Genus Xiphophorus Research Articles

Phylogenomic analyses of all species of swordtail fishes (genus Xiphophorus) show that hybridization preceded speciation

Hybridization has been recognized to play important roles in evolution, however studies of the genetic consequence are still lagging behind in vertebrates due to the lack of appropriate experimental systems. Fish of the genus Xiphophorus are proposed to have evolved with multiple ancient and ongoing hybridization events. They have served as an informative research model in evolutionary biology and in biomedical research on human disease for more than a century. Here, we provide the complete genomic resource including annotations for all described 26 Xiphophorus species and three undescribed taxa and resolve all uncertain phylogenetic relationships. We investigate the molecular evolution of genes related to cancers such as melanoma and for the genetic control of puberty timing, focusing on genes that are predicted to be involved in pre-and postzygotic isolation and thus affect hybridization. We discovered dramatic size-variation of some gene families. These persisted despite reticulate evolution, rapid speciation and short divergence time. Finally, we clarify the hybridization history in the entire genus settling disputed hybridization history of two Southern swordtails. Our comparative genomic analyses revealed hybridization ancestries that are manifested in the mosaic fused genomes and show that hybridization often preceded speciation.

High resolution genomes of multiple Xiphophorus species provide new insights into microevolution, hybrid incompatibility, and epistasis.

Because of diverged adaptative phenotypes, fish species of the genus Xiphophorus have contributed to a wide range of research for a century. Existing Xiphophorus genome assemblies are not at the chromosomal level and are prone to sequence gaps, thus hindering advancement of the intra- and inter-species differences for evolutionary, comparative, and translational biomedical studies. Herein, we assembled high-quality chromosome-level genome assemblies for three distantly related Xiphophorus species, namely, X. maculatus, X. couchianus, and X. hellerii Our overall goal is to precisely assess microevolutionary processes in the clade to ascertain molecular events that led to the divergence of the Xiphophorus species and to progress understanding of genetic incompatibility to disease. In particular, we measured intra- and inter-species divergence and assessed gene expression dysregulation in reciprocal interspecies hybrids among the three species. We found expanded gene families and positively selected genes associated with live bearing, a special mode of reproduction. We also found positively selected gene families are significantly enriched in nonpolymorphic transposable elements, suggesting the dispersal of these nonpolymorphic transposable elements has accompanied the evolution of the genes, possibly by incorporating new regulatory elements in support of the Britten-Davidson hypothesis. We characterized inter-specific polymorphisms, structural variants, and polymorphic transposable element insertions and assessed their association to interspecies hybridization-induced gene expression dysregulation related to specific disease states in humans.

Enhanced growth without accelerated puberty in fish: A role for the melanocortin system

In two swordtail species of the genus Xiphophorus, onset of puberty in males and females, fecundity in females, and adult size in males are modulated by sequence polymorphism and gene copy-number variation at the P locus affecting the type 4 melanocortin hormone receptor (Mc4r). The involvement of Mc4r in regulating the onset of puberty outside the genus Xiphophorus remains unclear. In this study we used a transgenic line overexpressing asip1 (asip1-Tg), an endogenous antagonist of both type 1 melanocortin hormone receptor (Mc1r) and Mc4r, to investigate the relevance of the melanocortin system on the onset of puberty and adult reproductive performance in zebrafish (Danio rerio). Comparison of growth, puberty and reproductive performance between wild-type (WT) and asip1-Tg zebrafish revealed that a decreased activity of the melanocortin system did not change the timing of puberty but significantly delayed early growth of transgenic animals. Hatching time was postponed in asip1-Tg fish and they were significantly smaller than their WT siblings at 75 dpf, despite showing enhanced linear growth after having completed puberty. asip1-Tg females produced 1.38 times more eggs but spawned less frequently, and their eggs had showed a 0.89-fold smaller diameter but a 1.04-fold increase in larvae body length at hatching. Therefore, we demonstrate that asip-tg zebrafish do not reach puberty earlier than WT counterparts as it could be expected considering the enhanced length and weight growth during early adulthood. This is so because the effects of transgene on growth are noticeable after a threshold length, when puberty has already been reached. Data show that the inhibition of melanocortin system via asip overexpression is an excellent strategy to promote growth, in absence of obesity, by enhancing food efficiency but without accelerating puberty timing. Data here presented provides a deeper characterization of the phenotype induced by the decreased activity of the melanocortin system in fish thus providing an excellent strategy for future aquaculture especially because U.S. Food and Drug Association has recently approved transgenic fish trading.

