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Concept: Xiphophorus

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BACKGROUND: Males in some species of the genus Xiphophorus, small freshwater fishes from Meso-America, have an extended caudal fin, or sword – hence their common name “swordtails”. Longer swords are preferred by females from both sworded and – surprisingly also, non-sworded (platyfish) species that belong to the same genus. Swordtails have been studied widely as models in research on sexual selection. Specifically, the pre-existing bias hypothesis was interpreted to best explain the observed bias of females in presumed ancestral lineages of swordless species that show a preference for assumed derived males with swords over their conspecific swordless males. However, many of the phylogenetic relationships within this genus still remained unresolved. Here we construct a comprehensive molecular phylogeny of all 26 known Xiphophorus species, including the four recently described species (X. kallmani, X. mayae, X. mixei and X. monticolus). We use two mitochondrial and six new nuclear markers in an effort to increase the understanding of the evolutionary relationships among the species in this genus. Based on the phylogeny, the evolutionary history and character state evolution of the sword was reconstructed and found to have originated in the common ancestral lineage of the genus Xiphophorus and that it was lost again secondarily. RESULTS: We estimated the evolutionary relationships among all known species of the genus Xiphophorus based on the largest set of DNA markers so far. The phylogeny indicates that one of the newly described swordtail species, Xiphophorus monticolus, is likely to have arisen through hybridization since it is placed with the southern platyfish in the mitochondrial phylogeny, but with the southern swordtails in the nuclear phylogeny. Such discordance between these two types of markers is a strong indication for a hybrid origin. Additionally, by using a maximum likelihood approach the possession of the sexually selected sword trait is shown to be the most likely ancestral state for the genus Xiphophorus. Further, we provide a well supported estimation of the phylogenetic relationships between the previously unresolved northern swordtail groups. CONCLUSIONS: This comprehensive molecular phylogeny of the entire genus Xiphophorus provides evidence that a second swordtail species, X. monticolus, arose through hybridization. Previously, we demonstrated that X. clemenciae, another southern swordtail species, arose via hybridization. These findings highlight the potential key role of hybridization in the evolution of this genus and suggest the need for further investigations into how hybridization contributes to speciation more generally.

Concepts: Evolution, Biology, Species, Phylogenetic tree, Phylogenetics, Green swordtail, Xiphophorus, Southern platyfish

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Morphological symmetry is a correlate of fitness-related traits or even a direct target of mate choice in a variety of taxa. In these taxa, when females discriminate among potential mates, increased selection on males should reduce fluctuating asymmetry (FA). Hybrid populations of the swordtails Xiphophorus birchmanni and Xiphophorus malinche vary from panmictic (unstructured) to highly structured, in which reproductive isolation is maintained among hybrids and parental species. We predicted that FA in flanking vertical bars used in sexual signalling should be lower in structured populations, where non-random mating patterns are observed. FA in vertical bars was markedly lower in structured populations than in parental and unstructured hybrid populations. There was no difference in FA between parentals and hybrids, suggesting that hybridisation does not directly affect FA. Rather, variation in FA likely results from contrasting mating patterns in unstructured and structured populations.

Concepts: Natural selection, Reproduction, Gender, Sex, Asymmetry, Green swordtail, Xiphophorus, Southern platyfish

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Poeciliid fish, freshwater fish with internal fertilization, are known for the diversity of structures on the male intromittent organ, the gonopodium. Prominent among these, in some species, is a pair of claws at its tip. We conducted a manipulative study of these claws in the guppy, Poecilia reticulata, to determine if these aid in transferring sperm to resistant females. We compared the sperm transfer rates of clawed versus surgically declawed males attempting to mate with either receptive or unreceptive (i.e. resistant) females. Our analyses demonstrate that the gonopodial claws function to increase sperm transfer to unreceptive females during uncooperative matings but not during receptive matings. Up to threefold more sperm were transferred to unreceptive females by clawed than declawed males. These data suggest that the claw is a sexually antagonistic trait, functioning to aid in transferring sperm to resistant females, and implicate sexual conflict as a selective force in the diversification of the gonopodium in the Poeciliidae.

