Abstract
BackgroundStudies of closely related species with different sex chromosome systems can provide insights into the processes of sex chromosome differentiation and evolution. To investigate the potential utility of molecular markers in studying sex chromosome differentiation at early stages of their divergence, we examined the levels and patterns of genetic differentiation between sex chromosomes in nine-spined (Pungitius pungitius) and three-spined sticklebacks (Gasterosteus aculeatus) using microsatellite markers.ResultsA set of novel microsatellite markers spanning the entire length of the sex chromosomes were developed for nine-spined sticklebacks using the sequenced genomes of other fish species. Sex-specific patterns of genetic variability and male-specific alleles were identified at most of these loci, indicating a high degree of differentiation between the X and Y chromosomes in nine-spined sticklebacks. In three-spined sticklebacks, male-specific alleles were detected at some loci confined to two chromosomal regions. In addition, male-specific null alleles were identified at several other loci, implying the absence of Y chromosomal alleles at these loci. Overall, male-specific alleles and null alleles were found over a region spanning 81% of the sex chromosomes in three-spined sticklebacks.ConclusionsHigh levels but distinct patterns of sex chromosome differentiation were uncovered in the stickleback species that diverged 13 million years ago. Our results suggest that the Y chromosome is highly degenerate in three-spined sticklebacks, but not in nine-spined sticklebacks. In general, the results demonstrate that microsatellites can be useful in identifying the degree and patterns of sex chromosome differentiation in species at initial stages of sex chromosome evolution.
Highlights
Studies of closely related species with different sex chromosome systems can provide insights into the processes of sex chromosome differentiation and evolution
Our results suggest that the Y chromosome is highly degenerate in three-spined sticklebacks
Our results further imply that the Y chromosome is highly degenerate in three-spined sticklebacks, but not in nine-spined sticklebacks
Summary
Studies of closely related species with different sex chromosome systems can provide insights into the processes of sex chromosome differentiation and evolution. Different heterogametic sex Despite the substantial progress in understanding the evolutionary history of sex chromosomes, there are still gaps in our knowledge about the process of sex chromosome differentiation. It has been hypothesized that sex chromosomes would typically evolve from a pair of autosomes that cease to recombine with each other after acquiring a sex determining role [13,14,15]. A heteromorphic sex chromosome pair is generally thought to have evolved through increasing stages of differentiation [14]. Neither suppressed recombination nor heteromorphism always evolve in old-established sex chromosomes [2]
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