A genomic analysis of the annual fish Nothobranchius furzeri, a vertebrate with the shortest known life span in captivity and which may provide a new model organism for aging research.

Sex chromosomes differ from autosomes by dissimilar gene content and, at a more advanced stage of their evolution, also in structure and size. This is driven by the divergence of the Y or W from their counterparts, X and Z, due to reduced recombination and the resulting degeneration as well as the accumulation of sex-specific and sexually antagonistic genes. A paradigmatic example for Y-chromosome evolution is found in guppies. In these fishes, conflicting data exist for a morphological and molecular differentiation of sex chromosomes. Using molecular probes and the previously established linkage map, we performed a cytogenetic analysis of sex chromosomes. We show that the Y chromosome has a very large pseudoautosomal region, which is followed by a heterochromatin block (HCY) separating the subtelomeric male-specific region from the rest of the chromosome. Interestingly, the size of the HCY is highly variable between individuals from different population. The largest HCY was found in one population of Poecilia wingei, making the Y almost double the size of the X and the largest chromosome of the complement. Comparative analysis revealed that the Y chromosomes of different guppy species are homologous and share the same structure and organization. The observed size differences are explained by an expansion of the HCY, which is due to increased amounts of repetitive DNA. In one population, we observed also a polymorphism of the X chromosome. We suggest that sex chromosome-linked color patterns and other sexually selected genes are important for maintaining the observed structural polymorphism of sex chromosomes.

Background: Sex determination relies on a hierarchically structured network of genes, and is one of the most plastic processes in evolution. The evolution of sex-determining genes within a network, by neo-or sub-functionalization, also requires the regulatory landscape to be rewired to accommodate these novel gene functions. We previously showed that in medaka fish, the regulatory landscape of the master male-determining gene dmrt1bY underwent a profound rearrangement, concomitantly with acquiring a dominant position within the sex-determining network. This rewiring was brought about by the exaptation of a transposable element (TE) called Izanagi, which is co-opted to act as a silencer to turn off the dmrt1bY gene after it performed its function in sex determination. Results: We now show that a second TE, Rex1, has been incorporated into Izanagi. The insertion of Rex1 brought in a preformed regulatory element for the transcription factor Sox5, which here functions in establishing the temporal and cell-type-specific expression pattern of dmrt1bY. Mutant analysis demonstrates the importance of Sox5 in the gonadal development of medaka, and possibly in mice, in a dmrt1bY-independent manner. Moreover, Sox5 medaka mutants have complete female-to-male sex reversal. Conclusions: Our work reveals an unexpected complexity in TE-mediated transcriptional rewiring, with the exaptation of a second TE into a network already rewired by a TE. We also show a dual role for Sox5 during sex determination: first, as an evolutionarily conserved regulator of germ-cell number in medaka, and second, by de novo regulation of dmrt1 transcriptional activity during primary sex determination due to exaptation of the Rex1 transposable element.

Background: Unisexuality, or all female reproduction, is rare among vertebrates. Studying these exceptional organisms may give useful information with respect to the evolution and maintenance of sexual reproduction. Poecilia formosa was the first unisexual vertebrate species to be detected and since then has served as a paradigmatic organism for unisexuality and studies on the evolution of sex. It reproduces through gynogenesis, using sperm of males from related species to trigger parthenogenetic development of the unreduced diploid eggs. Like in other unisexual vertebrates, triploids occur in a certain range of P. formosa. It has been suggested that the addition of the host species derived third chromosome set is evolutionary important. Clonal organisms lack sufficient genotypic diversity for adaptive changes to variable environments. Also non-recombining genomes cannot purge deleterious mutations and therefore unisexual organisms should suffer from a genomic decay. Thus, polyploidization leading to triploidy should bring "fresh" genetic material into the asexual lineage. To evaluate the importance of triploidy for maintaining the asexual species, it is important to know whether such an introgression event happens at a reasonable frequency.

Poecilia obscura, new species, is described from the Oropuche system, Trinidad. A mitochondrial DNA-sequence based molecular phylogenetic analysis revealed the status of the new species as a separate taxon. It is most closely related to the Common guppy, P. reticulata and to the recently described species, P. wingei. It can also be distinguished by morphometrics and gonopodial characteristics from these two species, although the ranges for all values overlap. A definition of the new species on morphology criteria alone is thus impossible. Therefore, P. obscura forms a cryptic species complex with the two other species. P. wingei is now unequivocally defined by the molecular phylogeny as a valid species. The three guppy species are included in the subgenus Acanthophacelus Eigenmann (1907), which is considered as generically different from all other taxa of the Poeciliinae sensu Parenti (1981).