An evaluation of the hybrid speciation hypothesis for Xiphophorus clemenciae based on whole genome sequences

Once thought rare in animal taxa, hybridization has been increasingly recognized as an important and common force in animal evolution. In the past decade, a number of studies have suggested that hybridization has driven speciation in some animal groups. We investigate the signature of hybridization in the genome of a putative hybrid species, Xiphophorus clemenciae, through whole genome sequencing of this species and its hypothesized progenitors. Based on analysis of this data, we find that X. clemenciae is unlikely to have been derived from admixture between its proposed parental species. However, we find significant evidence for recent gene flow between Xiphophorus species. Although we detect genetic exchange in two pairs of species analyzed, the proportion of genomic regions that can be attributed to hybrid origin is small, suggesting that strong behavioral premating isolation prevents frequent hybridization in Xiphophorus. The direction of gene flow between species is potentially consistent with a role for sexual selection in mediating hybridization.

Effects of 17β-trenbolone on Eastern and Western mosquitofish (Gambusia holbrooki and G. affinis) anal fin growth and gene expression patterns

The Eastern and Western mosquitofish (Gambusia holbrooki and G. affinis) are potential bioindicator organisms for endocrine disruptors. Male mosquitofish have an elongated anal fin (gonopodium) used for internal fertilization whose formation is driven by androgens. Normal female mosquitofish have a normal, rounded anal fin which undergoes elongation into a gonopodium structure when female mosquitofish are exposed to androgenic chemicals. Significant issues with using mosquitofish as a bioindicator include the lack of knowledge on how anal fin growth in females corresponds to endpoints relevant to biological integrity and the lack of information on the molecular pathways that regulate anal fin growth. The objectives of this study were to understand how androgen-induced anal fin elongation relates to changes in endpoints related to the female reproductive system and to understand how anal fin elongation occurs in androgen-exposed female mosquitofish. To achieve these objectives, adult female G. holbrooki were exposed to a vehicle control or one of three doses of the androgen 17β-trenbolone (TB) at nominal concentrations of 0.1, 1 or 10 μg TB/L. Anal fin measurements were taken and livers were used for quantitative polymerase chain reaction analysis of vitellogenin (vtg) mRNA expression at multiple time points. 10 μg TB/L induced anal fin elongation after 7 days of treatment (one-way ANOVA, p<0.05) as did 0.1 and 1 μg TB/L at later time points (one-way ANOVA, p<0.05). 10 μg TB/L significantly reduced hepatic vtg gene expression at all time points assessed (one-way ANOVA, p<0.05). There was no correlation between anal fin elongation levels and vtg gene expression (Spearman's ρ, p>0.05). In a separate experiment, female G. holbrooki and G. affinis were exposed to the vehicle control or 1 μg TB/L. Anal fins were used for qualitative gene expression analysis of the genes sonic hedgehog (shh), muscle segment homeobox C (msxC), and fibroblast growth factor receptor 1 (fgfr1) by in situ hybridization. Shh was expressed in the distal tip of the gonopodium while msxC and fgfr1 were more widely expressed along the same anal fin rays during androgen exposure. These data provide insight into the molecular pathways involved in anal fin elongation and pave the way for future work toward developing the mosquitofish into a bioindicator organism for endocrine disruptors.

Evaluation of the genotoxic and cytotoxic effects of glyphosate-based herbicides in the ten spotted live-bearer fish Cnesterodon decemmaculatus (Jenyns, 1842)

Mortality, genotoxicity, and cytotoxicity of the 48% glyphosate-based formulations Panzer and Credit(®) were evaluated on Cnesterodon decemmaculatus (Jenyns, 1842) (Pisces, Poeciliidae) under laboratory conditions. Induction of micronuclei (MN) and alterations in the erythrocytes:erythroblasts ratio were employed as end points for genotoxicity and cytotoxicity, respectively. For Panzer(®), mean values of 16.70 and 15.68 mg/L were determined for LC(50) at 24 and 96 h, respectively, and these concentrations reached mean values of 98.50 and 91.73 mg/L for Credit(®). LC(50) values decreased as a negative linear function of Panzer(®) exposure time within the 0-96 h period, but not for Credit(®). LC(50) values indicated that the fish were more sensitive to Panzer(®) than to Credit(®). Both 3.9 and 7.8 mg/L of Panzer(®) increased MN frequency at 48 and 96 h of treatment. When fish were exposed to Credit(®), an increased frequency of MN over control values was found after 96 h for all concentrations assayed, but not after 48 h. No cellular cytotoxicity was found after Panzer(®) and Credit(®) treatment, regardless of both the concentration and the sampling time. Furthermore, our results demonstrated that Panzer(®) and Credit(®) should be considered as glyphosate-based commercial formulations with genotoxic but not cytotoxic effect properties.

