Evolutionary Strata in a Small Mating-Type-Specific Region of the Smut Fungus Microbotryum violaceum

Abstract

DNA sequence analysis and genetic mapping of loci from mating-type-specific chromosomes of the smut fungus Microbotryum violaceum demonstrated that the nonrecombining mating-type-specific region in this species comprises 25% (1 Mb) of the chromosome length. Divergence between homologous mating-type-linked genes in this region varies between 0 and 8.6%, resembling the evolutionary strata of vertebrate and plant sex chromosomes.

No amicable divorce? Challenging the notion that sexual antagonism drives sex chromosome evolution

“…However, in many cases, each haploid individual has a mating type, genetically determined by the mating type locus, and individuals must be of opposite mating types to mate successfully. In fungi such as Neurospora tetrasperma, Microbotryum violaceum and Cryptococcus neoformans, each haploid genome contains a single copy of the mating type locus and meiotic recombination within the mating type locus is suppressed [110][111][112]. In these fungal mating type chromosomes, recombination is suppressed over a region extending far beyond the mating type genes.…”

“…In these fungal mating type chromosomes, recombination is suppressed over a region extending far beyond the mating type genes. Although they do not show the levels of gross heteromorphy observed in some animal and plant sex chromosomes, the occurrence of strata [111,112] indicates that expansion of the non-recombining region in these fungal chromosomes has occurred in several stages. In the absence of distinct sexes, expansion of the nonrecombining regions of fungal mating type chromosomes cannot be attributed to sexual antagonism as such, although, in theory, antagonistic processes may occur due to differences in ....Pr ospects & Overviews J. E. Ironside fitness optima between fungal mating types.…”

DNA polymorphism in recombining and non-recombing mating-type-specific loci of the smut fungus Microbotryum

“…Similar processes are expected to occur in other nonrecombining regions, such as plant self-incompatibility loci and fungal mating-type loci. Indeed, it has been suggested that mating-type-determining regions of fungi, such as Cryptococcus neoformans (Lengeler et al, 2002), Neurospora tetrasperma (Menkis et al, 2008) and Microbotryum violaceum (Hood, 2002;Votintseva and Filatov, 2009), represent fungal 'sex chromosomes'. However, at present there is no solid evidence for genetic degeneration in fungal mating-type-determining regions.…”

“…The fungus has a bipolar heterothallic breeding system, and conjugation occurs only between sporidia of opposite mating types. The two mating types, A1 and A2, are determined by a mating-type-specific region, which is similar to the Y chromosomes of animals and plants, as recombination is suppressed in a large region around the mating-type-determining genes (Hood, 2002;Votintseva and Filatov, 2009).…”

“…However, a lower gene density may be due to the addition of junk DNA to the mating-type-specific chromosomes rather than the loss of functional genes. Recent segregation analysis of the mating-type specificity of 86 fragments isolated from the A1 and A2 chromosomes revealed that only about a quarter are located in the non-recombining mating-type-specific regions, whereas the rest mapped to recombining regions and are not expected to undergo genetic degeneration because of a lack of recombination (Votintseva and Filatov, 2009). Based on this analysis, the size of the nonrecombining regions was estimated to be about 1 Mb long on each of the mating-type-specific chromosomes; however, this estimate is very approximate.…”