Melanocortin 4 receptor signaling and puberty onset regulation in Xiphophorus swordtails

Fish of the genus Xiphophorus provide a prominent example of genetic control of male body size and reproductive tactics. In X.nigrensis and X.multilineatus, puberty onset and body length are determined by melanocortin 4 receptor (Mc4r) allelic and copy number variations which were proposed to fine-tune the signaling output of the system. Accessory protein Mrap2 is required for growth across species by affecting Mc4r signaling. The molecular mechanism how Mc4r signaling controls puberty regulation in Xiphophorus and whether the interaction with Mrap2 is also involved was so far unclear. Hence, we examined Mc4r and Mrap2 in X.nigrensis and X.multilineatus, in comparison to a more distantly related species, X.hellerii. mc4r and mrap2 transcripts co-localized in the hypothalamus and preoptic regions in large males, small males and females of X.nigrensis, with similar signal strength for mrap2 but higher expression of mc4r in large males. This overexpression is constituted by wild-type and one subtype of mutant alleles. In vitro studies revealed that Mrap2 co-expressed with Mc4r increased cAMP production but did not change EC50. Cells co-expressing the wild-type and one mutant allele showed lower cAMP signaling than Mc4r wild-type cells. This indicates a role of Mc4r alleles, but not Mrap2, in puberty signaling. Different from X.nigrensis and X.multilineatus, X.hellerii has only wild-type alleles, but also shows a puberty onset and body length polymorphism, despite the absence of mutant alleles. Like in the two other species, mc4r and mrap2 transcripts colocalized and mc4r is expressed at substantially higher levels in large males. This demonstrates that puberty and growth regulation mechanism may not be identical even within same genus.

On-Site Capabilities of a Mobile Laboratory for Aquatic Germplasm Cryopreservation.

Cryopreservation of genetic material can become an important tool for user groups in imperiled fishes, wild fisheries, aquaculture, and biomedical research. Persistent challenges within aquatic species cryopreservation are standardization and reliable collection of diverse, high quality samples. The overall goal of this study was to work with different user groups and cryopreserve sperm on-site at their facilities to evaluate the uses and challenges of a mobile laboratory with high-throughput and quality control capabilities comparable to those of a specialized centralized facility. The objectives were to demonstrate collection and cryopreservation of sperm of: 1) large-bodied freshwater Blue Catfish (Ictalurus furcatus) for aquaculture; 2) small-bodied freshwater Xiphophorus for biomedical and imperiled repository development, and 3) saltwater Red Snapper (Lutjanus campechanus) for wild fisheries research. Over the course of this project, the mobile laboratory traveled more than 4,000 km collecting germplasm from more than 650 male fishes. A total of 136 Blue Catfish were processed in 2015 and 2016 resulting in a total of 6,146 0.5-mL French straws. A total of 521 males from 11 different species in the genus Xiphophorus were processed over 4 d in 2015 resulting in a total of 488 0.25-mL French straws. And, a total of 17 Red Snapper males were processed during 2015 resulting in a total of 316 0.5-mL French straws. This is the first development of a mobile laboratory with high-throughput capability for aquatic species. User groups would no longer be limited to germplasm resources that can only be shipped as samples or transported as live animals to a central cryopreservation facility. Mobile laboratories create opportunities to collect higher quality germplasm, provide access to new species, and enable direct cooperation, including training, with a wide variety of user groups and applications.

Analysis of the Role of the Mc4r System in Development, Growth, and Puberty of Medaka.

