Why sex chromosomes turn over and remain undifferentiated in some taxa, whereas they degenerate in others, is still an area of ongoing research. The recurrent occurrence of homologous and homomorphic sex chromosomes in distantly related taxa suggests their independent evolution or continued recombination since their first emergence. Fishes display a great diversity of sex-determining systems. Here, we focus on sex chromosome evolution in haplochromines, the most species-rich lineage of cichlid fishes. We investigate sex-specific signatures in the Pseudocrenilabrus philander species complex, which belongs to a haplochromine genus found in many river systems and ichthyogeographic regions in northern, eastern, central, and southern Africa. Using whole-genome sequencing and population genetic, phylogenetic, and read-coverage analyses, we show that one population of P. philander has an XX–XY sex-determining system on LG7 with a large region of suppressed recombination. However, in a second bottlenecked population, we did not find any sign of a sex chromosome. Interestingly, LG7 also carries an XX–XY system in the phylogenetically more derived Lake Malawi haplochromine cichlids. Although the genomic regions determining sex are the same in Lake Malawi cichlids and P. philander , we did not find evidence for shared ancestry, suggesting that LG7 evolved as sex chromosome at least twice in haplochromine cichlids. Hence, our work provides further evidence for the labile nature of sex determination in fishes and supports the hypothesis that the same genomic regions can repeatedly and rapidly be recruited as sex chromosomes in more distantly related lineages.

Sturgeons (family Acipenseridae) are one of the most endangered groups of animals. Two hundred million years of evolution and multiple ploidy levels make this group a unique subject for studying the evolution of polyploidy in animals. As most sturgeon species have gone through significant functional diploidization, 2 scales of ploidy levels can be distinguished: the "evolutionary scale," which indicates the maximum ploidy level achieved and the "recent scale," which indicates the current functional ploidy level. This study analyzes published and new microsatellites to check the ploidy level and to determine the degrees of functional diploidization in 10 sturgeon species from Europe and Asia. We screened 50 primer pairs newly developed for Acipenser gueldenstaedtii and 40 primer pairs previously developed in other studies for other sturgeon species. The maximal number of alleles per individual of a given species was assessed at 20 microsatellite loci, which showed consistent amplification in most of the 10 analyzed species. Taken together, our data on the percentage of disomic loci in different species suggest that functional diploidization is an ongoing process in sturgeons. We observed lower levels of diploidization in tetraploid species from the Atlantic clade than in the species from the Pacific clade, which can be explained by the more recent genome duplication in tetraploid species from the Atlantic clade. Based on the recent findings and results of this study, we propose that the evolution of sturgeons has been affected by at least 3 different polyploidization events.