Having sex, yes, but with whom? Inferences from fungi on the evolution of anisogamy and mating types

Abstract

The advantage of sex has been among the most debated issues in biology. Surprisingly, the question of why sexual reproduction generally requires the combination of distinct gamete classes, such as small and large gametes, or gametes with different mating types, has been much less investigated. Why do systems with alternative gamete classes (i.e. systems with either anisogamy or mating types or both) appear even though they restrict the probability of finding a compatible mating partner? Why does the number of gamete classes vary from zero to thousands, with most often only two classes? We review here the hypotheses proposed to explain the origin, maintenance, number, and loss of gamete classes. We argue that fungi represent highly suitable models to help resolve issues related to the evolution of distinct gamete classes, because the number of mating types vary from zero to thousands across taxa, anisogamy is present or not, and because there are frequent transitions between these conditions. We review the nature and number of gamete classes in fungi, and we attempt to draw inferences from these data on the evolutionary forces responsible for their appearance, loss or maintenance, and number.

Extremely Reduced Levels of Heterozygosity in the Vertebrate Pathogen Encephalitozoon cuniculi

“…One possibility is that the extremely low heterozygosity found in E. cuniculi is the result of self-reproduction (selfing), a mechanism that is known to heavily reduce the amount of heterozygosity in many other vertebrate parasites (22,34,35,40) and to produce clones that are optimally adapted to certain hosts (i.e., highly homozygous individuals) but that can still maintain the capacity to undergo outcrossing and create new variation following rare changes in environmental conditions (i.e., opportunistic sexuals). However, given the source of the E. cuniculi strains we have analyzed, the extreme reduction in heterozygosity we have observed may have been exacerbated by the methodology used to originate and propagate these strains under laboratory conditions.…”

Recent advances in population genetics of ectomycorrhizal mushroomsRussulaspp.

“…Following vegetative growth and if the environmental conditions are favorable, the dikaryotic mycelia can produce fruit bodies. In this process, a crucial factor is the mating compatibility between monokaryons (Moore and Frazer 2002; Billiard et al 2011). For monokaryotic cells, even if they belong to the same species and are located in the same community close to each other, they can only mate and form fertile mycelia when they have different alleles at the mating type locus (loci) (Casselton and Olesnicky 1998; Moore and Frazer 2002; Billiard et al 2011).…”

Evolution of Mating Systems in Basidiomycetes and the Genetic Architecture Underlying Mating-Type Determination in the YeastLeucosporidium scottii

“…It is thus not surprising that recent advances in genome sequencing have revealed that most fungal species have retained the machinery for sexual reproduction and meiosis (Halary et al 2011;Dyer and O'Gorman 2012;Gioti et al 2013;Heitman et al 2014). However, this resilience to keep sexual competence intact comes with an extremely dynamic evolution of sexual behaviors and mating type-determining mechanisms, which may influence important evolutionary and ecological processes, such as adaptation and speciation (Billiard et al 2011;Heitman et al 2013;Nieuwenhuis et al 2013).In the phylum Basidiomycota, sexual reproduction is often dictated by two independent sets of mating-type (MAT)-specific genes that control different stages of the sexual cycle. These genes encode premating lipopeptide pheromones and their cognate receptors (P/R), which mediate recognition of mating partners and cell fusion, and homeodomain transcription factors (HD1 and HD2) that form heterodimers that regulate postmating behavior (Kües et al 2011;Nieuwenhuis et al 2013).…”

“…It is thus not surprising that recent advances in genome sequencing have revealed that most fungal species have retained the machinery for sexual reproduction and meiosis (Halary et al 2011;Dyer and O'Gorman 2012;Gioti et al 2013;Heitman et al 2014). However, this resilience to keep sexual competence intact comes with an extremely dynamic evolution of sexual behaviors and mating type-determining mechanisms, which may influence important evolutionary and ecological processes, such as adaptation and speciation (Billiard et al 2011;Heitman et al 2013;Nieuwenhuis et al 2013).…”