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

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.

Genetic and morphological studies of Nothobranchius (Cyprinodontiformes) from Malawi with description of Nothobranchius wattersi sp. nov

Molecular and morphological data were used to explore evolutionary differentiation among populations of Nothobranchius in the Lake Malawi-upper Shire River and the Lakes Chilwa-Chiuta drainage systems in Malawi. The aim of the study was to test the hypothesis that Nothobranchius of the Malawi-Shire system constitute a separate evolutionary group from Nothobranchius kirki. Mitochondrial and nuclear sequence data show a strongly supported phylogenetic split into two monophyletic groups separating the Lake Malawi basin fish from N. kirki. Unlike N. kirki, Lake Malawi-Shire fish do not deviate from neutrality and express an excess of rare haplotypes and mutations in terminal branches, characteristic of recently expanded populations. Further, the two groups significantly differ in morphology. Two body characters (dorsal-fin base length and pre-pelvic-pre-anal distance) are significantly different between the two species in both sexes. Several other characters are significantly different in either male or female comparisons with respect to both standard and head lengths, and robust morphological differentiation is detected by multivariate analysis. The two groups are readily distinguished on the basis of male colouration, especially in scale centres and the caudal fin. On the basis of this differentiation at the molecular and morphological levels, in addition to colouration, the Lake Malawi-Shire fish are hereby formally recognized as constituting a new species, Nothobranchius wattersi. This distinction is in agreement with the geomorphologic and recent climatic history in the region.

Assessment of toxic effects of triclosan on the swordtail fish (Xiphophorus helleri) by a multi-biomarker approach

The toxic effects of triclosan (TCS) on the swordtail fish (Xiphophorus helleri) were assessed based on various biomarkers including enzymatic activities of ethoxyresorufin O-deethylase (EROD), erythromycin N-demethylase (ERND) and glutathione-s-transferase (GST) and mRNA expression levels of CYP1A, CYP3A, glutathione S-transferase (GST) and P-glycoprotein (P-gp). The acute toxicity test showed the LC(50) value of 1.47 mg L(-1) for TCS. The mRNA expressions of CYP1A, CYP3A, GST and P-gp showed dose-effect relationships in female swordtail fish when exposed to TCS, These mRNA expression levels were found more sensitive to TCS exposure than the enzymatic activities of EROD, ERND and GST do. In addition, the male fish displayed higher gene expression levels and more dramatic changes in enzyme activities than the females did. Our data further demonstrated that TCS was a typical inducer to Phase I and Phase II metabolism enzymes and genes, suggesting it is a potential ecotoxicological risk to aquatic ecosystems.

Maintaining functional major histocompatibility complex diversity under inbreeding: the case of a selfing vertebrate

Major histocompatibility complex (MHC) genes encode proteins that present pathogen-derived antigens to T-cells, initiating the adaptive immune response in vertebrates. Although populations with low MHC diversity tend to be more susceptible to pathogens, some bottlenecked populations persist and even increase in numbers despite low MHC diversity. Thus, the relative importance of MHC diversity versus genome-wide variability for the long-term viability of populations after bottlenecks and/or under high inbreeding is controversial. We tested the hypothesis that genome-wide inbreeding (estimated using microsatellites) should be more critical than MHC diversity alone in determining pathogen resistance in the self-fertilizing fish Kryptolebias marmoratus by analysing MHC diversity and parasite loads in natural and laboratory populations with different degrees of inbreeding. Both MHC and neutral diversities were lost after several generations of selfing, but we also found evidence of parasite selection acting on MHC diversity and of non-random loss of alleles, suggesting a possible selective advantage of those individuals with functionally divergent MHC, in accordance with the hypothesis of divergent allele advantage. Moreover, we found that parasite loads were better explained by including MHC diversity in the model than by genome-wide (microsatellites) heterozygosity alone. Our results suggest that immune-related overdominance could be the key in maintaining variables rates of selfing and outcrossing in K. marmoratus and other mixed-mating species.