Concepts: Reproduction, Guppy, Sex, Spermatozoon, Gamete, Poeciliidae, Poecilia, Xiphophorus

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Near-infrared (NIR) light constitutes an integrated part of solar radiation. The principal ability to sense NIR under laboratory conditions has previously been demonstrated in fish. The availability of NIR in aquatic habitats, and thus its potential use as a cue for distinct behaviors such as orientation and detection of prey, however, depends on physical and environmental parameters. In clear water, blue and green light represents the dominating part of the illumination. In turbid waters, in contrast, the relative content of red and NIR radiation is enhanced, due to increased scattering and absorption of short and middle range wavelengths by suspended particles and dissolved colored materials. We have studied NIR detection thresholds using a phototactic swimming assay in five fish species, which are exposed to different NIR conditions in their natural habitats. Nile and Mozambique tilapia, which inhabit waters with increased turbidity, displayed the highest spectral sensitivity, with thresholds at wavelengths above 930 nm. Zebrafish, guppy and green swordtail, which prefer clearer waters, revealed significantly lower thresholds of spectral sensitivity with 825-845 nm for green swordtail and 845-910 nm for zebrafish and guppy. The present study revealed a clear correlation between NIR sensation thresholds and availability of NIR in the natural habitats, suggesting that NIR vision, as an integral part of the whole spectrum of visual abilities, can serve as an evolutionarily adaptable trait in fish.

Concepts: Ultraviolet, Light, Fish, Electromagnetic radiation, Sun, Color, Spectrum, Xiphophorus

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Competition for resources including food, physical space, and potential mates is a fundamental ecological process shaping variation in individual phenotype and fitness. The evolution of competitive ability, in particular social dominance, depends on genetic (co)variation among traits causal (e.g., behaviour) or consequent (e.g. growth) to competitive outcomes. If dominance is heritable, it will generate both direct and indirect genetic effects (IGE) on resource dependent traits. The latter are expected to impose evolutionary constraint because winners necessarily gain resources at the expense of losers. We varied competition in a population of sheepshead swordtails, Xiphophorus birchmanni, to investigate effects on behaviour, size, growth, and survival. We then applied quantitative genetic analyses to determine (i) whether competition leads to phenotypic and/or genetic integration of behaviour with life history and (ii) the potential for IGE to constrain life history evolution. Size, growth and survival were reduced at high competition. Male dominance was repeatable and dominant individuals show higher growth and survival. Additive genetic contributions to phenotypic covariance were significant, with the G matrix largely recapitulating phenotypic relationships. Social dominance has a low but significant heritability and is strongly genetically correlated with size and growth. Assuming causal dependence of growth on dominance, hidden IGE will therefore reduce evolutionary potential. This article is protected by copyright. All rights reserved.

Concepts: Gene, Genetics, Natural selection, Genotype, Evolution, Heredity, Francis Crick, Xiphophorus

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The regulation of energy in fish, like most vertebrates, is a complex process that involves a number of brain and peripheral hormones. These signals include anorexigenic [e.g. cholecystokinin (CCK) and cocaine -and amphetamine-regulated transcript (CART)] as well as orexigenic [e.g. orexin and neuropeptide Y (NPY)] peptides. Platyfish, Xiphophorus maculatus, are freshwater viviparous fish for which little is known about the endocrine mechanisms regulating feeding. In order to elucidate the role of these peptides in the regulation of feeding of platyfish, we examined the effects of peripheral injections of CCK and orexin on feeding behavior and food intake. Injections of CCK decreased both food intake and searching behavior, while injections of orexin increased searching behavior but did not affect food consumption. In order to better characterize these peptides, we examined their mRNA tissue distribution and assessed the effects of a 10-day fast on their brain and intestine expressions in both males and females. CCK, CART, NPY and orexin all show widespread distributions in brain and several peripheral tissues, including intestine and gonads. Fasting induced decreases in both CCK and CART and an increase in orexin mRNA expressions in the brain and a decrease in CCK expression in the intestine, but did not affect either expressions of NPY. There were no significant sex-specific differences in either the behavioral responses to injections or the expression responses to fasting. The widespread distribution and the fasting-induced changes in expression of these peptides suggest that they might have several physiological roles in platyfish, including the regulation of feeding.

