Abstract
BackgroundSex chromosomes change more frequently in fish than in mammals or birds. However, certain chromosomes or genes are repeatedly used as sex determinants in different members of the teleostean lineage. East African cichlids are an enigmatic model system in evolutionary biology representing some of the most diverse extant vertebrate adaptive radiations. How sex is determined and if different sex-determining mechanisms contribute to speciation is unknown for almost all of the over 1,500 cichlid species of the Great Lakes. Here, we investigated the genetic basis of sex determination in a cichlid from Lake Tanganyika, Astatotilapia burtoni, a member of the most species-rich cichlid lineage, the haplochromines.ResultsWe used RAD-sequencing of crosses for two populations of A. burtoni, a lab strain and fish caught at the south of Lake Tanganyika. Using association mapping and comparative genomics, we confirmed male heterogamety in A. burtoni and identified different sex chromosomes (LG5 and LG18) in the two populations of the same species. LG5, the sex chromosome of the lab strain, is a fusion chromosome in A. burtoni. Wnt4 is located on this chromosome, representing the best candidate identified so far for the master sex-determining gene in our lab strain of A. burtoni.ConclusionsCichlids exemplify the high turnover rate of sex chromosomes in fish with two different chromosomes, LG5 and LG18, containing major sex-determining loci in the two populations of A. burtoni examined here. However, they also illustrate that particular chromosomes are more likely to be used as sex chromosomes. Chromosome 5 is such a chromosome, which has evolved several times as a sex chromosome, both in haplochromine cichlids from all Great Lakes and also in other teleost fishes.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-3178-0) contains supplementary material, which is available to authorized users.
Highlights
Sex chromosomes change more frequently in fish than in mammals or birds
We ran “populations” separately for the lab strain and wild fish, each with defining minimum number of individuals required for a locus in each population as 1 (-p 1) and minimum stack depth required for individuals at a locus as 3 (-m 3) excluding over-merged loci (-B blacklisted markers, generated with a custom python script [31])
At 52 of these Single nucleotide polymorphism (SNP), father and sons are heterozygous whereas daughters and the mother are homozygous; meaning that only sons inherit specific alleles from the father, as expected from an XX-XY sex-determining system
Summary
Sex chromosomes change more frequently in fish than in mammals or birds. certain chromosomes or genes are repeatedly used as sex determinants in different members of the teleostean lineage. We investigated the genetic basis of sex determination in a cichlid from Lake Tanganyika, Astatotilapia burtoni, a member of the most species-rich cichlid lineage, the haplochromines. Sexual reproduction originated in the last common ancestor of eukaryotes and is almost ubiquitous in the animal kingdom [1] For such an old trait, the initial triggers of sex determination (SD), the process driving the undifferentiated embryo towards a female or male phenotype, are not conserved. The homolog carrying the SD locus may progressively degenerate with the expansion of the non-recombining segment facilitated by linkage disequilibrium and sexually antagonistic selection [4].
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