Artificial selection on laterality in the teleost fish Girardinus falcatus

We performed five generations of artificial selection on laterality of eye preference in Girardinus falcatus using a detour test. Two lines were selected for right turning when encountering a potential predator, two for left turning, one for no turning bias and one unselected line was used as control. We observed a prompt response to directional selection in all lines and the response was approximately symmetrical in left and right turning lines. However, the response to selection ceased after the first or the second generation and unexpectedly in all lines the average laterality score slowly decreased in subsequent generations. After selection was suspended for three generations, no significant variation in mean laterality was observed in most cases, indicating that natural selection was not actively opposing artificial selection during the experiment. After five generations, selected lines maintained substantial additive variance as evidenced by the possibility of rapidly reversing the direction of laterality bias in just one generation of counter-selection.

A sex-linked allele, autosomal modifiers and temperature-dependence appear to regulate melanism in male mosquitofish (Gambusia holbrooki)

About 1% of male mosquitofish (Gambusia holbrooki) express melanic (mottled-black) body coloration, which differs dramatically from the wild-type, silvery-gray coloration. Here, I report on the genetic inheritance pattern of melanic coloration, which indicates Y-linkage, and at least one autosomal modifier. Phenotypic expression of melanism is also affected by temperature. Expression is constitutive (temperature insensitive) in some populations, inducible (temperature sensitive) in others. Constitutive and inducible expression occur among geographically proximal populations. However, males from any single population demonstrate the same constitutive or inducible expression pattern as one another. The F1 males from inter-population crosses demonstrate temperature-related expression patterns like their sires'. As well, the sex ratio of melanic males' progeny differs among populations. Here, inter-population crosses demonstrate a sex-ratio bias in the same direction as intra-population crosses of the sire population. About 20% of the male progeny of melanic sires express the wild-type phenotype. These silver F1 males sire only silver offspring, suggestive of loss of the melanin gene in F1 males from crossover between sex chromosomes, or control by additional modifiers, or involvement of additional factors. In nature, melanic males persist at very low frequencies. The data collected here on heritability indicate that genetic factors contribute to the rarity of melanic male mosquitofish.

Beyond the point of no return? A comparison of genetic diversity in captive and wild populations of two nearly extinct species of Goodeid fish reveals that one is inbred in the wild

The relative importance of genetic and non-genetic factors in extinction liability has been extensively debated. Here, we examine the levels of genetic variability at 13 (seven informative) loci in wild and captive populations of two endangered species of Mexican Goodeid fish, Ameca splendens and Zoogoneticus tequila. Allelic diversity was higher in the wild populations, and F(IS) lower. Values of theta (=4Nemu) were estimated using a coalescent approach. These implied that the effective population size of all captive populations of A. splendens were smaller than that of the wild population; qualitatively similar results were obtained using an analytical method based on within-population gene identity disequilibrium. However, the wild population of Z. tequila did not show a significantly greater estimate of theta. We used the Beaumont approach to infer population declines, and found that both species showed clear evidence of a decline in effective population size, although this was stronger and probably occurred over a longer period of time in Z. tequila than in A. splendens. The decline in Z. tequila probably occurred before captive populations were established. We discuss implications for the conservation of critically endangered populations.

