With more than 25,000 species, fish represent more than half of extant vertebrate species. Certain traits such as sex determination, which are very stable in most vertebrate lineages, display an amazing level of diversity in fish. Such observations strongly suggest that fish possess very powerful evolutionary tools in order to modify the structure and regulation of their genes, or even to rearrange more profoundly larger parts of their genomes. We are interested in understanding the molecular basis, biological significance and evolutionary consequences of genetic diversity in fish and other vertebrates.
Despite its major importance in aquaculture, almost nothing is known about the basis of genetic sex determination in fish. In contrast to the situation observed in mammals, no simple model of genetic sex determination can be generalised in fish. Both male (XX/XY) and female heterogamety (WZ/ZZ) has been observed, as well as more complicated situations involving multiple sex chromosomes, polygenic sex determination and autosomal modifiers. Different variations of hermaphroditism are also found. Gonadal sex differentiation can be strongly influenced by environmental factors including temperature. Sex inversion can be obtained spontaneously or after treatment by steroid hormones. Determination and manipulation of sex in fish are key economical challenges for aquaculture, particularly because of the general advantage of monosex populations. We are currently identifying sex-linked markers, sex chromosomes and sex-determining genes in different aquarium fish species (platyfish, medaka, zebrafish) and aquaculture species (carp, tilapia, turbot) in order to understand the molecular basis and the biological significance of sex determination variability in fish.
The poeciliid fish Xiphophorus is one of the most advanced systems to study the evolution of sex determination, sex chromosomes and sex-linked functions in lower vertebrates. A high number of sex-linked pigmentation and hereditary melanoma patterns have been described, and bacterial artificial chromosome contigs covering about one megabase of both X and Y sex chromosomes of X. maculatus have been constructed by our group. Interestingly, almost all traits encoded by gene loci located in the sex determination region are extremely polymorphic. This phenomenon is associated with a high level of genomic plasticity and gene turnover. Rearrangements including duplications, amplifications, deletions and transpositions frequently occur and can modify the copy number, structure and transcriptional context of genes located in this region. This situation is very reminiscent of that observed in the SRY region of the human Y chromosome. We are analyzing the candidate genes located on the sex chromosomes of the platyfish, and are investigating the influence of genomic plasticity on gene function and sex chromosome evolution.
Retrotransposable elements are thought to play an important role in genome evolution. Autonomous retroelements are able to integrate into new genomic locations or to mobilize non-autonomous elements, and can disrupt genes, regulatory sequences and other important genomic structures. Retrotransposable elements generally carry regulatory sequences or can attract methylation and consequently influence the transcription of flanking genes. Importantly, retroelements increase their copy number by retrotransposition and can subsequently be substrates for homologous recombination to form various DNA rearrangements such as deletions, inversions, translocations, duplications and amplifications. Recent evidence, particularly from our lab, pointed out that fish apparently possess more retrotransposon families in their genomes than mammals. We are characterizing fish retrotransposable elements at both evolutionary and functional levels, and are studying their role in genome plasticity, genetic diversity and speciation. Particularly, we explore the intriguing possibility that some retrotransposons, generally considered as genome parasites, might have undertaken new cellular functions useful for their hosts.
New nuclear markers, particularly retroelement sequences, will be used to resolve recent and deep controversial fish phylogenies, and to develop new universal fingerprinting methods with biotechnological applications.