Sex Chromosome Evolution with Haploid Sex Determination

在单倍体交配生物体（如真菌）中的性染色体（或交配型）与大多数植物和动物的性染色体之间经常进行类比。 然而，分歧的机制，从而性染色体的内容，可能是非常不同的交配兼容性时，确定在单倍体阶段。 特别是，在“高等”二倍体中，Y染色体的隐蔽变性被认为是可能的，因为只有Y是永久杂合的。 然而，在单倍体交配的生物体中没有这种不对称的遮蔽，但它们也有性染色体大小的二态性。 在真菌中，这是第一次在担子菌Microbotryum violaceum中观察到。 微葡萄孢属性染色体大小的变异可能是由易位或重复元件的增量插入（例如，转座子积累）引起的。 在这个项目中，性染色体特异性文库将被筛选，使用染色体分离的常染色体，用于识别易位的杂交模式。 重复DNA的积累是否是二态性的原因将通过对来自每条性染色体的大量DNA片段样品进行测序来确定。 将测试这些序列与已知基因的同源性以及它们之间的相似性。 微葡萄孢属性染色体上的几个反转录转座子相关序列已经用这种方法鉴定出来。 性染色体变化的方向和速率将通过使用来自与交配型基因座完全连锁的区域的DNA序列数据重建种内同源性来研究。 染色体二态性的性别特异性将通过表征来自欧洲和北美的广泛差异的样品的性染色体来研究。尽管我们对动植物性染色体的理解在理论和实验上都取得了巨大的进步，但在大多数真菌和苔藓等以单倍体交配的生物体中，性染色体的存在一直是一个长期存在的谜。 这项研究，使用真菌Microbotryum，将测试特定的遗传假说有关的组成，起源和进化方向的单倍体性染色体。 这项研究也将有助于了解同一物种成员之间的遗传信息如何在细胞核内组织的变化。 目前，人们正致力于构建完整的基因组序列，了解物种内的遗传变异对于解释这些数据至关重要，特别是当它与特定染色体相关时。 此外，由于染色体的结构变化往往被认为是相容交配的障碍，对促进这种变异的力量的研究将增加我们对生殖隔离新物种起源的理解。

An analogy is often drawn between sex (or mating-type) chromosomes in haploid-mating organisms, such as fungi, and the sex chromosomes in the majority of plants and animals. However, the mechanisms of divergence, and thus the content of sex chromosomes, are likely to be very different when mating compatibility is determined during the haploid stage. In particular, sheltered degeneration of the Y-chromosome in "higher" diploids is thought to be possible because only the Y is permanently heterozygous. However, no such asymmetrical sheltering occurs in haploid-mating organisms, yet they also have sex chromosomes dimorphic for size. In fungi, this was first observed in the basidiomycete Microbotryum violaceum. Variation in the size of sex chromosomes of Microbotryum may be caused by translocations or by incremental insertions of repetitive elements (for example, transposon accumulation). In this project, sex chromosome-specific libraries will be screened, using electrophoretically isolated autosomes, for hybridization patterns that identify translocations. Whether accumulation of repetitive DNA is a cause of the dimorphism will be determined by sequencing a large sample of DNA fragments from each sex chromosome. These sequences will be tested for homology to known genes and for similarity among them. Several retrotransposon-related sequences on the sex chromosomes of Microbotryum have already been identified using this approach. The directions and rates of sex chromosome change will be investigated by reconstructing intra-specific phylogenies using DNA sequence data from regions completely linked to the mating type locus. Sex specificity of the chromosome dimorphism will be investigated by characterizing sex chromosomes from widely divergent samples from Europe and North America. Although there have been enormous theoretical and empirical advances in our understanding of sex chromosomes in plants and animals, their occurrence has been a long-standing enigma in organisms that mate as haploids, such as most fungi and mosses. This research, using the fungus Microbotryum, will test specific genetic hypotheses regarding the composition, origin, and evolutionary directions of haploid sex chromosomes. This research will also contribute to understanding variation among members of the same species for how genetic information is organized within the nucleus. Great effort is currently being devoted to the construction of complete genome sequences, and an understanding of genetic variation within species is essential to interpreting such data, especially when it is associated with particular chromosomes. In addition, because structural changes in chromosomes are often implicated as barriers to compatible mating, the study of forces that promote such variation will increase our understanding of the origins of new species by reproductive isolation.