Perturbation of DNA repair gene expression due to interspecies hybridization

The effect of interspecies hybridization on gene regulation was examined using real-time polymerase chain reaction (RT-PCR) to measure the expression of five base-excision repair genes in brain, eye, gill, liver, and tailfin tissues from Xiphophorus parental species and F(1) hybrids. Relative mRNA levels of uracil N-glycosylase (Ung), Apurinic/apyrimidinic endonuclease (Ape1), polymerase-beta (Polb), flap endonuclease (Fen1), and DNA ligase (Lig1) were measured in three parental Xiphophorus species (X. maculatus Jp 163 B, X. helleri Sarabia, and X. andersi andC) and in two interspecies F(1) hybrids, the Sp-helleri hybrid (X. maculatus Jp 163 BxX. helleri Sarabia) and the Sp-andersi hybrid (X. maculatus Jp 163 BxX. andersi) to identify genes that undergo changes in expression levels upon interspecies hybridization. Significant differences in gene expression were observed between parental animals and their respective F(1) hybrids in both interspecies crosses. Generally, marked increases in DNA repair gene mRNA levels were observed across all tissues in F(1) hybrid animals from the Sp-helleri cross compared to either X. maculatus or X. helleri parents. In contrast, the Sp-andersi F(1) hybrid animals generally exhibited decreased base-excision repair gene expression, although this trend was more specific to individual tissues than observed for Sp-helleri hybrids.

Regulation of CDKN2A/B and Retinoblastoma genes in Xiphophorus melanoma

Xiphophorus interspecies hybrids provide several well-characterized genetic models of melanoma susceptibility. The Xiphophorus CDKN2A/B gene, homologous to mammalian CDKN2A/B cyclin-dependent kinase inhibitors (p16 and p15), is a candidate tumor susceptibility gene in these models. Using real-time PCR and Western blot analysis, we analyzed expression of CDKN2A/B in spontaneous and UV-induced primary melanomas from individual backcross hybrid fish. We found that CDKN2A/B mRNA is highly expressed in melanomas (18-fold), relative to other fish tissues. Expression is also elevated, to a lesser extent (9.5-fold), in melanized skin from tumor-bearing fish. However, quantitative levels of CDKN2A/B mRNA in tumors varied considerably and positively correlated with expression of the Xmrk oncogene, suggesting possible functional interaction between Xmrk and CDKN2A/B expression. As a homolog corresponding to members of the mammalian CDKN2 family which regulate cell cycle progression at the G1 checkpoint, the CDKN2A/B p13 protein is a putative regulator of the G1 checkpoint apparatus in Xiphophorus. Since CDKN2A is often observed to be inversely regulated compared to RB in some human tumors, and is capable of transcriptionally regulating RB in human ovarian tumors, we cloned the Xiphophorus maculatus RB cDNA and analyzed RB expression by real-time PCR and Western blot analysis in the fish melanomas. These experiments were designed to ascertain whether CDKN2A/B and RB expression were inversely correlated. Our results indicate that RB mRNA was consistently expressed at only a 2-fold higher level in both tumors and melanized skin than in muscle. Qualitatively similar results were obtained for protein expression. These results collectively suggest that (i) Xmrk and CDKN2A/B may be co-regulated at the transcriptional level, and (ii) there is little, if any, alteration of RB expression in Xiphophorus melanomas.

Human impacts on drainages of the Mesa Central, Mexico, and its genetic effects on an endangered fish, Zoogoneticus quitzeoensis

The Mesa Central of Mexico is of special conservation interest due to its high richness of freshwater fish species, of which the goodeines are one of the most representative groups. Through an integrated approach, we determined conservation priorities for goodeine populations. We based our recommendations on the genetic diversity (variation in five microsatellite DNA loci) in 10 populations of Zoogoneticus quitzeoensis and on an analysis of ecological (e.g., presence of exotic species), social (e.g., political situation), and environmental (e.g., pollution) information for 52 historical occurrence points for species in the genus Zoogoneticus. Patterns of genetic erosion and genetic diversity indices were closely associated with human impact. Recent bottleneck events were most evident in the populations from remnants of the lakes drained at the beginning of the twentieth century. We identified seven operational conservation units (OCUs), all of which should be conserved because they contain unique portions of the total variation of the species. Special attention needs to be given to increase genetic variability, recover population sizes, and reestablish contact among populations within OCUs. It is imperative to create an integrative and effective approach for the recovery and conservation of the freshwater fish diversity of Central Mexico that is based on social and natural sciences.