In mammals the melanocortin 4 receptor (Mc4r) signaling system has been mainly associated with the regulation of appetite and energy homeostasis. In fish of the genus Xiphophorus (platyfish and swordtails) puberty onset is genetically determined by a single locus, which encodes the mc4r. Wild populations of Xiphophorus are polymorphic for early and late-maturing individuals. Copy number variation of different mc4r alleles is responsible for the difference in puberty onset. To answer whether this is a special adaptation of the Mc4r signaling system in the lineage of Xiphophorus or a more widely conserved mechanism in teleosts, we studied the role of Mc4r in reproductive biology of medaka (Oryzias latipes), a close relative to Xiphophorus and a well-established model to study gonadal development. To understand the potential role of Mc4r in medaka, we characterized the major features of the Mc4r signaling system (mc4r, mrap2, pomc, agrp1). In medaka, all these genes are expressed before hatching. In adults, they are mainly expressed in the brain. The transcript of the receptor accessory protein mrap2 co-localizes with mc4r in the hypothalamus in adult brains indicating a conserved function of modulating Mc4r signaling. Comparing growth and puberty between wild-type and mc4r knockout medaka revealed that absence of Mc4r does not change puberty timing but significantly delays hatching. Embryonic development of knockout animals is retarded compared to wild-types. In conclusion, the Mc4r system in medaka is involved in regulation of growth rather than puberty.

Design and Cost Analysis of a Self-contained Mobile Laboratory for Commercial-scale Aquatic Species Cryopreservation.

Although aquatic species cryopreservation protocols have been studied around the world over the past 60 yr., germplasm repository development efforts and commercialization have begun only recently. The goal of this project was to develop a self-contained mobile laboratory for on-site high-throughput cryopreservation of aquatic species. The objectives of this study were to: (1) identify how a mobile laboratory would function in different operational scenarios, (2) customize an enclosed cargo trailer to function as a mobile laboratory, (3) evaluate the laboratory layout and ability of cryopreservation equipment to operate from generator power, and (4) document the investment costs for private and public groups to integrate a mobile laboratory into an existing cryopreservation facility at three levels of automation and estimate the total cost per trip based on hypothetical assumptions for two scenarios (aquaculture production and repository development). There were three operational designs identified for the mobile laboratory: (1) self-contained work inside the unit using generator power, (2) work inside the unit using external facility power, and (3) using the equipment inside of a host facility. The investment costs for a base-level mobile laboratory ranged between US$5670 and US$5787 for private groups and between US$5208 and US$5315 for public groups. With the addition of a range of automated processing equipment, total investment costs ranged from US$13,616 to US$103,529 for private groups and US$12,494 to US$94,891 for public groups. The total cost per trip to cryopreserve sperm of 59 blue catfish, Ictalurus furcatus, males to produce 6300 0.5-mL French straws was estimated to range from US$6089 to US$14,633 for private and between US$5703 and US$16,938 for public groups depending on the level of automation. Total cost per trip to cryopreserve sperm of 500 males of five different species in the genus Xiphophorus to produce 641 0.25-mL French straws was estimated to range from US$6653 to US$7640 for private and US$7582 to US$8088 for public groups depending on level of automation. Overall, a commercial-scale mobile laboratory was developed that can assist current germplasm activities and support future repository and industry development, and the layout information provided can help others to design and build comparable units.

Risk assessment of the ornamental fish trade in Mexico: analysis of freshwater species and effectiveness of the FISK (Fish Invasiveness Screening Kit)

Aquarium trade has been recognized as one of the major pathways of introduction of non-native fishes into new regions. Nearly 43 million freshwater ornamental fish of different species and varieties are annually commercialized in Mexico and there is a high probability for the establishment of some of these species because of their invasive attributes and the diverse climatic zones existing in the country. Within this context, the identification of high-risk species is of paramount importance considering the potential threat to Mexico’s great biological diversity. In the present study 700 freshwater aquarium fish species commonly imported and produced in the country were filtered for synonyms/varieties, resulting in 368 species, which where submitted to revision for invasive reports using specialized invasive species databases. This allowed ranking the main invasive species and the top 30 were subjected to risk analysis using the Fish Invasiveness Screening Kit (FISK). Calibration of FISK was carried out after generating the reports. A receiver operating characteristic curve was made to determine the FISK ability to discriminate between invasive and non-invasive species in Mexico. Thereafter, Youden’s index was calculated and a threshold of 24 was obtained, representing the cut-off value for defining high-risk species. A total of 17 species out of 30 were classified under a high-risk category, among them several species of the genus Xiphophorus, Pterygoplichthys and Poecilia. Most of the species were native to Asia, Central and South America. Some of these species are already invasive in Mexico.

Transcriptomics of two evolutionary novelties: how to make a sperm-transfer organ out of an anal fin and a sexually selected "sword" out of a caudal fin.

