Intratetrad mating and its genetic and evolutionary consequences

Abstract

Genetic characteristics of intratetrad mating, i.e., fusion of haploid products of one meiotic division, are considered. Upon intratetrad mating, the probability of homozygotization is lower than that upon self-fertilization, while heterozygosity at genes linked to the mating-type locus, which determines the possibility of cell fusion, is preserved. If the mating-type locus is linked to the centromere, the genome regions adjoining the centromeres of all chromosomes remain heterozygous. Intratetrad mating is characteristic of a number of fungi ( Saccharomyces cerevisiae, Saccharomycodes ludwigii, Neurospora tetrasperma, Agaricus bisporus, Microbotryum violaceum , and others). Parthenogenetic reproduction in some insects also involves this type of fusion of nuclei. Intratetrad mating leads to the accumulation of haplolethals (i.e., lethals manifesting in haploid cells but not hindering their mating) in pericentric chromosome regions. Since heterozygosity increases viability of an organism, recombination has been suppressed during evolution in fungi characterized by intratetrad mating, which ensures heterozygosity of the most part of the genome. THEORETICAL PAPERS AND REVIEWS

Recent and Massive Expansion of the Mating-Type-Specific Region in the Smut Fungus Microbotryum

“…The most recent stratum, apparently comprising most of the SR region in M. lychnis-dioicae, appears to have formed only a few million years ago, perhaps under selection forlinkage of the mating-type locus with the centromere (Antonovics and Abrams 2004;Zakharov 2005;Giraud et al 2008;Abbate and Hood 2010). Selection for linkage to the centromere was hypothesized to be responsible for evolution of large SR regions on mat chromosomes of N. tetrasperma (Ellison et al 2011).…”

Mating System of the Anther Smut Fungus Microbotryum violaceum : Selfing under Heterothallism

“…4) (41, 53, 54) and should perhaps be considered a special form of selfing because of the important consequences for the maintenance of genetic variation and heterozygosity (56). Indeed, the restriction of a haploid mating pool to the four products of a single meiosis leads to a slightly slower loss of heterozygosity than mating among products of separate meioses from the same diploid genotype (66,111). But more importantly, fungi exhibiting intratetrad mating also tend to show first-division segregation of mating type, for which M. violaceum is an illustrative example (66,110,111).…”

Population genomics of the wild yeast Saccharomyces paradoxus : Quantifying the life cycle

“…Because of the self-incompatibility of MAT, matings only occur between individuals of different mating types, even when otherwise highly inbred, thus making the MAT locus the most outbred region of the genome. With haplo-selfing, the rest of the genome is made completely homozygous, whereas with other forms of inbreeding the increase in homozygosity is partial and depends on distance from MAT (21)(22)(23). That is, different regions of the genome will have different levels of heterozygosity, which in turn should produce different estimates of .…”

“…That is, different regions of the genome will have different levels of heterozygosity, which in turn should produce different estimates of . If a fraction s h of zygotes is formed by haplo-selfing, s i by intratetrad mating, and t ϭ 1 Ϫ s h Ϫ s i by random mating, then the equilibrium heterozygosity at a locus (relative to Hardy-Weinberg proportions) will be 1 Ϫ F ϭ 3t/(3 Ϫ (2 ϩ e Ϫ3x )s i ), where x is the map distance (in Morgans) between the locus and MAT (21,23). Because is a linear function of 1 Ϫ F, we can therefore estimate the frequency of intratetrad mating by comparing near the MAT to for the whole chromosome [supporting information (SI) Appendix].…”