Methyltestosterone efficiently induces male development in the self-fertilizing hermaphrodite fish, Kryptolebias marmoratus

A hermaphrodite fish, Kryptolebias marmoratus, is the only known vertebrate that reproduces by self-fertilization. In nature, males have been rarely observed. Low-temperature treatment during late embryonic stages is known to induce males but its efficacy is variable. Here we report that 17alpha-methyltestosterone (MT) treatment of the embryos converted most of the fish to males. We examined a time course of this male induction with histological and marker gene expression analyses. Oogenesis started in the gonads of the control embryo at hatching; spermatogenesis did not start until two months after hatching. In the MT-treated fish, oogenesis started initially as in the control but stopped completely within one month after hatching. Instead, spermatogonial proliferation started earlier than in the control fish and progressed to full spermatogenesis. Expression profiles of the sex-specific marker genes corresponded well with histological observations. From one month after hatching, expression of an oocyte-specific marker, figalpha, and a testicular somatic cell marker, dmrt1, started to increase in the control and in the MT-treated fish, respectively.

Melanoma susceptibility and cell cycle genes inXiphophorus hybrids

Xiphophorus interspecies hybrids provide genetically defined models of both spontaneous and inducible melanomagenesis. In both models, backcrossing F(1) hybrids of different strains of X. maculatus and X. helleri to a X. helleri parental fish results in segregation of melanoma susceptibility, fitting a Mendelian two-gene inheritance model. The sex-linked Xmrk oncogene is required for melanoma development in both crosses. The Xiphophorus CDKN2A/B gene, which is homologous to mammalian CDKN2A/B cyclin-dependent kinase inhibitors (p16 and p15), is a candidate melanoma susceptibility gene. In this model, tumor susceptibility segregates with homozgyosity for CDKN2A/B from the recurrent X. helleri parent in backcross hybrids. We found that both CDKN2A/B mRNA and protein are highly overexpressed in melanoma. Because the p13 protein product of CDKN2A/B is a putative regulator of the G1 checkpoint, we investigated expression of other components of Xiphophorus G1 checkpoint control. By real-time PCR analysis, retinoblastoma gene (RB) is consistently expressed twofold higher in both tumors and melanized skin than in normal tissue, indicating that RB is not downregulated by the overexpression of CDKN2A/B in Xiphophorus melanoma. We also found a significant correlation between the quantitative level of CDKN2A/B and Xmrk RNA in tumors, suggesting a functional relationship between Xmrk and CDKN2A/B expression. Although X. helleri CDKN2A/B protein contains a non-conservative substitution, the biochemical function appears to show little overt defect. These studies indicate that in Xiphophorus melanoma, CDKN2A/B is functionally insufficient to mediate cell-cycle arrest in the presence of Xmrk.

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.

From Mendelian to molecular genetics: the Xiphophorus melanoma model

In human tumor biopsies it is almost impossible to pinpoint the particular molecular abnormalities that determine neoplasia. In animal models where tumorigenesis is initiated by clearly defined genetic events, it is possible to study the genes and their functions that make a normal cell become a fully malignant cancer cell. In the fish Xiphophorus, melanoma can be initiated by simple crossings, and the signaling pathways that govern tumor growth and progression can be delineated. This model offers the prospect of obtaining a complete picture of the molecular changes and regulatory networks underlying tumor formation, which should contribute to a better understanding of some general principles of cancer biology, and identify new targets for melanoma research in particular.

Cloning and differential expression of estrogen receptor and aromatase genes in the self-fertilizing hermaphrodite and male mangrove rivulus, Kryptolebias marmoratus