Swords are exaggerated male ornaments of swordtail fishes that have been of great interest to evolutionary biologists ever since Darwin described them in the Descent of Man (1871). They are a novel sexually selected trait derived from modified ventral caudal fin rays and are only found in the genus Xiphophorus. Another phylogenetically more widespread and older male trait is the gonopodium, an intromittent organ found in all poeciliid fishes, that is derived from a modified anal fin. Despite many evolutionary and behavioral studies on both traits, little is known so far about the molecular mechanisms underlying their development. By investigating transcriptomic changes (utilizing a RNA-Seq approach) in response to testosterone treatment in the swordtail fish, Xiphophorus hellerii, we aimed to better understand the architecture of the gene regulatory networks underpinning the development of these two evolutionary novelties. Large numbers of genes with tissue-specific expression patterns were identified. Among the “sword genes” those involved in embryonic organ development, sexual character development and coloration were highly expressed, while in the gonopodium rather more morphogenesis-related genes were found. Interestingly, many genes and genetic pathways are shared between both developing novel traits derived from median fins: the sword and the gonopodium. Our analyses show that a larger set of gene networks was co-opted during the development and evolution of the “older” gonopodium than in the “younger,” and morphologically less complex trait, the sword. We provide a catalog of candidate genes for future efforts to dissect the development of those sexually selected exaggerated male traits in swordtails.

Interspecific variation in the repair of UV damaged DNA in the genus Xiphophorus as a factor in the decline of the Rio Grande Platyfish.

The fish genus Xiphophorus consists of 26 species distributed along the eastern slopes of mountain ranges extending from northern Mexico to Belize and Nicaragua. We analyzed light-dependent repair of UV-induced DNA damage in at least two species from each of the four monophyletic Xiphophorus groups. We found that the northern platyfish had significantly reduced photoenzymatic repair compared to the other three groups, including the northern swordtails, southern platyfish and southern swordtails. All of the species of the northern platyfish, including the Marbled (meyeri), Northern (gordoni) and Monterrey Platyfish (couchianus) are the northernmost species in the genus and are the only three species in the genus that are currently found on the IUCN Red List of Threatened Species. Satellite data from the past 30years (1979-2008) correlate greater increases in shorter wavelength UVB with higher latitudes within the Xiphophorus range. We suggest that, combined with other consequences of human population growth, anthropogenic deozonation resulting in a disproportionate increase in UVB in temperate latitudes may be a contributing factor in the decline and extirpation of the northern platyfish.

Assortative mating and the maintenance of population structure in a natural hybrid zone.

Understanding the factors that give rise to natural hybrid zones and govern their dynamics and structure is important to predicting the evolutionary consequences of hybridization. Here we use a combination of multigenerational population genetic data, mating patterns from a natural population, behavioral assays, and mark-recapture data within clinal hybrid zones of the genus Xiphophorus to test the role of assortative mating in maintaining population structure and the potential for ongoing genetic exchange between heterospecifics. Our data demonstrate that population structure is temporally robust and driven largely by assortative mating stemming from precopulatory isolation between pure species. Furthermore, mark-recapture data revealed that rates of migration within the same stream reach are far below the level needed to support population structure. In contrast to many empirical studies of natural hybrid zones, there appeared to be no hybrid male dysfunction or discrimination against hybrid males by pure parental females, and hybrid females mated and associated with pure species and hybrid males at random. Despite strong isolation between pure parentals, hybrids therefore can act as a conduit for genetic exchange between heterospecifics, which has been shown to increase the tempo of evolutionary change. Additionally, our findings highlight the complexity of natural hybrid zone dynamics, demonstrating that sexual and ecological selection together can give rise to patterns that do not fit classical models of hybrid zone evolution.

Copy number variation in the melanocortin 4 receptor gene and alternative reproductive tactics the swordtail Xiphophorus multilineatus

Alternative reproductive tactics are powerful examples of how variation in genetics and physiology among individuals can lead to striking diversity in phenotype. In the swordtails (genus Xiphophorus), copy number variation (CNV) at the melanocortin 4 receptor (mc4r) locus is correlated with male body size, which in turn is correlated with male mating behavior. We measured the relationship between mc4r CNV, behavior, and 11-ketotesterone (11-KT) in X. multilineatus to determine whether mc4r CNV was associated with other components of male tactics in addition to body size. We confirmed the results of previous studies, showing that male size increases with mc4r CNV and that mating behavior toward females was size-dependent. We also examined agonistic behavior by exposing males to their mirror image and found that male-male displays behavior were size-dependent. Small males were less likely to exhibit an agonistic response, suggesting that alternative reproductive tactics span intrasexual and intersexual contexts. There was no significant association between mc4r CNV and behavior or 11-KT hormone titer. Mc4r CNV is thus associated with the variation in male body size, but not with other traits independent of body size.

