Menkis Audrius, Jacobson David J, Gustafsson Tim, Johannesson Hanna
Department of Forest Mycology and Pathology, Swedish University of Agricultural Sciences, Uppsala, Sweden.
PLoS Genet. 2008 Mar 14;4(3):e1000030. doi: 10.1371/journal.pgen.1000030.
We combined gene divergence data, classical genetics, and phylogenetics to study the evolution of the mating-type chromosome in the filamentous ascomycete Neurospora tetrasperma. In this species, a large non-recombining region of the mating-type chromosome is associated with a unique fungal life cycle where self-fertility is enforced by maintenance of a constant state of heterokaryosis. Sequence divergence between alleles of 35 genes from the two single mating-type component strains (i.e. the homokaryotic mat A or mat a-strains), derived from one N. tetrasperma heterokaryon (mat A+mat a), was analyzed. By this approach we were able to identify the boundaries and size of the non-recombining region, and reveal insight into the history of recombination cessation. The non-recombining region covers almost 7 Mbp, over 75% of the chromosome, and we hypothesize that the evolution of the mating-type chromosome in this lineage involved two successive events. The first event was contemporaneous with the split of N. tetrasperma from a common ancestor with its outcrossing relative N. crassa and suppressed recombination over at least 6.6 Mbp, and the second was confined to a smaller region in which recombination ceased more recently. In spite of the early origin of the first "evolutionary stratum", genealogies of five genes from strains belonging to an additional N. tetrasperma lineage indicate independent initiations of suppressed recombination in different phylogenetic lineages. This study highlights the shared features between the sex chromosomes found in the animal and plant kingdoms and the fungal mating-type chromosome, despite fungi having no separate sexes. As is often found in sex chromosomes of plants and animals, recombination suppression of the mating-type chromosome of N. tetrasperma involved more than one evolutionary event, covers the majority of the mating-type chromosome and is flanked by distal regions with obligate crossovers.
我们结合基因分歧数据、经典遗传学和系统发育学，研究丝状子囊菌四孢脉孢菌（Neurospora tetrasperma）中交配型染色体的进化。在这个物种中，交配型染色体的一个大的非重组区域与独特的真菌生命周期相关，在这个周期中，异核体的恒定状态维持着自我育性。分析了来自一个四孢脉孢菌异核体（mat A+mat a）的两个单交配型组成菌株（即同核体mat A或mat a菌株）中35个基因的等位基因之间的序列分歧。通过这种方法，我们能够确定非重组区域的边界和大小，并深入了解重组停止的历史。非重组区域覆盖了近7兆碱基对，超过染色体的75%，我们推测该谱系中交配型染色体的进化涉及两个连续事件。第一个事件与四孢脉孢菌与其异交相对物种粗糙脉孢菌（N. crassa）从共同祖先中分离同时发生，并至少在6.6兆碱基对区域抑制了重组，第二个事件局限于一个较小的区域，该区域的重组在最近才停止。尽管第一个“进化层”起源较早，但来自另一个四孢脉孢菌谱系菌株的五个基因的系谱表明，不同系统发育谱系中抑制重组是独立起始的。这项研究突出了动植物王国中发现的性染色体与真菌交配型染色体之间的共同特征，尽管真菌没有单独的性别。正如在动植物性染色体中经常发现的那样，四孢脉孢菌交配型染色体的重组抑制涉及多个进化事件，覆盖了交配型染色体的大部分，并由有 obligate 交叉的远端区域侧翼包围。 （注：“obligate”此处可能有误，结合语境推测可能是“obligatory”，但按照要求未修改原文翻译）