The mechanisms underlying sex determination and differentiation in fishes are labile in response to environmental parameters. Sex-specific phenotypes are largely regulated by sex steroids, and the inhibition or the stimulation of aromatase can reverse sex as well as alter secondary sexual characteristics in fishes. Among vertebrates, the mangrove rivulus is the only known self-fertilizing hermaphrodite. Throughout most of its range, rivulus appear to exist as clonally reproducing hermaphrodites. However, outcrossing has been documented in Belize, where up to 25% of rivulus collected are males. The direct development of (primary) males occurs when embryos are incubated at 18 degrees C and hermaphrodites develop into secondary males when held at 28 degrees C. Given the importance of sex steroids, their receptors, and aromatase in sex determination and differentiation of fishes, we cloned, sequenced, and quantified the expression of estrogen receptors (ERalpha, ERbeta) and ovarian (AroA) and brain (AroB) aromatase genes. Hermaphrodites had increased ERalpha, ERbeta, AroA, and AroB gene expression in the liver, gonad, gonad, and brain respectively, compared to males. These data are consistent with the gene expression data reported for other species and are reflective of the presence of ovarian tissue in the hermaphrodites. Interestingly, we show the elevated expression of brain aromatase in the hermaphrodite brain. The role of the dimorphic expression of brain aromatase in the regulation of sex-specific characteristics is intriguing and requires further research. Because of the uniqueness of its reproductive biology, rivulus is an excellent model for elucidating the mechanisms regulating vertebrate sex determination and sexual differentiation.

Molecular cloning and characterization of θ-class glutathione S-transferase (GST-T) from the hermaphroditic fish Rivulus marmoratus and biochemical comparisons with α-class glutathione S-transferase (GST-A)

We cloned and sequenced full-length cDNA of a theta-class-like glutathione S-transferase (GST-T) from liver tissue of the self-fertilizing fish Rivulus marmoratus. The full-length cDNA of rm-GST-T was 907 bp in length containing an open reading frame of 666 bp that encoded a 221-amino acid putative protein. Its derived amino acid sequence was clustered with other vertebrate theta-class GSTs in a phylogenetic tree. The deduced amino acid sequence of theta-like rm-GST (rm-GST-T) was compared with both classes (alpha and theta) of GST and alpha-class rm-GST (rm-GST-A). Tissue-specific expression of two rm-GST mRNAs was investigated using real-time RT-PCR. To further characterize the catalytic properties of this enzyme along with rm-GST-A, we constructed the recombinant theta-like rm-GST plasmid with a 6 x His-Tag at the N-terminal of rm-GST-T cDNA. Recombinant rm-GST-T was highly expressed in transformed Escherichia coli, and its soluble fraction was purified by His-Tag affinity column chromatography. The kinetic properties and effects of pH and temperature on rm-GST-T were further studied, along with enzyme activity and inhibition effects, and compared with recombinant rm-GST-A. These results suggest that recombinant rm-GSTs such as rm-GST-A and rm-GST-T play a conserved functional role in R. marmoratus.

Phylogenetic analysis of the evolution of a signal of aggressive intent in northern swordtail fishes

The initiation of a coevolutionary relationship between signal and response can be explained by either the receiver taking advantage of information inadvertently provided by the sender or the sender taking advantage of a perceptual bias in the receiver. In addition, once both signal and response are present, the exchange of information may or may not be cooperative. We examined the evolution of a signal of aggressive intent (expression of vertical bars) across all the northern swordtail fishes (Xiphophorus) in a phylogenetic context. We found that the signal was present before responses evolved, which suggests that this coevolutionary relationship was initiated by the receiver taking advantage of information inadvertently provided by the signaler. In addition, we introduce a novel method for examining the cooperative nature of signaling systems and provide some evidence to suggest that in this signaling system, receivers may be exploiting an honest signal in some species.

