinity) selective agents. In contrast, magnitudes of divergence among selective agents were similar. Finally, there was a negative, asymmetric relationship between offspring‐number and offspring‐size divergence, suggesting greater costs of increasing offspring size than number. Ultimately, these results provide strong evidence for predictable and consistent patterns of reproductive life‐history divergence and highlight the importance of comparing phenotypic divergence across species and ecological selective agents.     

What can phylogeographic breaks in the livebearer Poeciliopsis turrubarensis tell about fish community boundaries in western Costa Rica?

This study asks if the geographic boundary delineating two fish communities in western Costa Rica is congruent with a phylogeographic break in a single widespread fish species Poeciliopsis turrubarensis (Poeciliidae) that spans this area. Such congruence would suggest that a common historical event (e.g. geological or climatic) could be responsible for both patterns. It was found that there was a shared break across a region in central Costa Rica suggesting a common cause may be responsible for both the abrupt shift in fish community composition and the genetic break in P. turrubarensis.     

Selection from parasites favours immunogenetic diversity but not divergence among locally adapted host populations

The unprecedented polymorphism in the major histocompatibility complex (MHC) genes is thought to be maintained by balancing selection from parasites. However, do parasites also drive divergence at MHC loci between host populations, or do the effects of balancing selection maintain similarities among populations? We examined MHC variation in populations of the livebearing fish Poecilia mexicana and characterized their parasite communities. Poecilia mexicana populations in the Cueva del Azufre system are locally adapted to darkness and the presence of toxic hydrogen sulphide, representing highly divergent ecotypes or incipient species. Parasite communities differed significantly across populations, and populations with higher parasite loads had higher levels of diversity at class II MHC genes. However, despite different parasite communities, marked divergence in adaptive traits and in neutral genetic markers, we found MHC alleles to be remarkably similar among host populations. Our findings indicate that balancing selection from parasites maintains immunogenetic diversity of hosts, but this process does not promote MHC divergence in this system. On the contrary, we suggest that balancing selection on immunogenetic loci may outweigh divergent selection causing divergence, thereby hindering host divergence and speciation. Our findings support the hypothesis that balancing selection maintains MHC similarities among lineages during and after speciation (trans‐species evolution).     

Vertical bars on male Xiphophorus multilineatus: a signal that deters rival males and attracts females

We examined the function of the vertical bar pattern on maleswordtails (Xiphophorus multilinneatus) as a signal in bothmale-male competition and female choice. This pattern had previouslybeen described as an aggressive signal because males intensifiedthe bars during male-male encounters in the laboratory. Ourfield observations supported this observation and also showedthat bars intensified when males courted females. The intensityof bars was correlated with access to females in the field.Within the size range of males that have bars, however, neitherbar number nor male size appeared to influence access to females.We used freeze-branding to remove the bars from males in thelaboratory so that we could control for characters correlatedwith bar intensity, and tested males and females separatelyso that we could separate the influence of these two componentsof sexual selection. We compared the responses of males andfemales to males that had their bars removed and control malesfreeze-branded between the bars. Test males responded more aggressivelyto males without bars as compared to control males. In addition,females showed a preference for control males over males thathad their bars removed. These results suggest that the barsmay function as a signal that deters rival males and attractsfemales.     

Replicated hybrid zones of Xiphophorus swordtails along an elevational gradient

Natural hybrid zones provide opportunities to study a range of evolutionary phenomena from speciation to the genetic basis of fitness-related traits. We show that widespread hybridization has occurred between two neo-tropical stream fishes with partial reproductive isolation. Phylogenetic analyses of mitochondrial sequence data showed that the swordtail fish Xiphophorus birchmanni is monophyletic and that X. malinche is part of an independent monophyletic clade with other species. Using informative single nucleotide polymorphisms in one mitochondrial and three nuclear intron loci, we genotyped 776 specimens collected from twenty-three sites along seven separate stream reaches. Hybrid zones occurred in replicated fashion in all stream reaches along a gradient from high to low elevation. Genotyping revealed substantial variation in parental and hybrid frequencies among localities. Tests of F(IS) and linkage disequilibrium (LD) revealed generally low F(IS) and LD except in five populations where both parental species and hybrids were found suggesting incomplete reproductive isolation. In these locations, heterozygote deficiency and LD were high, which suggests either selection against early generation hybrids or assortative mating. These data lay the foundation to study the adaptive basis of the replicated hybrid zone structure and for future integration of behaviour and genetics to determine the processes that lead to the population genetic patterns observed in these hybrid zones.     

