Intratetrad mating as the driving force behind the formation of sex chromosomes in fungi

Abstract

In some fungi-ascomycetes and basidiomycetes, the 4 haploid spores formed as a result of meiosis can fuse in pairs, forming a dikaryon or diploid. The consequence of such intratetrad mating is the preservation of heterozygosity of genes linked to the mating-type (MAT) locus. If the MAT is linked to the centromere physically or genetically (as a result of the suppression of recombination), the centromere regions of all chromosomes are preserved in a heterozygous state. Suppression of recombination in the MAT chromosome contributes to the accumulation of lethal mutations and chromosomal rearrangements in it. Two MAT chromosomes cease to be homologous and become analogues of the sex chromosomes of animals and plants.

Repeated loss of function at HD mating-type genes and of recombination suppression without mating-type locus linkage in anther-smut fungi

“…Microbotryum fungi are model organisms in as diverse fields as ecology, disease transmission, host specialization and reproductive isolation evolution, host-pathogen costructure, mating system and sex-related chromosome evolution 52 ,54 ,55 ,56 ,57 ,58 ,59 ,60,61 ,62 ,63 . Microbotryum fungi mostly undergo selfing via intra-tetrad mating 59,64,65 , in which case carrying a single mating-type locus is advantageous because it maximizes the odds of gamete compatibility 18 . Microbotryum fungi have in fact repeatedly evolved HD-PR linkage, with at least nine events of chromosomal rearrangements and recombination suppression between the two mating-type loci 18,19,40,66 .…”