title: Selective Advantages of a Parasexual Cycle for the Yeast Candida albicans.
abstract: The yeast Candida albicans can mate. However, in the natural environment mating may generate progeny (fusants) fitter than clonal lineages too rarely to render mating biologically significant: C. albicans has never been observed to mate in its natural environment, the human host, and the population structure of the species is largely clonal. It seems incapable of meiosis, and most isolates are diploid and carry both mating-type-like (MTL) locus alleles, preventing mating. Only chromosome loss or localized loss of heterozygosity can generate mating-competent cells, and recombination of parental alleles is limited. To determine if mating is a biologically significant process, we investigated if mating is under selection. The ratio of nonsynonymous to synonymous mutations in mating genes and the frequency of mutations abolishing mating indicated that mating is under selection. The MTL locus is located on chromosome 5, and when we induced chromosome 5 loss in 10 clinical isolates, most of the resulting MTL-homozygotes could mate with each other, producing fusants. In laboratory culture, a novel environment favoring novel genotypes, some fusants grew faster than their parents, in which loss of heterozygosity had reduced growth rates, and also faster than their MTL-heterozygous ancestors-albeit often only after serial propagation. In a small number of experiments in which co-inoculation of an oral colonization model with MTL-homozygotes yielded small numbers of fusants, their numbers declined over time relative to those of the parents. Overall, our results indicate that mating generates genotypes superior to existing MTL-heterozygotes often enough to be under selection.
authors.affiliation: Institute of Fundamental Sciences, Massey University, Palmerston North 4410, New Zealand.
authors.forename: Ningxin
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Department of Genetics, Cell Biology, and Development, University of Minnesota, Minneapolis, Minnesota 55455.
authors.forename: Beatrice B
authors.initials: BB
authors.lastname: Magee
authors.affiliation: Department of Genetics, Cell Biology, and Development, University of Minnesota, Minneapolis, Minnesota 55455.
authors.forename: Paul T
authors.initials: PT
authors.lastname: Magee
authors.affiliation: School of Physical Sciences, University of Tasmania, Hobart 7001, Australia.
authors.forename: Barbara R
authors.identifier: http://orcid.org/0000-0002-4628-7938
authors.initials: BR
authors.lastname: Holland
authors.affiliation: Department of Oral Sciences, University of Otago, Dunedin 9054, New Zealand.
authors.forename: Ely
authors.initials: E
authors.lastname: Rodrigues
authors.affiliation: Department of Oral Sciences, University of Otago, Dunedin 9054, New Zealand.
authors.forename: Ann R
authors.initials: AR
authors.lastname: Holmes
authors.affiliation: Department of Oral Sciences, University of Otago, Dunedin 9054, New Zealand.
authors.forename: Richard D
authors.identifier: http://orcid.org/0000-0002-5398-2066
authors.initials: RD
authors.lastname: Cannon
authors.affiliation: Institute of Fundamental Sciences, Massey University, Palmerston North 4410, New Zealand j.schmid@massey.ac.nz.
authors.forename: Jan
authors.identifier: http://orcid.org/0000-0001-8794-4175
authors.initials: J
authors.lastname: Schmid
mesh_terms: D000818:Animals; D002176:Candida albicans / Q000235:genetics* / Q000254:growth & development / Q000502:physiology*; D019143:Evolution, Molecular; D049770:Genes, Mating Type, Fungal / Q000235:genetics; D006720:Homozygote; D006801:Humans; D008297:Male; D009154:Mutation; D051381:Rats; D012098:Reproduction / Q000235:genetics; D012641:Selection, Genetic
keywords: Candida albicans; cryptic sexual cycle; mating; parasexual cycle