MAT heterozygosity and the second sterility barrier in the reproductive isolation of Saccharomyces species

Abstract

The genetic analysis of large numbers of Saccharomyces cerevisiae × S. uvarum ("cevarum") and S. kudriavzevii × S. uvarum ("kudvarum") hybrids in our previous studies revealed that these species are isolated by a postzygotic double-sterility barrier. We proposed a model in which the first barrier is due to the abruption of the meiotic process by the failure of the chromosomes of the subgenomes to pair (and recombine) in meiosis and the second barrier is assumed to be the result of the suppression of mating by allospecific MAT heterozygosity. While the former is analogous to the major mechanism of postzygotic reproductive isolation in plants and animals, the latter seems to be Saccharomyces specific. To bolster the assumed involvement of MAT in the second sterility barrier, we produced synthetic alloploid two-species cevarum and kudvarum hybrids with homo- and heterothallic backgrounds as well as three-species S. cerevisiae × S. kudvarum × S. uvarum ("cekudvarum") hybrids by mass-mating and examined their MAT loci using species- and cassette-specific primer pairs. We found that the allospecific MAT heterozygosity repressed MAT switching and mating in the hybrids and in the viable but sterile spores produced by the cevarum hybrids that had increased (allotetraploid) genomes. The loss of heterozygosity by meiotic malsegregation of MAT-carrying chromosomes in the latter hybrids broke down the sterility barrier. The resulting spores nullisomic for the S. uvarum chromosome produced vegetative cells capable of MAT switching and conjugation, opening the way for GARMe (Genome Autoreduction in Meiosis), the process that leads to chimeric genomes.

Keywords: Interspecies hybridisation; MAT; Mating; Sporulation; Sterility barrier; Yeast.

Fig. 1

Karyotypes of parental strains, hybrids and spore clones. a Karyotypes of a two-species kudvarum hybrid, a three-species cekudvarum hybrid and the parental strains. Only the regions of smaller chromosomes are shown in which individual bands can be distinguished in the cekudvarum karyotype. b Karyotypes of two-species cevarum hybrids and spore clones. Only the regions containing the bands of the MAT-carrying chromosomes are shown. The columns on the left sides of the gel photographs show the conventional numbering of chromosomes in the S. cerevisiae, S. uvarum and S. kudriavzevii genomes. The S. uvarum chromosomes are numbered according to Nguyen et al (2000). Sc: S. cerevisiae 10-170; Sk: S. kudriavzevii 10-1653; Su: S. uvarum 10-522; II/6: kudvarum hybrid; II/6.1: cekudvarum hybrid; A27 and A3: cevarum hybrids, A3/1a: sterile spore clone of A3; A3/1b: fertile spore clone of A3. Arrowhead marks the position of the missing Chr III^Su (Chr 2) in the fertile spore clone A3/1b

Fig. 2

Microscopic images of cultures tested for mating. a Cells of the sterile spore clone A3/1a. b Cells of the fertile spore clone A3/1b. Arrowheads point to germinating zygotes

Fig. 3

Selection of binding segments for primers that can be used to amplify species-specific sections of the MAT loci. a Structures of the MATalpha and MATa cassettes of S. cerevisiae distinguished by their Ya or Yalpha regions differing from each other both in length and in nucleotide sequence. W, X, Z1 and Z2 are conserved regions present both in the MATalpha and MATa cassettes (Haber 1998). b Locations of primers used for the amplification of species-specific segments of the MAT alleles

Fig. 4

Verification of the specificity of primers. Amplification of MAT segments of homo- and heterothallic strains of S. cerevisiae (10-170, 10-642), S. kudriavzevii (10-643, 10-1652, 10-1653) and S. uvarum (10-512, 10-522, 10-1650, 10-1651) with primer pairs specific for MATa^Sc, MATalpha^Sc, MATa^Sk, MATalpha^Sk, MATa^Su and MATalpha^Su. Sc: S. cerevisiae; Sk: S. kudriavzevii; Su: S. uvarum

Fig. 5

Examination of the MAT genotypes of hybrids and spore clones with species-specific primers. a HO/ho cevarum hybrids (A2, A3 and A27), the sterile spore clone A3/1a and the fertile spore clone A3/1b. All hybrids and the sterile spore clone have MATa^Sc/MATalpha^Su genotypes. The fertile spore clone lacks MATalpha^Su but has MATalpha^Sc. b The ho/ho kudvarum hybrid II/6 and the ho/ho/ho cekudvarum hybrid II/6.1. The kudvarum hybrid has MATa^Sk/MATalpha^Su genotype whereas the cekudvarum hybrid also has a MATa^Sc cassette

Fig. 6

A general scheme showing the experimental strategy and the detected correlation between MAT genotypes, alloploid sterility and the break-down of the sterility barrier. Hybrids and viable spore clones are grey. ↔ : MAT switching. GARMe genome autoreduction by meiosis (Karanyicz et al. 2017)