Nonylphenol modulates expression of androgen receptor and estrogen receptor genes differently in gender types of the hermaphroditic fish Rivulus marmoratus

To uncover the effect of estrogenic chemicals [4-nonylphenol (NP) and bisphenol A (BisA)] on the expression of androgen receptor (AR) and estrogen receptors (ERalpha and ERbeta) in the hermaphroditic fish Rivulus marmoratus, we cloned the full length of the cDNAs encoding AR, ERalpha, and ERbeta from gonadal tissue of R. marmoratus and analyzed the modulation of expression of these genes following exposure to estrogenic chemicals using real-time RT-PCR. R. marmoratus AR, ERalpha, and ERbeta genes showed a high similarity to the relevant fish species on amino acid residues, respectively. Rm-ERalpha and Rm-ERbeta cDNAs included a serine-rich region when compared to other teleost fish ER genes. Tissue-specific expression of Rm-AR and Rm-ERbeta mRNAs in adult hermaphrodite R. marmoratus was high in the gonad, while Rm-ERalpha mRNA was high in the liver based on real-time RT-PCR. In addition, Rm-AR and Rm-ERalpha mRNAs increased along with developmental stage from stage 3 (5 dpf) to hatching, while Rm-ERbeta mRNA increased from stage 2 (2 dpf). To uncover the effect of estrogenic chemicals on R. marmoratus, we exposed the fish to NP (300 microg/l) and BisA (600 microg/l) for 96 h. Significant down-regulation of Rm-AR, Rm-ERalpha, and Rm-ERbeta mRNA was observed in gonadal tissue after exposure to NP but not BisA. In the liver, there were gender differences in gene expression after EDC exposure. These results demonstrate that expression patterns of the Rm-AR, Rm-ERalpha, and Rm-ERbeta genes in the hermaphroditic fish, R. marmoratus, vary according to tissue and developmental stage as well as the specificity of environmental estrogenic chemicals. These genes can be useful as molecular biomarkers in assessing the potential impact of estrogenic compounds using this species as a model system.

Cloning of Ki-ras and Ha-ras cDNAs from the hermaphroditic fish Rivulus marmoratus (Cyprinodontiformes, Rivulidae) and its expression after exposure to 4-nonylphenol

Previous studies on ras proto-oncogene genes in fish have been focused on chemical-associated carcinogenesis, and the expression of fish ras genes was not well-characterized. We investigated Ki- and Ha-ras genes from the hermaphroditic fish Rivulus marmoratus to understand better their expression patterns in specific tissues, as well as their responses to endocrine-disrupting chemicals such as 4-nonylphenol (4-NP). By investigating expression patterns, we found that the R. marmoratus Ki-ras (Rm Ki-ras) gene showed an alternative splicing event between exons 4A and 4B according to tissue types, which is different from the expression pattern of mammalian Ki-ras genes. In the Rm Ki-ras gene, there were two different expressed types, with exons 1-2-3-4A-4B (long form) and with exons 1-2-3-4B (short form). In the Rm Ki-ras gene, the long form was expressed strongly in the gonad and intestine, and the short form was expressed ubiquitously, except for a low level of expression in the liver. Following 4-NP exposure (300 microg/L), the Rm Ki-ras long form in the liver was significantly expressed, while it was expressed moderately in the ovaries. However, the Rm Ha-ras gene was significantly over-expressed in the brain, while its expression in the gonad was down-regulated. In relation to these modulations after 4-NP exposure, we searched the Rm Ha- and Ki-ras promoter regions and found several ERE-half sites, that may be involved in the modulation of ras gene expression following 4-NP exposure. These genes could be applicable as new biomarker genes for assessing exposure to endocrine-disrupting chemicals (EDCs). Further, this implies the disturbance of ras-dependent signal transduction following EDC exposure.

Cytogenetic analysis of platyfish (Xiphophorus maculatus) shows location of major and minor rDNA on chromosomes

Chromosome distribution of nucleolus organizer regions (NORs) and 5S rDNA loci were investigated in platyfish (Xiphophorus maculates) using conventional and molecular cytogenetic techniques. Chromomycin A3 (CMA3) staining of GC- rich heterochromatin provided four CMA3 cytotypes in studied platyfish. As in fish, CMA3 positive sites are related to NORs, chromosomes sequentially stained by CMA3 and AgNO3 showed bright blocks that were transcriptionaly active NORs. Additionally, PRINS with primers enabling amplification of 5S rDNA and CMA3 staining revealed existence of 5S rDNA sites located close to GC rich regions. The analysis could suggests that minor and major rDNAs may be linked.

