The distribution of mating-type bias in natural populations of the anther-smutUstilago violaceaonSilene albain Virginia

Abstract

Complete individual-wide mating-type bias (retrieval of sporidia of only one mating type from germinated teliospores of one fungal individual) was observed to be a common and widespread feature of the anther-smut fungus, Ustilago violacea, collected from natural populations of its host, Silene alba. The bias was usually to mating type Al, but the frequency of bias and its spatial distribution varied from region to region. Populations with high frequencies of bias still showed high rates of disease transmission. Crosses between Al mating type sporidial lines from completely biased individuals and A2 mating types from unbiased individuals showed no bias in the progeny. During teliospore germination, biased individuals often showed conjugation among adjacent cells of the promycelium, suggesting that both mating types are present in the germinating teliospore but one mating type is unable to grow as free-living sporidia. The complete bias was most readily interpreted as evidence of "haploid lethals" linked to mating type that cause poor survival or growth of the sporidial stage. The results show that such "haploid lethals" may be a common occurrence in natural populations, and that fungal mating systems may vary considerably over short distances.

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

“…The presence of lethals linked to the mating type locus or with centromeres of other chromosomes was shown when studying natural populations of the fungus Microbotryum sp. [20] .…”

Little Evidence of Antagonistic Selection in the Evolutionary Strata of Fungal Mating-Type Chromosomes (Microbotryum lychnidis-dioicae)

“…Most tellingly, alleles that are lethal in the haploid phase and linked to mating-type loci have been found in up to 50% of M. lychnidis-dioicae strains in natural populations (Kaltz and Shykoff 1997; Oudemans et al 1998; Thomas et al 2003). The maintenance of such alleles can only be explained by a lack of the free-living haploid phase in nature and mating within the tetrad.…”