Pigmentation inXiphophorus: An Emerging System in Ecological and Evolutionary Genetics

The genus Xiphophorus has great potential to contribute to the study of vertebrate pigmentation and elucidating the relative influence of ecology, physiology, and behavior on evolution at the molecular level. More importantly, the association between pigmentation and a functional oncogene offers the potential to understand the evolution and maintenance of cancer-causing genetic elements. Using criteria laid out recently in the literature, I demonstrate the power of the Xiphophorus system for studying pigment evolution through integrative organismal biology. Using the most recent phylogeny, the phylogenetic distribution of several important pigmentation loci are reevaluated. I then review support for existing hypotheses of the functional importance of pigmentation. Finally, new observations and hypotheses regarding some of the characteristics of pigment patterns in natural populations and open questions and future directions in the study of the evolution of these traits are discussed.

Gene Amplification and Functional Diversification of Melanocortin 4 Receptor at an Extremely Polymorphic Locus Controlling Sexual Maturation in the Platyfish

In two swordtail species of the genus Xiphophorus, the onset of puberty has been shown to be modulated at the P locus by sequence polymorphism and gene copy-number variation affecting the type 4 melanocortin hormone receptor Mc4r. The system works through the interaction of two allelic types, one encoding wild type and the other dominant-negative receptors. We have analyzed the structure and evolution of the P locus in the platyfish Xiphophorus maculatus, where as many as nine alleles of P determining the onset of sexual maturity in males and females, fecundity in females, and adult size in males are located on both the X and Y chromosomes in a region linked to the master sex-determining locus. In this species, mc4r has been amplified to up to 10 copies on both the X and Y chromosomes through recent large serial duplications. Subsequently, mc4r paralogues have diverged considerably into many different subtypes. Certain copies have acquired new untranslated regions through genomic rearrangements, and transposable element insertions and other mutations have accumulated in promoter regions, possibly explaining observed deviations from the classical mc4r transcriptional pattern. In the mc4r-coding sequence, in-frame insertions and deletions as well as nonsense and missense mutations have generated a high diversity of Mc4r-predicted proteins. Most of these variants are expressed in embryos, adults, and/or tumors. Functional receptor characterization demonstrated major divergence in pharmacological behavior for Mc4r receptors encoded by different copies of platyfish mc4r, with differences in constitutive activity as well as binding and stimulation by hormones. The high degree of allelic and copy-number variation observed between individuals can explain the high level of polymorphism for sexual maturation, fecundity, and body size in the platyfish: multiple combinations of Mc4r variants with different biochemical properties might interact to modulate the melanocortin signaling that regulates the hypothalamus-pituitary-gonadal axis.

Evolution of a Genetic Incompatibility in the Genus Xiphophorus

Genetic incompatibilities are commonly observed between hybridizing species. Although this type of isolating mechanism has received considerable attention, we have few examples describing how genetic incompatibilities evolve. We investigated the evolution of two loci involved in a classic example of a Bateson-Dobzhansky-Muller (BDM) incompatibility in Xiphophorus, a genus of freshwater fishes from northern Central America. Hybrids develop a lethal melanoma due to the interaction of two loci, an oncogene and its repressor. We cloned and sequenced the putative repressor locus in 25 Xiphophorus species and an outgroup species, and determined the status of the oncogene in those species from the literature. Using phylogenetic analyses, we find evidence that a repeat region in the proximal promoter of the repressor is coevolving with the oncogene. The data support a hypothesis that departs from the standard BDM model: it appears the alleles that cause the incompatibilities have coevolved simultaneously within lineages, rather than in allopatric or temporal isolation.

