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Intratetrad mating, heterozygosity, and the maintenance of deleterious alleles

Summary: Intratetrad mating, heterozygosity,
and the maintenance of deleterious alleles
in Microbotryum violaceum
(=Ustilago violacea)
Department of Biology, University of Virginia, Charlottesville, VA 22903, USA
The mating system of Microbotryum violaceum was investigated in populations that are polymorphic
for mating-type bias, where individuals produce viable haploids of only one of the two required
mating types. The cause of mating-type bias was identi®ed as deleterious recessive alleles linked to
mating type. Maintenance of the deleterious alleles was promoted by early conjugation among
products of single meioses, such that the duration of the free-living haploid stage is minimized. This
development was also observed in nonbiased isolates. As a consequence, the mating system tends
toward mating within the tetrad, which might be expected to reduce heterozygosity. However,
complete centromere linkage of mating type ensures conjugation between ®rst division meiotic
products, such that mating in M. violaceum is analogous to forms of meiotic parthenogenesis with ®rst
division restitution (i.e. automixis with central fusion). This fungus was used to test the prediction that
this mating system would maintain heterozygosity in regions of the genome linked to centromeres.
Therefore, populations were screened for additional heterozygous lethal recessive alleles linked to
centromeres, and several examples were found. Furthermore, the occurrence of intratetrad mating in
M. violaceum provides an explanation for low variation among individuals within populations,


Source: Antonovics, Janis - Department of Biology, University of Virginia


Collections: Biology and Medicine