Sex Chromosome Evolution with Haploid Sex Determination

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

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.