The evolutionary history of Xiphophorus fish and their sexually selected sword: a genome‐wide approach using restriction site‐associated DNA sequencing

Next-generation sequencing (NGS) techniques are now key tools in the detection of population genomic and gene expression differences in a large array of organisms. However, so far few studies have utilized such data for phylogenetic estimations. Here, we use NGS data obtained from genome-wide restriction site-associated DNA (RAD) (∼66000 SNPs) to estimate the phylogenetic relationships among all 26 species of swordtail and platyfish (genus Xiphophorus) from Central America. Past studies, both sequence and morphology-based, have differed in their inferences of the evolutionary relationships within this genus, particularly at the species-level and among monophyletic groupings. We show that using a large number of markers throughout the genome, we are able to infer the phylogenetic relationships with unparalleled resolution for this genus. The relationships among all three major clades and species within each of them are highly resolved and consistent under maximum likelihood, Bayesian inference and maximum parsimony. However, we also highlight the current cautions with this data type and analyses. This genus exhibits a particularly interesting evolutionary history where at least two species may have arisen through hybridization events. Here, we are able to infer the paternal lineages of these putative hybrid species. Using the RAD-marker-based tree we reconstruct the evolutionary history of the sexually selected sword trait and show that it may have been present in the common ancestor of the genus. Together our results highlight the outstanding capacity that RAD sequencing data has for resolving previously problematic phylogenetic relationships, particularly among relatively closely related species.

Comprehensive phylogenetic analysis of all species of swordtails and platies (Pisces: Genus Xiphophorus) uncovers a hybrid origin of a swordtail fish, Xiphophorus monticolus, and demonstrates that the sexually selected sword originated in the ancestral lineage of the genus, but was lost again secondarily.

BackgroundMales 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.ResultsWe 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.ConclusionsThis 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.

DNA barcoding of the ichthyofauna of Pánuco–Tamesí complex: Evidence for taxonomic conflicts in some groups

The Pánuco–Tamesí complex in eastern Mexico is globally recognized as an important ecoregion due to its high level of endemism. In this study, DNA barcodes were generated for 152 individuals of 31 species. Additionally, 170 DNA barcodes for the related species available in the Barcode of Life Database (BOLD) system were included to test the ability of barcoding technique to discriminate between the closely related species. DNA barcoding allowed the discrimination of 79.2% of the analyzed species; poor resolution was observed in four genera in which the levels of resolution ranged from 16.6% in the genus Herichthys to 77.7% in the genus Xiphophorus. The results of this study demonstrate that DNA barcoding is a useful exploratory tool but fails to discriminate between closely related species.

Natural versus sexual selection: predation risk in relation to body size and sexual ornaments in the green swordtail

In general, we assume that natural (predation-mediated) and sexual selection have opposing effects on the evolution of characters that serve as ornaments. Males of most swordtail fishes (genus Xiphophorus) have an elongation of the caudal fin known as the sword that is used to attract females, as it increases apparent body size of males. By increasing apparent body size, the sword may also attract the attention of predators or decrease the likelihood that a predator will attack. Using the green swordtail, Xiphophorus hellerii, we evaluated separately the effect of body size and the presence of the sword on the likelihood of being attacked by a predator. We conducted preference tests using a sympatric cichlid, Thorichthys ellioti, as a predator. For the effect of body size we used live pairs of male swordtails of different sizes without swords (surgically removed) as stimuli, and for the effect of the sword we used videos of male swordtails with and without their sword (digitally removed). We found no effect of body size, but the cichlid predators directed more bites towards individuals with swords. Despite their benefits in terms of attracting mates, the sword seems to increase the risk of being attacked by a predator and thus represents a trade-off between natural selection (mediated by predation) and sexual selection.

Growth and male ornamentation in Xiphophorus montezumae

Juvenile growth rate is an important life-history trait that affects the size at maturity, and may influence the development of sexual ornamentation. The sword of several species of the genus Xiphophorus (Teleostei: Poeciliidae) is an elaborate secondary sexual trait that confers an advantage in terms of sexual selection, counterbalanced by locomotive and predatory costs. This study assesses differences in male juvenile growth, age, and size at maturation and their relationship with maximum body size and sword elongation in Montezuma swordtails (Xiphophorus montezumae). Fish size and sword length were measured over the course of 20 months. The size at which individual males initiated sword elongation varied considerably among individuals. Male maximum body size was correlated with early growth rate and age of maturation, and sword growth rate was positively correlated with male size at the onset of sword growth. Early growth in X. montezumae may provide valuable information about performance in subsequent stages and about probable future reproductive success.