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Heritable color variations in the Mexican Swrdtail fish
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TWO TYPES OF MELANOMAS IN A NEW EXPERIMENTAL SYSTEM OF PLATYFISH SWORDTAIL HYBRIDS
A new experimental system for investigating platyfish-swordtail hybrid melanomas was
developed using ornamental platyfish (Xiphophorus maculatus) and swordtails (Xiphophorus helleri).
The incidence of melanomas in the hybrids was studied in relation to individual age factors. Based
on the age of the onset of the melanomas, two types of the melanomas were discerned and were
designated as the fry and the adult melanomas, respectively. The fry melanomas were characterized
by early onset, wide-ranged lesions, an invasive nature owing to densely pigmented cells, and a
tendency toward regression. The adult melanomas were characterized by occurrence in the later
stage of life, a small number of primary sites, and rapid, expanding, and invasive growth due to
sparsely pigmented cells.
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GENETIC BASIS FOR ALTERNATIVE REPRODUCTIVE TACTICS IN THE PYGMY SWORDTAIL, XIPHOPHORUS NIGRENSIS
Differences in adult male size and age at sexual maturity in the Río Coy (Mexico) population of Xiphophorus nigrensis (Pisces; Poeciliidae) are controlled by genetic variation at a Y-linked locus. Four genetic size-classes have been identified. The mating behavior of the males of the three largest size-classes consists exclusively of an elaborate courtship display, whereas that of the genetically small males ranges from display to a sneak-chase attempt at copulation. In the presence of large males, small males switch to the sneak-chase behavior. Females prefer the display of large males. In mating-competition experiments (two females with one large male and one small male), the large males are dominant and deny the small males access to females. From 20 such experiments, 601 large-male and 200 small-male progeny were obtained, indicating that the switch to sneak-chase behavior by small males is not particularly effective in overcoming the large-male advantage. By using the largest males of the genetically smallest size class and the smallest males of the genetically next-larger size-class, size was kept constant, whereas genotype was varied. When these males were tested in competition with genetically large males, only the males of the genetically smallest size class showed sneak-chase behavior. These observations suggest that the difference in mating behavior is not an indirect developmental effect of size but, rather, is under direct genetic control.
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A Genetic Polymorphism in the Swordtail Xiphophorus nigrensis: Testing the Prediction of Equal Fitnesses
When a genetically determined polymorphism is selectively maintained in a population, the different morphs should have equal fitnesses at equilibrium. We empirically examined this prediction for the size polymorphism of the swordtail Xiphophorus nigrensis, in which a single locus on the Y chromosome controls male size. Small males mature earlier and chase females, whereas large males mature later and court females. We analyze our data with a model that uses the differential mating success and the ages at sexual maturity of the two morphs to calculate the per capita death rate necessary for them to have equal fitness. We demonstrate how female fecundity data can be used to determine whether the estimated death rate is biologically realistic. Our data support the hypothesis that morph fitnesses are equal, and the model is fairly robust to changes in population growth rate and differential death rates of morphs. However, the confidence intervals for our estimates are large, which suggests that the null hypothesis only be accepted with caution. We show that in many circumstances very large sample sizes will be needed to distinguish between alternative hypotheses concerning the relative fitnesses of the two morphs. We emphasize that despite the popularity of alternative mating behaviors, specifically, and mixed evolutionarily stable strategies, in general, there is almost no empirical evidence that alternative behavioral morphs have equal fitnesses. Also, the conclusion that morph fitnesses are equal does not address the hypothesis that frequency-dependent mating success is the mechanism maintaining the equilibrium of fitnesses. This requires additional evidence directly demonstrating the fitness effect of changes in morph frequency.
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Mhc class I genes of swordtail fishes, Xiphophorus: variation in the number of loci and existence of ancient gene families
Swordtail fishes and platies in the genus Xiphophorus (order Cyprinodontiformes, Teleostei) encompass 22 closely related species which are the products of a recent adaptive radiation in the streams of Central America. To investigate the evolution of the major histocompatibility complex (Mhc) genes in the period immediately following speciation, the class I genes from 20 of the 22 species were cloned and characterized by sequencing. The analysis revealed the existence of multiple loci (at least seven in some individuals) whose numbers vary among the different species and probably also among individuals of the same species. The variation does not seem to bear any relationship to the taxonomy of the genus. Genes at the different loci are distinguished by their intron sequences and by the presence of characteristic motifs in exons 2 and 3. The variation in copy number of loci may have been effected in part by unequal crossing over occurring between introns of misaligned closely related genes. The sequences of the genes fall into two groups, A and B, which represent ancient lineages. The groups define two families of loci, which diverged from each other an estimated 85 million years ago, before the separation of the Acanthopterygii from the Paracanthopterygii of the advanced bony fishes. Evolution of the genes within each family can be explained by the birth-and-death process driven by gene duplications and mutational differentiation.