Independent evolution of complex life history adaptations in two families of fishes, live-bearing halfbeaks (zenarchopteridae, beloniformes) and poeciliidae (cyprinodontiformes)

We have previously documented multiple, independent origins of placentas in the fish family Poeciliidae. Here we summarize similar analyses of fishes in the family Zenarchopteridae. This family includes three live-bearing genera. Earlier studies documented the presence of superfetation, or the ability to carry multiple litters of young in different stages of development in the same ovary, in some species in all three genera. There is also one earlier report of matrotrophy, or extensive postfertilization maternal provisioning, in two of these genera. We present detailed life-history data for approximately half of the species in all three genera and combine them with the best available phylogeny to make inferences about the pattern of life-history evolution within this family. Three species of Hemirhamphodon have superfetation but lack matrotrophy. Most species in Nomorhamphus and Dermogenys either lack superfetation and matrotrophy or have both superfetation and matrotrophy. Our phylogenetic analysis shows that matrotrophy may have evolved independently in each genus. In Dermogenys, matrotrophic species produce fewer, larger offspring than nonmatrotrophic species. In Nomorhamphus; matrotrophic species instead produce more and smaller offspring than lecithotrophic species. However, the matrotrophic species in both genera have significantly smaller masses of reproductive tissue relative to their body sizes. All aspects of these results are duplicated in the fish family Poeciliidae. We discuss the possible adaptive significance of matrotrophy in the light of these new results. The two families together present a remarkable opportunity to study the evolution of a complex trait because they contain multiple, independent origins of the trait that often include close relatives that vary in either the presence or absence of the matrotrophy or in the degree to which matrotrophy is developed. These are the raw materials that are required for either an analysis of the adaptive significance of the trait or for studies of the genetic mechanisms that underlie the evolution of the trait.     

Repeated evolution of local adaptation in swimming performance: population‐level trade‐offs between burst and endurance swimming in Brachyrhaphis freshwater fish

Specialization is fundamentally important in biology because specialized traits allow species to expand into new environments, in turn promoting population differentiation and speciation. Specialization often results in trade‐offs between traits that maximize fitness in one environment but not others. Despite the ubiquity of trade‐offs, we know relatively little about how consistently trade‐offs evolve between populations when multiple sets of populations experience similarly divergent selective regimes. In the present study, we report a case study on Brachyrhaphis fishes from different predation environments. We evaluate apparent within/between population trade‐offs in burst‐speed and endurance at two levels of evolutionary diversification: high‐ and low‐predation populations of Brachyrhaphis rhabdophora, and sister species Brachyrhaphis roseni and Brachyrhaphis terrabensis, which occur in high‐ and low‐predation environments, respectively. Populations of Brachyrhaphis experiencing different predation regimes consistently evolved swimming specializations indicative of a trade‐off between two swimming forms that are likely highly adaptive in the environment in which they occur. We show that populations have become similarly locally adapted at both levels of diversification, suggesting that swimming specialization has evolved rather rapidly and persisted post‐speciation. Our findings provide valuable insight into how local adaptation evolves at different stages of evolutionary divergence.     

Ejaculate traits in relation to male body size in the eastern mosquitofish Gambusia holbrooki

In the present study, the correlation between sperm number, sperm quality (speed, viability, longevity and length), sperm bundles quality (size and dissolving rate) and male body size has been tested in the eastern mosquitofish Gambusia holbrooki a poecilid species characterized by coercive mating tactics where males do not possess obvious ornaments, and the body size is the key determinant of pre‐copulatory male mating success. The results do not tally with theoretical predictions. Indeed, no correlation between male body size and either sperm or sperm‐bundle traits has been found, evidencing the lack of the theoretically expected trade‐off between the investment in characters involved in mate acquisition and the investment in ejaculate quality. An explanation for the observed pattern comes from the extremely dynamic mating system of G. holbrooki, characterized by variable size‐related male mating success and strong post‐copulatory selective pressure, with all males facing a similar high level of sperm competition. In this situation, a higher investment in growth and maintenance at the expense of ejaculate quality is not expected. These results underscore the necessity to comprehend detailed information on species’ reproductive biology and reproductive environment to understand both the evolution of ejaculate characteristics and possible deviations from theoretical predictions.