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).

Rapid and socially induced change of a badge of status.

This paper reports a new aspect of male signalling in the green swordtail Xiphophorus hellerii. Males are highly ornamented and possess a sword (an extension of the caudal fin), as well as a lateral stripe that has been shown to be a badge of status. Males of one natural population, however, were found to quickly change the colour of their lateral stripe to red when socially dominant over other males and black when subdominant, thereby exploiting the female preference for red stripes while dominant, and avoiding the costs of male aggression while subdominant.

Toxic hydrogen sulphide and dark caves: pronounced male life-history divergence among locally adapted Poecilia mexicana (Poeciliidae).

Chronic environmental stress is known to induce evolutionary change. Here, we assessed male life-history trait divergence in the neotropical fish Poecilia mexicana from a system that has been described to undergo incipient ecological speciation in adjacent, but reproductively isolated toxic/nontoxic and surface/cave habitats. Examining both field-caught and common garden-reared specimens, we investigated the extent of differentiation and plasticity of life-history strategies employed by male P. mexicana. We found strong site-specific life-history divergence in traits such as fat content, standard length and gonadosomatic index. The majority of site-specific life-history differences were also expressed under common garden-rearing conditions. We propose that apparent conservatism of male life histories is the result of other (genetically based) changes in physiology and behaviour between populations. Together with the results from previous studies, this is strong evidence for local adaptation as a result of ecologically based divergent selection.

Morphology, testes development and behaviour of unusual triploid males in microchromosome-carrying clones of Poecilia formosa.

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.

Feeding rates in the sailfin molly Poecilia latipinna and its coexisting sexual parasite, the gynogenetic Amazon molly Poecilia formosa.

Feeding rates of the gynogenetic Amazon molly Poecilia formosa and one of its sexual hosts, the sailfin molly Poecilia latipinna, were measured under winter and summer temperature conditions. Food consumption of the unisexual P. formosa in winter conditions was significantly higher than that of P. latipinna, and it is hypothesized that the resulting food stress might have an important influence on the population composition of these closely related fishes via higher winter mortality in P. formosa.

Using video playback to study the effect of an audience on male mating behavior in the Sailfin molly (Poecilia latipinna).

Sexual conflict in poeciliid fishes is well-documented, particularly male sexual harassment and its effects on females. For instance, male attempts to force copulations influence female feeding, energy allocation, and preference for shoaling partners. However, there has been little research conducted to determine how the social environment shapes the occurrence and intensity of sexual harassment. In this study we ask whether an audience male influences the sexual behaviors of a focal male, the correlated feeding time reduction of female Poecilia latipinna, and if the size of the audience male (larger or smaller than the focal male) influences these behaviors. We presented a video of a male, either smaller or larger than the focal male, or an empty tank (control) to a female interacting with a male or female partner and measured feeding times and sexual behaviors. We found that male sexual behaviors increased in the presence of an audience male, especially if the audience male was larger than the focal male. Females fed more in the presence of a partner female than in the presence of a male, which was independent of the audience (i.e., video treatment). Focal female aggression towards the partner female increased with the size of the audience male. The present study shows that an audience male has multiple interacting influences on both male and female behavior.

Differences in thermal tolerance in coexisting sexual and asexual mollies (Poecilia, Poeciliidae, Teleostei).

This study reports significant differences between the gynogenetic Amazon molly Poecilia formosa and one of its sperm hosts, and the sexual sailfin molly Poecilia latipinna in the critical temperatures at which individual fishes lost motion control. Based on these measurements, it is suggested that cold snaps occurring in winter, but not summer temperatures, can significantly change population composition of these closely related fishes by inflicting higher mortality on P. formosa.

Local adaptation and pronounced genetic differentiation in an extremophile fish, Poecilia mexicana, inhabiting a Mexican cave with toxic hydrogen sulphide.