Concepts: Gene expression, Nutrition, Eating, Food, Anorectic, Poeciliidae, Xiphophorus, Southern platyfish

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A rapidly increasing body of work is revealing that the genomes of distinct species often exhibit hybrid ancestry, presumably due to post-speciation hybridization between closely related species. Despite the growing number of documented cases, we still know relatively little about how genomes evolve and stabilize following hybridization, and to what extent hybridization is functionally relevant. Here we examine the case of Xiphophorus nezahualcoyotl, a teleost fish whose genome exhibits significant hybrid ancestry. We show that hybridization was relatively ancient and is unlikely to be ongoing. Strikingly, the genome of X. nezahualcoyotl has largely stabilized following hybridization, distinguishing it from examples such as human-Neandertal hybridization. Hybridization-derived regions are remarkably distinct from other regions of the genome, tending to be enriched in genomic regions with reduced constraint. These results suggest that selection has played a role in removing hybrid ancestry from certain functionally important regions. Combined with findings in other systems, our results raise many questions about the process of genomic stabilization and the role of selection in shaping patterns of hybrid ancestry in the genome. This article is protected by copyright. All rights reserved.

Concepts: Gene, Genome, Stabilizer, Copyright, Green swordtail, Xiphophorus

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The presence of antibiotics including norfloxacin in the aquatic environment may cause adverse effects in non-target organisms. But the toxic mechanisms of fluoroquinolone to fish species are still not completely elucidated. Thus, it is essential to investigate the response of fish to the exposure of fluoroquinolone at molecular or cellular level for better and earlier prediction of these environmental pollutants toxicity. The sub-chronic toxic effects of norfloxacin (NOR) on swordtail fish (Xiphophoru s helleri) were investigated by measuring mRNA expression of cytochrome P450 1A (CYP1A), cytochrome P450 3A (CYP3A), glutathione S-transferase (GST) and P-glycoprotein (P-gp) and their corresponding enzyme activities (including ethoxyresorufin O-deethylase, erythromycin N-demethylase and GST. Results showed that NOR significantly affected the expression of CYP1A, CYP3A, GST and P-gp genes in swordtails. The gene expressions were more responsive to NOR exposure than their corresponding enzyme activities. Moreover, sexual differences were found in gene expression and enzyme activities of swordtails exposed to NOR. Females displayed more dramatic changes than males. The study further demonstrated that the combined biochemical and molecular parameters were considered as useful biomarkers to improve our understanding of potential ecotoxicological risks of NOR exposure to aquatic organisms.

Concepts: DNA, Gene, Gene expression, Bacteria, Molecular biology, Cytochrome P450, Green swordtail, Xiphophorus

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The Monterrey platyfish (Xiphophorus couchianus) is native to the vicinity of Monterrey, Mexico and is listed as Critically Endangered by International Union for Conservation of Nature (IUCN). Here, we describe the complete mitochondrial genome (mitogenome) sequence of Monterrey platyfish. The genome is 16 638 bp in length, consisting of 13 protein-coding genes, 22 transfer RNA (tRNA) genes, 2 ribosomal RNA genes and 1 non-coding control region. Phylogenetic tree was constructed based on the complete mitogenome of X. couchianus and closely related five Poeciliid species to estimate their phylogenic relationship. The complete mitochondrial genome of X. couchianus would provide more valuable information to assess the evolution of family Poeciliidae.

Concepts: DNA, Archaea, RNA, Species, Ribosome, Non-coding RNA, Poeciliidae, Xiphophorus

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Just as mating patterns can promote speciation or hybridization, the presence of hybridization can shape mating patterns within a population. In this study, we characterized patterns of multiple mating and reproductive skew in a naturally hybridizing swordtail fish species, Xiphophorus birchmanni. We quantified multiple mating using microsatellite markers to genotype embryos from 43 females collected from 2 wild populations. We also used a suite of single-nucleotide polymorphism markers to categorize females and their inferred mates as either parental X. birchmanni or as introgressed individuals, which carried alleles from a sister species, X. malinche. We found that parental and introgressed X. birchmanni females mated multiply with both parental and introgressed males. We found no difference in mating patterns or reproductive skew between parental and introgressed X. birchmanni females. However, nonintrogressed X. birchmanni males mated more often with large, fecund females. These females also had the greatest levels of skew in fertilization success of males. Thus, our results show that X. birchmanni has a polygynandrous mating system and that introgression of X. malinche alleles has only subtle effects on mating patterns in this species.

Concepts: Human, Reproduction, Evolution, Sex, Fertility, Live-bearing fish, Green swordtail, Xiphophorus