MAT heterozygosity and the second sterility barrier in the reproductive isolation of Saccharomyces species
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ORIGINAL ARTICLE
MAT heterozygosity and the second sterility barrier in the reproductive isolation of Saccharomyces species Matthias Sipiczki1 · Zsuzsa Antunovics1 · Adrienne Szabo1 Received: 23 March 2020 / Revised: 16 April 2020 / Accepted: 24 April 2020 © The Author(s) 2020
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 doublesterility 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 · Sterility barrier · Yeast · MAT · Mating · Sporulation
Introduction Nature employs various ‘barriers’ to keep closely related species distinct from one another. One of these barriers is reproductive isolation that maintains the integrity of species over time by preventing genetic admixture (Coyne and Orr 2004). The reproductive isolation mechanisms can be prezygotic or postzygotic depending on whether they operate before or after fertilisation (Seehausen et al. 2014). The former prevents the formation of viable hybrid zygotes; while, the latter results in hybrids in which the incompatibility of the parental genomes causes aberrant development, low fitness, and/or sterility (Ouyang and Zhang Communicated by M. Kupiec. * Matthias Sipiczki [email protected] 1
Department of Genetics and Applied Microbiology, University of Debrecen, Debrecen, Hungary
2013). Hybrid sterility is one of the earliest reproductive isolation mechanisms to evolve between two recently diverged species (Coyne and Orr 2004). The hybrids of such species are viable but incapable of sexual r
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