The possibility of de novo assembly of the genome and population genomics of the mangrove rivulus, Kryptolebias marmoratus

How organisms adapt to the range of environments they encounter is a fundamental question in biology. Elucidating the genetic basis of adaptation is a difficult task, especially when the targets of selection are not known. Emerging sequencing technologies and assembly algorithms facilitate the genomic dissection of adaptation and population differentiation in a vast array of organisms. Here we describe the attributes of Kryptolebias marmoratus, one of two known self-fertilizing hermaphroditic vertebrates that make this fish an attractive genetic system and a model for understanding the genomics of adaptation. Long periods of selfing have resulted in populations composed of many distinct naturally homozygous strains with a variety of identifiable, and apparently heritable, phenotypes. There also is strong population genetic structure across a diverse range of mangrove habitats, making this a tractable system in which to study differentiation both within and among populations. The ability to rear K. marmoratus in the laboratory contributes further to its value as a model for understanding the genetic drivers for adaptation. To date, microsatellite markers distinguish wild isogenic strains but the naturally high homozygosity improves the quality of de novo assembly of the genome and facilitates the identification of genetic variants associated with phenotypes. Gene annotation can be accomplished with RNA-sequencing data in combination with de novo genome assembly. By combining genomic information with extensive laboratory-based phenotyping, it becomes possible to map genetic variants underlying differences in behavioral, life-history, and other potentially adaptive traits. Emerging genomic technologies provide the required resources for establishing K. marmoratus as a new model organism for behavioral genetics and evolutionary genetics research.

Genetic diversity shaped by historical and recent factors in the live-bearing twoline skiffia Neotoca bilineata

The endangered twoline skiffia Neotoca bilineata, a viviparous fish of the subfamily Goodeinae, endemic to central Mexico (inhabiting two basins, Cuitzeo and Lerma-Santiago) was evaluated using genetic and habitat information. The genetic variation of all remaining populations of the species was analysed using both mitochondrial and microsatellite markers and their habitat conditions were assessed using a water quality index (I(WQ)). An 80% local extinction was found across the distribution of N. bilineata. The species was found in three of the 16 historical localities plus one previously unreported site. Most areas inhabited by the remaining populations had I(WQ) scores unsuitable for the conservation of freshwater biodiversity. Populations showed low but significant genetic differentiation with both markers (mtDNA φ(ST) = 0.076, P < 0.001; microsatellite F(ST) = 0.314, P < 0.001). Borbollon, in the Cuitzeo Basin, showed the highest level of differentiation and was identified as a single genetic unit by Bayesian assignment methods. Rio Grande de Morelia and Salamanca populations showed the highest genetic diversity and also a high migration rate facilitated by an artificial channel that connected the two basins. Overall, high genetic diversity values were observed compared with other freshwater fishes (average N(a) = 16 alleles and loci and mean ±S.D. H(o) = 0.63 ± 0.10 and nucleotide diversity π = 0.006). This suggests that the observed genetic diversity has not diminished as rapidly as the species' habitat destruction. No evidence of correlation between habitat conditions and genetic diversity was found. The current pattern of genetic diversity may be the result of both historical factors and recent modifications of the hydrological system. The main threat to the species may be the rapid habitat deterioration and associated demographic stochasticity rather than genetic factors.

