Figure 3.

Fertility (spore viability) vs. sequence divergence. The biological species definition is based on within-species fertility but between-species sterility after mating. The potential causes of postzygotic reproductive isolation include GCRs, B-D-M incompatibilities, and sequence divergence itself acted upon by the mismatch-repair system during meiotic recombination. Most of the interspecies hybrids exhibit levels of spore viability that are in the range of random segregation of chromosomes leading to an accidental viable combination of <1%. In populations that have diverged yet are not separate species [three populations of S. paradoxus (Sp a, b, and c) and three populations of S. kudriavzevii (Sk a, b, and c)] as well as in the hybrid of the very closely related species S. uvarum and S. eubayanus, there is a linear relationship between sequence divergence and spore viability (Liti et al. 2006; Hittinger 2013). Some close strains by sequence do exhibit spore viability less than expected by the linear relationship; however, when corrected for known translocations, the viability returns to the expected relationship. This is particularly relevant for the S. cariocanus by North American S. paradoxus (1) where the sequence divergence is small and there are four previously described reciprocal translocations (Fischer et al. 2000) and other large GCRs discovered by complete assembly of the genome (D. Delneri, personal communication). Other examples where the correction of GCRs reestablishes the sequence divergence-spore viability relationship include strains of S. paradoxus from the Far East population (2) where one has a translocation, and strains from the European population and Far East populations differing by one translocation (3).