Extensive outcrossing and androdioecy in a vertebrate species that otherwise reproduces as a self-fertilizing hermaphrodite

The mangrove killifish (Kryptolebias marmoratus) is the only vertebrate known to be capable of self-fertilization. Its gonad is typically an ovotestis that simultaneously produces eggs and sperm, and fertilization is internal. Although most populations of this species consist primarily or exclusively of hermaphroditic individuals, gonochoristic males occur at approximately 20% frequency in a natural population at Twin Cays, Belize. Here we use a battery of 36 microsatellite loci to document a striking genetic pattern (high intraspecimen heterozygosities and low within-population linkage disequilibria) that differs qualitatively from the highly homozygous (or "clonal") genetic architecture characteristic of killifish populations previously studied in Florida, where males are much rarer. These findings document that outcrossing (probably between gonochoristic males and hermaphrodites) is common at the Belize site, and, more importantly, they demonstrate the dramatic impact that functional androdioecy can have on the population genetic architecture of this reproductively unique vertebrate species.

Endocrine disrupting chemicals (bisphenol A, 4-nonylphenol, 4-tert-octylphenol) modulate expression of two distinct cytochrome P450 aromatase genes differently in gender types of the hermaphroditic fish Rivulus marmoratus

To understand the effect of endocrine-disrupting chemicals (EDCs) on cytochrome P450 aromatase (rm-cyp19) gene expression between gender types in the hermaphroditic fish Rivulus marmoratus, we cloned two distinct rm-cyp19 genes using RT-PCR with degenerative primers, obtained full-length cDNAs using 5'- and 3'-RACE-PCR methods, and completely sequenced them. The brain aromatase (rm-cyp19b) cDNA consisted of 2,124 bp including the open reading frame (ORF), which encoded a putative protein of 505 amino acids. The ovarian aromatase (rm-cyp19a) cDNA consisted of 2,075 bp, including the ORF encoding a putative protein of 516 amino acids. Expression patterns of rm-cyp19b and rm-cyp19a mRNAs were investigated in embryos of different developmental stages and in seven different tissues of adult fish. The rm-cyp19b gene in hermaphrodite and secondary male R. marmoratus was predominantly expressed in the brain, while the rm-cyp19a gene was expressed gender-specifically in the gonad. The expression of rm-cyp19b mRNA increased from stage 1 (2 d post fertilization) to stage 4 (12 d post fertilization) in a developmental stage-dependent manner but steeply decreased in the hatching stage. Compared to the rm-cyp19b gene, the abundance of ovarian aromatase rm-cyp19a transcripts was very low, and its expression was first detected at stage 3 and then decreased gradually to the hatching stage. Alteration of rm-cyp19b and rm-cyp19a gene expression was further analyzed in the brain and gonad by real-time RT-PCR 96 h after EDC exposure in hermaphrodites and secondary males. The brain aromatase rm-cyp19b gene was up-regulated in the brain after 4-nonylphenol (4-NP)-exposure, while the ovarian aromatase rm-cyp19a gene was significantly down-regulated in the gonad. In 300 microg/L 4-tert octylphenol (4-tert-OP), or 600 microg/L bisphenol A-exposed brain and gonad, both rm-cyp19b and rm-cyp19a genes were up-regulated. In the case of secondary males, the rm-cyp19b gene was highly expressed in the 4-NP-exposed brain, while expression of the rm-cyp19a gene was not detected in the gonad. These results indicate that the expression of rm-cyp19a and rm-cyp19b genes is differently modulated according to estrogenic compounds and gender type of R. marmoratus.

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 for through 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.