Gambusia quadruncus (Cyprinodontiformes: Poeciliidae): a new species of mosquitofish from east-central México

Gambusia quadruncus n. sp., the llanos mosquitofish, is described from east-central México. The region inhabited by the species represents a hotspot of diversity of Gambusia, and G. quadruncus sometimes coexists with at least three congeners. The species differs from its closest relative, Gambusia affinis, in several characteristics with plausible effects on reproductive isolation, e.g. body size, body and fin morphology, male genital morphology (distal tip of gonopodium) and female anal spot morphology (colouration near the urogenital sinus). Moreover, combined analysis of mitochondrial and nuclear gene sequence data (c. 2158 total base pairs) indicates reciprocal monophyly of G. quadruncus and its sister species G. affinis, with levels of genetic divergence suggesting the two species diverged from one another over a million years ago. The origin of G. quadruncus may reflect a vicariant event associated with Pliocene orogenesis in the Tamaulipas Arch and a frontal section of the Sierra Madre Oriental (Lleran Mesas). Gambusia quadruncus inhabits a variety of freshwater habitats across several river drainages, with its range spanning at least 350 km from north to south, covering over 25 000 km(2). A key to aid identification of the species is provided.

Evidence of melanoma in wild marine fish populations

The increase in reports of novel diseases in a wide range of ecosystems, both terrestrial and marine, has been linked to many factors including exposure to novel pathogens and changes in the global climate. Prevalence of skin cancer in particular has been found to be increasing in humans, but has not been reported in wild fish before. Here we report extensive melanosis and melanoma (skin cancer) in wild populations of an iconic, commercially-important marine fish, the coral trout Plectropomus leopardus. The syndrome reported here has strong similarities to previous studies associated with UV induced melanomas in the well-established laboratory fish model Xiphophorus. Relatively high prevalence rates of this syndrome (15%) were recorded at two offshore sites in the Great Barrier Reef Marine Park (GBRMP). In the absence of microbial pathogens and given the strong similarities to the UV-induced melanomas, we conclude that the likely cause was environmental exposure to UV radiation. Further studies are needed to establish the large scale distribution of the syndrome and confirm that the lesions reported here are the same as the melanoma in Xiphophorus, by assessing mutation of the EGFR gene, Xmrk. Furthermore, research on the potential links of this syndrome to increases in UV radiation from stratospheric ozone depletion needs to be completed.

What can phylogeographic breaks in the livebearer Poeciliopsis turrubarensis tell about fish community boundaries in western Costa Rica?

This study asks if the geographic boundary delineating two fish communities in western Costa Rica is congruent with a phylogeographic break in a single widespread fish species Poeciliopsis turrubarensis (Poeciliidae) that spans this area. Such congruence would suggest that a common historical event (e.g. geological or climatic) could be responsible for both patterns. It was found that there was a shared break across a region in central Costa Rica suggesting a common cause may be responsible for both the abrupt shift in fish community composition and the genetic break in P. turrubarensis.

Identification of transcriptome SNPs between Xiphophorus lines and species for assessing allele specific gene expression within F₁ interspecies hybrids

Variations in gene expression are essential for the evolution of novel phenotypes and for speciation. Studying allelic specific gene expression (ASGE) within interspecies hybrids provides a unique opportunity to reveal underlying mechanisms of genetic variation. Using Xiphophorus interspecies hybrid fishes and high-throughput next generation sequencing technology, we were able to assess variations between two closely related vertebrate species, Xiphophorus maculatus and Xiphophorus couchianus, and their F(1) interspecies hybrids. We constructed transcriptome-wide SNP polymorphism sets between two highly inbred X. maculatus lines (JP 163 A and B), and between X. maculatus and a second species, X. couchianus. The X. maculatus JP 163 A and B parental lines have been separated in the laboratory for ≈70 years and we were able to identify SNPs at a resolution of 1 SNP per 49 kb of transcriptome. In contrast, SNP polymorphisms between X. couchianus and X. maculatus species, which diverged ≈5-10 million years ago, were identified about every 700 bp. Using 6524 transcripts with identified SNPs between the two parental species (X. maculatus and X. couchianus), we mapped RNA-seq reads to determine ASGE within F(1) interspecies hybrids. We developed an in silico X. couchianus transcriptome by replacing 90,788 SNP bases for X. maculatus transcriptome with the consensus X. couchianus SNP bases and provide evidence that this procedure overcomes read mapping biases. Employment of the in silico reference transcriptome and tolerating 5 mismatches during read mapping allow direct assessment of ASGE in the F(1) interspecies hybrids. Overall, these results show that Xiphophorus is a tractable vertebrate experimental model to investigate how genetic variations that occur during speciation may affect gene interactions and the regulation of gene expression.

