Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, North Carolina, United States of America.
PLoS Genet. 2012 Jul;8(7):e1002810. doi: 10.1371/journal.pgen.1002810. Epub 2012 Jul 5.
Meiotic recombination of sex chromosomes is thought to be repressed in organisms with heterogametic sex determination (e.g. mammalian X/Y chromosomes), due to extensive divergence and chromosomal rearrangements between the two chromosomes. However, proper segregation of sex chromosomes during meiosis requires crossing-over occurring within the pseudoautosomal regions (PAR). Recent studies reveal that recombination, in the form of gene conversion, is widely distributed within and may have played important roles in the evolution of some chromosomal regions within which recombination was thought to be repressed, such as the centromere cores of maize. Cryptococcus neoformans, a major human pathogenic fungus, has an unusually large mating-type locus (MAT, >100 kb), and the MAT alleles from the two opposite mating-types show extensive nucleotide sequence divergence and chromosomal rearrangements, mirroring characteristics of sex chromosomes. Meiotic recombination was assumed to be repressed within the C. neoformans MAT locus. A previous study identified recombination hot spots flanking the C. neoformans MAT, and these hot spots are associated with high GC content. Here, we investigated a GC-rich intergenic region located within the MAT locus of C. neoformans to establish if this region also exhibits unique recombination behavior during meiosis. Population genetics analysis of natural C. neoformans isolates revealed signals of homogenization spanning this GC-rich intergenic region within different C. neoformans lineages, consistent with a model in which gene conversion of this region during meiosis prevents it from diversifying within each lineage. By analyzing meiotic progeny from laboratory crosses, we found that meiotic recombination (gene conversion) occurs around the GC-rich intergenic region at a frequency equal to or greater than the meiotic recombination frequency observed in other genomic regions. We discuss the implications of these findings with regards to the possible functional and evolutionary importance of gene conversion within the C. neoformans MAT locus and, more generally, in fungi.
性染色体的减数分裂重组被认为在具有异型性决定（例如哺乳动物的 X/Y 染色体）的生物体中受到抑制，这是由于两个染色体之间广泛的分歧和染色体重排。然而，减数分裂中性染色体的正确分离需要在假常染色体区域（PAR）内发生交叉。最近的研究表明，重组以基因转换的形式广泛分布于内，并且可能在某些染色体区域的进化中发挥了重要作用，这些区域的重组被认为受到抑制，例如玉米的着丝粒核心。新型隐球菌是一种主要的人类致病真菌，其交配型基因座（MAT，>100 kb）异常大，来自两个相反交配型的 MAT 等位基因显示出广泛的核苷酸序列分歧和染色体重排，反映了性染色体的特征。减数分裂重组被认为在新型隐球菌 MAT 基因座内受到抑制。之前的研究确定了新型隐球菌 MAT 侧翼的重组热点，这些热点与高 GC 含量有关。在这里，我们研究了新型隐球菌 MAT 基因座内的一个富含 GC 的基因间区，以确定该区域在减数分裂中是否也表现出独特的重组行为。对天然新型隐球菌分离株的群体遗传学分析显示，跨越不同新型隐球菌谱系的富含 GC 的基因间区存在均匀化信号，这与在减数分裂中该区域的基因转换防止其在每个谱系中多样化的模型一致。通过分析来自实验室杂交的减数分裂后代，我们发现减数分裂重组（基因转换）在富含 GC 的基因间区周围发生的频率与在其他基因组区域观察到的减数分裂重组频率相等或更高。我们讨论了这些发现对新型隐球菌 MAT 基因座内基因转换的可能功能和进化重要性的影响，更普遍地说，对真菌的影响。