Female Disdain for Swords in a Swordtail Fish

Studies of mate choice evolution tend to focus on how female mating preferences are acquired and how they select for greater elaboration of male traits. By contrast, far less is known about how female preferences might be lost or reversed. In swordtail fish Xiphophorus, female preference for the sword ornament is an ancestral trait. Xiphophorus birchmanni, however, is one species that has secondarily lost the sword. Using synthetic animation playback of "virtual" males, we found that female X. birchmanni preferred a swordless conspecific over a sworded heterospecific. Moreover, when offered the choice between a conspecific without a sword and one with a digitally attached sword, females preferred the former. These results suggest female preferences need not always select for elaboration of male traits, and they provide a plausible explanation for the lack of introgression of a sexual trait in a naturally occurring hybrid zone.

Karyotype Differentiation in Chromaphyosemion Killifishes (Cyprinodontiformes, Nothobranchiidae). I: Chromosome Banding Patterns of C. Alpha, C. Kouamense and C. Lugens

The karyotypes of three recently described species of Chromaphyosemion, namely C. lugens, C. alpha and C. kouamense, were analysed using conventional Giemsa staining, C-banding and sequential banding (fluorescence banding with 4', 6-diamidino-2-phenylindole (DAPI) and Chromomycin A(3) (CMA(3)), C-banding, AgNO(3)-staining). Diploid chromosome numbers ranged from 2n = 36 in C. lugens to 2n = 38 in C. alpha and C. kouamense. The karyotype of C. lugens consisted of one pair of metacentric (m) and 17 pairs of telocentric (t) chromosomes, that of C. alpha was composed of one pair of submetacentric (sm), six pairs of subtelocentric (st) and 12 pairs of t chromosomes, and that of C. kouamense comprised five pairs of st and 14 pairs of t chromosomes. In addition to the differences in karyotype structures and/or chromosome numbers, the karyotypes of the examined species differed with respect to NOR phenotype and distribution and base composition of heterochromatin. No heteromorphic sex chromosomes were detected in any of the species. Our findings provide cytotaxonomic evidence for the species distinctness of C. alpha, C. kouamense and C. lugens whose descriptions were based primarily on external morphology.

Premating, not postmating, barriers drive genetic dynamics in experimental hybrid populations of the endangered Sonoran topminnow

The timing and pattern of reproductive barrier formation in allopatric populations has received much less attention than the accumulation of reproductive barriers in sympatry. The theory of allopatric speciation suggests that reproductive barriers evolve simply as by-products of overall genetic divergence. However, observations of enhanced premating barriers in allopatric populations suggest that sexual selection driven by intraspecific competition for mates may enhance species-specific signals and accelerate the speciation process. In a previous series of laboratory trials, we examined the strength of premating and postmating barriers in an allopatric species pair of the endangered Sonoran topminnow, Poeciliopsis occidentalis and P. sonoriensis. Behavioral observations provided evidence of asymmetrical assortative mating, while reduced brood sizes and male-biased F(1) sex ratios suggest postmating incompatibilities. Here we examine the combined effects of premating and postmating barriers on the genetic makeup of mixed populations, using cytonuclear genotype frequencies of first- and second-generation offspring. Observed genotype frequencies strongly reflect the directional assortative mating observed in behavioral trials, illustrating how isolating barriers that act earlier in the reproductive cycle will have a greater effect on total reproductive isolation and may be more important to speciation than subsequent postmating reproductive barriers.