We investigated genetic differentiation and migration patterns in a small livebearing fish, Poecilia mexicana, inhabiting a sulfidic Mexican limestone cave (Cueva del Azufre). We examined fish from three different cave chambers, the sulfidic surface creek draining the cave (El Azufre) and a nearby surface creek without the toxic hydrogen sulphide (Arroyo Cristal). Using microsatellite analysis of 10 unlinked loci, we found pronounced genetic differentiation among the three major habitats: Arroyo Cristal, El Azufre and the cave. Genetic differentiation was also found within the cave between different pools. An estimation of first-generation migrants suggests that (i) migration is unidirectional, out of the cave, and (ii) migration among different cave chambers occurs to some extent. We investigated if the pattern of genetic differentiation is also reflected in a morphological trait, eye size. Relatively large eyes were found in surface habitats, small eyes in the anterior cave chambers, and the smallest eyes were detected in the innermost cave chamber (XIII). This pattern shows some congruence with a previously proposed morphocline in eye size. However, our data do not support the proposed mechanism for this morphocline, namely that it would be maintained by migration from both directions into the middle cave chambers. This would have led to an increased variance in eye size in the middle cave chambers, which we did not find. Restricted gene flow between the cave and the surface can be explained by local adaptations to extreme environmental conditions, namely H2S and absence of light. Within the cave system, habitat properties are patchy, and genetic differentiation between cave chambers despite migration could indicate local adaptation at an even smaller scale.

Male mate choice and sperm allocation in a sexual/asexual mating complex of Poecilia (Poeciliidae, Teleostei).

Male mate choice is critical for understanding the evolution and maintenance of sexual/asexual mating complexes involving sperm-dependent, gynogenetic species. Amazon mollies (Poecilia formosa) require sperm to trigger embryogenesis, but the males (e.g. Poecilia mexicana) do not contribute genes. Males benefit from mating with Amazon mollies, because such matings make males more attractive to conspecific females, but they might control the cost of such matings by providing less sperm to Amazon mollies. We examined this at the behavioural and sperm levels. P. mexicana males preferred to mate with, and transferred more sperm to conspecific females. However, if males mated with P. formosa, sperm was readily transferred. This underscores the importance of male choice in this system.

Distribution and stability of supernumerary microchromosomes in natural populations of the Amazon molly, Poecilia formosa.

In animals, supernumerary chromosomes and their evolution have mostly been studied in sexual reproducing species. In the present study, for the first time, the natural distribution and stability of supernumerary microchromosomes were investigated in the unisexual fish species Poecilia formosa. Natural habitats throughout the range of P. formosa were screened for the presence of microchromosomes over several years. A high frequency of microchromosomes was found in the Río Purificación river system. Evidence points to the presence of the same microchromosome lineage over many generations. No supernumerary chromosomes were found elsewhere than in the Río Purificación representing a significant difference in the distribution of microchromosome-bearing individuals between the Río Purificación and all other collection sites.

Using video playbacks to study visual communication in a marine fish, Salaria pavo.

Video playbacks have been successfully applied to the study of visual communication in several groups of animals. However, this technique is controversial as video monitors are designed with the human visual system in mind. Differences between the visual capabilities of humans and other animals will lead to perceptually different interpretations of video images. We simultaneously presented males and females of the peacock blenny, Salaria pavo, with a live conspecific male and an online video image of the same individual. Video images failed to elicit appropriate responses. Males were aggressive towards the live male but not towards video images of the same male. Similarly, females courted only the live male and spent more time near this stimulus. In contrast, females of the gynogenetic poecilid Poecilia formosa showed an equal preference for a live and video image of a P. mexicana male, suggesting a response to live animals as strong as to video images. We discuss differences between the species that may explain their opposite reaction to video images. Copyright 2000 The Association for the Study of Animal Behaviour.

Female preference for symmetrical vertical bars in male sailfin mollies.

Fluctuating asymmetry is defined as random deviation from perfect bilateral symmetry in otherwise symmetrical morphological traits and originates from developmental errors during ontogeny. It is thought to reflect the inability of a genotype to buffer itself effectively against environmental perturbations, thus providing an honest phenotypic indicator of genetic quality. It has been proposed that females use the degree of asymmetry in male ornaments to assess male quality in mate choice. We studied whether fluctuating asymmetry in vertical bars in the sailfin molly, Poecilia latipinna, is a criterion in female mate preference, as has been shown before in a swordtail, Xiphophorus cortezi. Using both live stimulus males and silicon models, we found that females have significant preferences both for bars per se and for males with symmetrical bars. The total number of bars and the degree of fluctuating asymmetry were negatively related so fluctuating asymmetry in bars may be an honest signal. This phenomenon may have influenced the evolution of the male courtship display in P. latipinna. Copyright 1998 The Association for the Study of Animal Behaviour.

