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Abstract:
Recombination increases dramatically during meiosis to promote genetic exchange and generate recombinant progeny. Interestingly, meiotic recombination is unevenly distributed throughout genomes and as a consequence genetic and physical map distances do not have a simple linear relationship. Recombination hotspots and coldspots have been described in many organisms and often reflect global features of chromosome structure. In particular, recombination frequencies are often distorted within or outside sex determining regions of the genome. Here we report that recombination is elevated adjacent to the mating-type locus (MAT) in the pathogenic basidiomycete Cryptococcus neoformans. Among fungi, C. neoformans has an unusually large MAT locus, and recombination is suppressed between the two >100 kb mating-type specific alleles. When genetic markers were introduced at defined physical distances from MAT, we found the meiotic recombination frequency to be ~20% between MAT and a flanking marker at 5, 10, 50 or 100 kb from the right border. As a result, the physical/genetic map ratio in the regions adjacent to MAT is distorted ~10- to 50- fold compared to the genome-wide average. Moreover, recombination frequently occurred on both sides of MAT and negative interference between crossovers was observed. Sequence analysis revealed a correlation between high G + C content and these hotspot regions. We hypothesize that the presence of recombinational activators may have driven several key events during the assembly and reshaping of the MAT locus, and may have played similar roles in the origins of both metabolic and biosynthetic gene clusters. Our findings suggest that during meiosis the MAT locus may be exchanged onto different genetic backgrounds and therefore have broad evolutionary implications with respect to mating-type switching in both model and and pathogenic yeasts.