Proteomic analyses of the Xiphophorus Gordon-Kosswig melanoma model

Interspecies hybridization between the platyfish X. maculatus Jp 163 A, and the swordtail X. helleri (Sarabia), generates F(1) hybrids with pronounced melanin pigmentation. Backcrossing of F(1) hybrids with the X. helleri parent results in 25% of progeny that will spontaneously develop melanoma. We have applied proteomic methods to this Gordon-Kosswig (G-K) melanoma model to identify candidate proteins that exhibit modulated expression in fin tissue due to interspecies hybridization and progression of hybrid tissues to spontaneous melanoma. Difference Gel Electrophoresis (DIGE) was used to minimize the variability commonly observed in quantitative analyses of comparative protein samples. Following identification of up- or down-regulated protein expression by DIGE, candidate protein spots were identified by mass spectrometric sequencing. Several protein expression differences displayed in interspecies hybrids were identified and compared to distinct differences that occur upon backcrossing and progression to melanoma. These studies are important for the identification of distinct biochemical pathways involved in the variety of Xiphophorus interspecies hybrid tumor models.

Recent advances in sunlight-induced carcinogenesis using the Xiphophorus melanoma model

Unlike breast and prostate cancers, the nature and sequence of critical genetic and epigenetic events involved in the initiation and progression of melanoma are not well understood. A contributing factor to this dilemma, especially given our current understanding of the importance of UV light in melanoma etiology, is the lack of quality UV-inducible melanoma animal models. In this study we elaborate on the capability of UV light to induce cutaneous malignant melanomas (CMM) in Xiphophorus fishes, which were previously found to develop melanomas after acute neonatal UVB irradiation. In two separate tumorigenesis experiments, we exposed adult Xiphophorus hybrids to either acute UVB irradiations (5 consecutive daily treatments) or chronic solar irradiations (continuous UVA/UVB treatment for 9 months). Acute adult UVB irradiation resulted in the significant induction of melanomas, and moreover, this induction rate is equivalent to that of animals exposed to acute neonatal UVB irradiation. This study represents the first evidence that acute adult UVB irradiation, in the absence of any early life exposures, induces CMM. Similar to the findings conducted on other divergent melanoma models, including HGF/SF transgenic mice and Monodelphis domestica, prolonged chronic solar UV was not a factor in melanomagenesis.

Effects of short read quality and quantity on a de novo vertebrate transcriptome assembly

For many researchers, next generation sequencing data holds the key to answering a category of questions previously unassailable. One of the important and challenging steps in achieving these goals is accurately assembling the massive quantity of short sequencing reads into full nucleic acid sequences. For research groups working with non-model or wild systems, short read assembly can pose a significant challenge due to the lack of pre-existing EST or genome reference libraries. While many publications describe the overall process of sequencing and assembly, few address the topic of how many and what types of reads are best for assembly. The goal of this project was use real world data to explore the effects of read quantity and short read quality scores on the resulting de novo assemblies. Using several samples of short reads of various sizes and qualities we produced many assemblies in an automated manner. We observe how the properties of read length, read quality, and read quantity affect the resulting assemblies and provide some general recommendations based on our real-world data set.

Expression regulation triggers oncogenicity of xmrk alleles in the Xiphophorus melanoma system

The Xiphophorus melanoma model has gained attention in biomedical research as a genetic model for tumor formation. Melanoma development in interspecific hybrids of Xiphophorus is connected to pigment cell specific overexpression of the mutationally activated receptor tyrosine kinase Xmrk. In purebred fish the oncogenic function of xmrk is suppressed by a so far unknown regulator locus R. To test the hypothesis that R is involved in transcriptional regulation of xmrk and consequently acts upstream of the xmrk signal, we performed a quantitative analysis of xmrk transcript levels in normal and melanoma tissues of different Xiphophorus genotypes carrying either a highly tumorigenic or a non-tumorigenic xmrk allele. Our results demonstrate that expression of the tumorigenic xmrk allele is highly increased in malignant melanomas compared to benign lesions, macromelanophore spots, and healthy skin. Transcription of the non-tumorigenic xmrk allele in pigment cells, in contrast, is not influenced by the presence or absence of R. These findings strongly indicate that differential transcriptional regulation of the xmrk promoter determines the tumorigenic potential of xmrk alleles in the Xiphophorus melanoma system, thereby supporting the hypothesis that R suppresses the oncogenic function of xmrk on the level of transcriptional control.