Dispensable and indispensable genes in an ameiotic fish, the Amazon molly Poecilia formosa.

All-female vertebrates are excellent model systems for studying many evolutionary problems. One of these is the Amazon molly. In this review, three aspects of its biology are discussed: (1) An important question is how dispensable genes, such as all male coding genes, evolve in this species. A number of studies found that most of these genes remain remarkably stable and functional. (2) The gynogenetic Amazon mollies have to live in sympatry with males of a gonochoristic species, because sperm are needed to trigger embryogenesis. Yet, Amazon mollies cannot replace their sexual competitors, because this would lead to their own extinction. Studies on the behavior of Amazon mollies and their sperm-donor species indicate that a number of behavior patterns stabilize the mating system by providing Amazon mollies with the copulations they need to reproduce. (3) The age of Amazon mollies has been estimated to be approximately 100,000 years. This is older than predicted by some theoretical models. In Amazon mollies two ways to occasionally incorporate fresh genetic material have evolved. One way is to add one complete set of paternal chromosomes, which, in nature, leads to stable triploid lineages. The second way is the incorporation of minute, centromere-containing microchromosomes. The evolutionary impact of these phenomena, however, is not resolved so far and needs further study.

Susceptibility to the development of pigment cell tumors in a clone of the Amazon molly, Poecilia formosa, introduced through a microchromosome.

The Amazon molly Poecilia formosa is a gynogenetic fish that reproduces through the development of ameiotic diploid eggs triggered by insemination by males of related species without following karyogamie. This leads to clonal offspring. In rare cases, however, this gynogenesis is leaky, and paternal DNA in the form of small supernumerary chromosomes is included into the maternal genome. We have obtained a clone where one such microchromosome contains a pigmentary locus, resulting in macromelanophore pigmentation of the carrier. Approximately 5% of these fish spontaneously develop exophytic nodular or papillomatous pigment cell tumors. The tumors display considerable differences with respect to growth characteristics and invasiveness, despite the genetic uniformity of the affected animals. Following transplantation to syngeneic hosts, a remarkable clonal variability was observed. Oncogenes that are involved in tumorigenesis in hereditary melanoma of the closely related fish Xiphophorus appear not to be instrumental for induction of the P. formosa pigment cell tumors. Moreover, a new genetic locus is defined that mediates susceptibility to pigment cell tumor development and leads to transformation of chromatoblasts.

Benefit to male sailfin mollies of mating with heterospecific females.

Female gynogens reproduce clonally but rely on sperm from heterospecific males to initiate embryogenesis. It has been assumed that males gain no benefit from such matings; thus, selection should favor males that avoid them. Here it is shown that males gain a benefit by mating with female gynogens in an asexual-sexual complex of fish. The sexual females increase their preference for males whom they observe consorting with female gynogens. Thus, gynogenetic species might persist because selection favors males to be sexually parasitized.

On the stability of dispensable constituents of the eukaryotic genome: stability of coding sequences versus truly hypervariable sequences in a clonal vertebrate, the amazon molly, Poecilia formosa.

In clonal unisexual vertebrates, the genes specifying the males become dispensable. To study the fate of such genes the gynogenetic all-female fish Poecilia formosa was treated with androgens. Phenotypic males were obtained that exhibited the complete set of male characteristics of closely related gonochoristic species, including body proportions, pigmentation, the extremely complex insemination apparatus of poeciliid fish, sexual behavior, and spermatogenesis. The apparent stability of such genic structures, including those involved in androgen regulation, is contrasted by high instability of noncoding sequences. Frequent mutations, their clonal transmission, and at least two truly hypervariable loci leading to individual differences between these otherwise clonal organisms were detected by DNA fingerprinting. These observations substantiate the concept that also in "ameiotic" vertebrates certain compartments of the genome are more prone to mutational alterations than others.