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Hybrid origin of a swordtail species (Teleostei: Xiphophorus
clemenciae) driven by sexual selection
The swordlike exaggerated caudal fin extensions of male swordtails are conspicuous traits
that are selected forthrough female choice. Swords are one of only few examples where the
hypothesis of a pre-existing bias is believed to apply for the evolution of a male trait. Previous
laboratory experiments demonstrated that females prefer males with longer swords
and even females from some swordless species show an affiliation for males of sworded
species. Earlier phylogenetic studies based on maternally inherited mitochondrial DNA
placed the sworded southern swordtail Xiphophorus clemenciae with swordless platies,
contradicting its morphology-based evolutionary affinities. The analyses of new nuclear
DNA markers now recover its traditional phylogenetic placement with other southern
swordtails, suggesting that this species was formed by an ancient hybridization event. We
propose that sexual selection through female choice was the likely process of hybrid speciation,
by mating of platy females with males of an ancestral swordtail lineage.In artificial
crosses of descendent species from the two potential ancestral lineages of X. clemenciae the
hybrid and backcross males have swords of intermediate lengths. Additionally, mate
choice experiments demonstrate that hybrid females prefer sworded males. These experimental
lines of evidence make hybridization through xeno-specific sexual selection by
female choice the likely mechanism of speciation
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Genetic variation and phylogeography of the swordtail fish Xiphophorus cortezi (Cyprinodontiformes, Poeciliidae)
Swordtail fish have been studied extensively in relation to diverse aspects of biology; however, little attention has been paid to the patterns of genetic variation within and among populations of swordtails. In this study, we sequenced the mtDNA control region from 65 individuals and 10 populations of Xiphophorus cortezi to investigate the genetic variation within and among populations, including tests for correlations between genetic and geographic distances and tests for species monophyly. We found low gene and nucleotide diversity within populations and high degrees of genetic differentiation among populations. Significant and positive correlations between genetic distance and both river and straight-line geographic distance indicate that genetic differentiation among X. cortezi populations can be explained, to some extent, by an isolation-by-distance model and provide evidence of stream capture. Phylogenetic analyses suggest that X. cortezi is paraphyletic relative to X. malinche, raising questions concerning the status of these taxa as separate species.
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The complete mitochondrial genome of the RR-B strain of swordtail (Xiphophorus helleri)
The genome comprises 13 protein-coding genes, 22 tRNAs, two rRNAs and a major non-coding region. The comparison of the two specimens' mitogenomes revealed a relatively low number (57) of single nucleotide polymorphisms—29 located in protein-coding genes, 11 in rRNA genes, six in tRNA genes, and six in the non-coding region.
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Natural hybridization reveals incompatible alleles that cause melanoma in swordtail fish
The establishment of reproductive barriers between populations can fuel the evolution of new species. A genetic framework for this process posits that “incompatible” interactions between genes can evolve that result in reduced survival or reproduction in hybrids. However, progress has been slow in identifying individual genes that underlie hybrid incompatibilities. We used a combination of approaches to map the genes that drive the development of an incompatibility that causes melanoma in swordtail fish hybrids. One of the genes involved in this incompatibility also causes melanoma in hybrids between distantly related species. Moreover, this melanoma reduces survival in the wild, likely because of progressive degradation of the fin. This work identifies genes underlying a vertebrate hybrid incompatibility and provides a glimpse into the action of these genes in natural hybrid populations.
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The developmental and genetic architecture of the sexually selected male ornament of swordtails
Sexual selection results in sex-specific characters like the conspicuously pigmented extension of the
ventral tip of the caudal fin - the “sword” - in males of several species of Xiphophorusfishes. To uncover
the genetic architecture underlying sword formation and to identify genes that are associated with its
development, we characterized the sword transcriptional profile and combined it with genetic mapping
approaches. Results showed that the male ornament of swordtails develops from a sexually non-
dimorphic prepattern of transcription factors in the caudal fin. Among genes that constitute the
exclusive sword transcriptome only two are located in the genomic region associated with this trait; the
chaperone, fkbp9, and the potassium channel, kcnh8 that in addition to its neural function performs a
role known to affect fin growth. This indicates that during evolution of swordtails a brain gene has been
recruited for an additional function in establishing a male ornament.
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Introducing-a-dominant-gene
I’m beginning a blog series on introducing new genes (technically alleles, but I’ll bow to common usage) into a xiphophorine population. In this blog I’ll discuss the mechanics of introducing a single dominant gene into a xiphophorine population. By the way, this doesn’t only apply to xiphophorines or even fish; it can be applied to any animal or plant.
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