A microsatellite genetic linkage map for Xiphophorus

Interspecies hybrids between distinct species of the genus Xiphophorus are often used in varied research investigations to identify genomic regions associated with the inheritance of complex traits. There are 24 described Xiphophorus species and a greater number of pedigreed strains; thus, the number of potential interspecies hybrid cross combinations is quite large. Previously, select Xiphophorus experimental crosses have been shown to exhibit differing characteristics between parental species and among the hybrid fishes derived from crossing them, such as widely differing susceptibilities to chemical or physical agents. For instance, genomic regions harboring tumor suppressor and oncogenes have been identified via linkage association of these loci with a small set of established genetic markers. The power of this experimental strategy is related to the number of genetic markers available in the Xiphophorus interspecies cross of interest. Thus, we have undertaken the task of expanding the suite of easily scored markers by characterization of Xiphophorus microsatellite sequences. Using a cross between Xiphophorus maculatus and X. andersi, we report a linkage map predominantly composed of microsatellite markers. All 24 acrocentric chromosome sets of Xiphophorus are represented in the assembled linkage map with an average intergenomic distance of 7.5 cM. Since both male and female F1 hybrids were used to produce backcross progeny, these recombination rates were compared between "male" and "female" maps. Although several genomic regions exhibit differences in map length, male- and female-derived maps are similar. Thus Xiphophorus, in contrast to zebrafish, Danio rerio, and several other vertebrate species, does not show sex-specific differences in recombination. The microsatellite markers we report can be easily adapted to any Xiphophorus interspecies and some intraspecies crosses, and thus provide a means to directly compare results derived from independent experiments.

Strong reproductive skew among males in the multiply mated swordtail Xiphophorus multilineatus (Teleostei)

Male swordtails in the genus Xiphophorus display a conspicuous ventral elongation of the caudal fin, the sword, which arose through sexual selection due to female preference. Females mate regularly and are able to store sperm for at least 6 months. If multiple mating is frequent, this would raise the intriguing question about the role of female choice and male-male competition in shaping the mating system of these fishes. Size-dependent alternate mating strategies occur in Xiphophorus; one such strategy is courtship with a sigmoid display by large dominant males, while the other is gonopodial thrusting, in which small subordinate males sneak copulations. Using microsatellite markers, we observed a frequency of multiple paternity in wild-caught Xiphophorus multilineatus in 28% of families analyzed, but the actual frequency of multiple mating suggested by the correction factor PrDM was 33%. The number of fathers contributing genetically to the brood ranged from one to three. Compared to other species in the family Poeciliidae, both frequency and degree of multiple paternity were low. Paternity was found to be highly skewed, with one male on average contributing more than 70% to the offspring. Hence in this Xiphophorus mating system, typically one male dominates and sneaker males do not appear to be particularly effective. Postcopulatory mechanisms, however, such as sperm competition, are also indicated by our data, using sex-linked phenotypes among the offspring.

An in silico mining for simple sequence repeats from expressed sequence tags of zebrafish, medaka, Fundulus, and Xiphophorus

Teleost fish genome projects involving model species are resulting in a rapid accumulation of genomic and expressed DNA sequences in public databases. The expressed sequence tags (ESTs) collected in the databases can be mined for the analysis of both structural and functional genomics. In this study, we in silico analyzed 49,430 unigenes representing a total of 692,654 ESTs from four model fish for their potential use in developing simple sequence repeats (SSRs), or microsatellites. After bioinformatical mining, a total of 3,018 EST derived SSRs (EST-SSRs) were identified for 2,335 SSR containing ESTs (SSR-ESTs). The frequency of identified SSR-ESTs ranged from 1.5% for Xiphophorus to 7.3% for zebrafish. The dinucleotide repeat motif is the most abundant SSR, accounting for 47%, 52%, 64%, and 78% for medaka, Fundulus, zebrafish, and Xiphophorus, respectively. Simulation analysis suggests that a majority of these EST-SSRs have sufficient flanking sequences for polymerase chain reaction (PCR) primer design. Comparative DNA sequence analyses of SSR-ESTs identified several cross-species SSRs and sequences that may be used as cross-reference genes in comparative studies. For example, the flanking sequences of one SSR (CTG)n within the pituitary tumor-transforming gene (PTTG) 1 interacting protein (PTTGIP), showed conservation spanning the medaka, Fundulus, human, and mouse genomes. This study provides a large body of information on EST-SSRs that can be useful for the development of polymorphic markers, gene mapping, and comparative genome analysis. Functional analysis of these SSR-ESTs may reveal their role in metabolism and gene evolution of these model species.