Characterization of telomeres and telomerase expression in Xiphophorus

Research investigating telomere lengths and telomerase expression in vertebrates has progressively become important due to the association of these two biological endpoints with cellular aging and cancer in humans. Studies that rely upon the traditional use of laboratory mice have been faced with limitations largely due to inbred mice possessing large telomeres and ubiquitous expression of telomerase. Recently, a number of small fish species have been shown to provide potentially informative models for examining the role of telomeres and telomerase within intact vertebrate animals. Xiphophorus fishes represent a new world live-bearing genus that has not previously been assessed for telomere length or telomerase expression. To add to the knowledge base of telomere and telomerase biology in vertebrates we assessed telomere length and telomerase expression among several species of Xiphophorus. The telomere lengths in several organs (gill, brain, eyes, testis, ovary and liver) in three species (Xiphophorus hellerii, Xiphophorus maculatus, Xiphophorus couchianus) and also in F(1) interspecies hybrids were approximately 2-6 kb. This size was consistent within the same organs of the same species, as well as between species and F(1) hybrids. Despite possessing relatively short telomere lengths compared to humans, the consistency of size among Xiphophorus species and organs may allow experimental detection of telomere shortening. The relative expression of telomerase reverse transcriptase (TERT) was determined by quantitative real-time PCR. Expression levels of TERT was measured in seven organs (ovary, testis, liver, gill, brain, heart, skin) from X. maculatus, X. hellerii and in control and ultraviolet light (UVB) exposed skin samples from X. maculatus, X. hellerii, and F(1) interspecies hybrids. TERT gene expression was significantly higher in ovary and testis, while all other organs showed low relative TERT expression. Detectable increases in TERT expression were found in skin samples upon UVB exposure. Our findings suggest that Xiphophorus may serve as a suitable model for future studies investigating the association of telomere length and telomerase expression in regard to aging and disease.

Extreme homogeneity and low genetic diversity in Kryptolebias ocellatus from south-eastern Brazil suggest a recent foundation for this androdioecious fish population

This study documents unexpectedly low levels of intra and interpopulation genetic diversity in Kryptolebias ocellatus, an androdioecious and predominantly self-fertilizing killifish from south-eastern Brazil. This finding generally is inconsistent with the established opinion that the K. ocellatus and K. marmoratus clade originated in this geographic region and later dispersed northward into the Caribbean.

Mitochondrial DNA copy number and function decrease with age in the short-lived fish Nothobranchius furzeri

Among vertebrates that can be kept in captivity, the annual fish Nothobranchius furzeri possesses the shortest known lifespan. It also shows typical signs of aging and is therefore an ideal model to assess the role of different physiological and environmental parameters on aging and lifespan determination. Here, we used Nothobranchius furzeri to study whether aging is associated with mitochondrial DNA (mtDNA) alterations and changes in mitochondrial function. We sequenced the complete mitochondrial genome of N. furzeri and found an extended control region. Large-scale mtDNA deletions have been frequently described to accumulate in other organisms with age, but there was no evidence for the presence of detectable age-related mtDNA deletions in N. furzeri. However, mtDNA copy number significantly decreased with age in skeletal muscle, brain, liver, skin and dorsal fin. Consistent with this finding, expression of Pgc-1α that encodes a transcriptional coactivator of mitochondrial biogenesis and expression of Tfam and mtSsbp both encoding mtDNA binding factors was downregulated with age. The investigation of possible changes in mitochondrial function revealed that the content of respiratory chain complexes III and IV was reduced in skeletal muscle with age. In addition, ADP-stimulated and succinate-dependent respiration was decreased in mitochondria of old fish. These findings suggest that despite the short lifespan, aging in N. furzeri is associated with a decline in mtDNA copy number, the downregulation of mtDNA-associated genes and an impairment of mitochondrial function.

Bisphenol A modulates expression of sex differentiation genes in the self-fertilizing fish, Kryptolebias marmoratus

Endocrine disrupting chemicals (EDCs) have been a major concern in the normal reproduction and development of aquatic organisms. In the teleost, steroid hormones are synthesized via the steroidogenesis pathway, and play a key physiological role in the regulation of gonadal sex differentiation. The protogynous hermaphroditic fish, Kryptolebias marmoratus is the only vertebrate capable of reproducing through internal self-fertilization. To uncover the effect of bisphenol A (BPA) on sex differentiation genes on transcription, we investigated the expression patterns of several sex differentiation-related genes such as dax1, dmrt1, mis, sf1, figlα, StAR and wt1 after BPA exposure with controls (E2 and TMX). In response to 17β-estradiol (E2) exposure, a testis-specific gene, dmrt1 mRNA was down-regulated in the gonad of the secondary male but the expression of the female-specific gene, dax1 mRNA was significantly elevated in the brain and gonad. A high level of StAR mRNA was detected in the brain and gonad of both hermaphrodite and secondary males, suggesting that the elevated expression of dax1 and StAR genes would be involved in E2 exposure. As expected, upon BPA exposure, the dmrt1 and MIS mRNA level decreased in both hermaphrodite and secondary males, while the female-specific gene, figlα mRNA level increased in the gonad of both genders. BPA showed an opposite mode of action on the expression of dax1 (induction, P>0.05) and sf1 mRNA (inhibition, P>0.05) in the brain and gonad against both genders. The sensitivity of dax1 to BPA on expression was relatively high in the secondary male. The wt1 mRNA was up-regulated in most tissues except in the liver of BPA-exposed secondary males. Regarding the time course study, the figlα mRNA level increased at 6 h after BPA exposure. In addition, BPA elevated the expression of StAR, dax1, and wt1 mRNA but repressed sf1 mRNA. In this paper, we demonstrated that BPA may modulate the expression of sex differentiation and steroidogenesis pathway genes, and this finding would provide a better understanding on the modulation of transcription upon BPA exposure in steroidogenesis and sex differentiation in the hermaphroditic fish, K. marmoratus.

Successful extraction of DNA from archived alcohol-fixed white-eye fish specimens using an ancient DNA protocol

A protocol used routinely for rapid ancient DNA extraction was applied to fish tissue archived over 80 years ago. The method proved successful, whereas other extraction protocols failed. Researchers working on DNA from older archived fish samples are encouraged to continue to concentrate their efforts on 'white-eye' specimens, which indicate an alcohol-based fixative and are thus likely to yield viable DNA.

Heritability of heat tolerance in a small livebearing fish, Heterandria formosa

Climate change is expected to result in an increased occurrence of heat stress. The long-term population-level impact of this stress would be lessened in populations able to genetically adapt to higher temperatures. Adaptation requires the presence of genetically-based variation. At-risk populations may undergo strong declines in population size that lower the amount of genetic variation. The objectives of this study were to quantify the heritability of heat tolerance in populations of the least killifish, Heterandria formosa, and to determine if heritabilities were reduced following a population bottleneck. Heritabilities of heat tolerance were determined for two lines of each of two source populations; two bottlenecked lines (established with one pair of fish) and two regular lines. Heat tolerance was quantified as temperature-at-death (TAD), when fish acclimated at 28 °C were subjected to an increase in water temperature of 2 °C/day. Mid-parent/mean offspring regressions and full-sib analyses were used to estimate the heritability of TAD. Heritability estimates from parent/offspring regressions ranged from 0.185 to 0.462, while those from sib analyses ranged from 0 to 0.324, with an overall estimate of 0.203 (0.230 for the regular lines, 0.168 for bottlenecked ones). Fish from the bottlenecked line from one source population (but not the other) had a lower heritability than did those from the regular line. These results show that the populations tested had some potential for adaptation to elevated water temperatures, and that this potential may be reduced following a population bottleneck. This should not be construed as evidence that natural populations will not suffer negative consequences from global warming; this study only showed that these specific populations have some potential to adapt under a very specific set of conditions.