Figure 1. Global phylogeny of sequenced yeasts. Tree topology, names of genera, and taxonomy are according to Kurtzman, Fell, and Boekshout, [ 2011]. Ancient taxonomy at the time of original publications is in brackets. Branch lengths are arbitrary. Dotted lines indicate uncertainty and/or incongruence between different published phylogenies. Deep branch separations are very ancient (Hedges et al., 2004; Taylor and Berbee, 2006). ^*Saccharomyces pastorianus and Millerozyma sorbitophila are hybrids. References to original publications can be found in Dujon [ 2010]. Figure adapted with modifications from Dujon, 2010.

Figure 2. Sexual cycle with limited genetic exchanges in Saccharomyces sensu stricto yeasts. Figures represent estimated frequencies of events in natural populations, considering that meiosis occurs on average only once for every 1000 mitoses.
(Source: Adapted from Zeyl, 2009..)

Figure 3. Model explaining the LOH and formation of chimeric genomes in diploid yeasts. The model is inspired from the observed presence of mosaics of homozygous and heterozygous regions in the genomes of several yeasts (see text), and from the properties of the break-induced replication mechanisms deduced from experiments in S. cerevisiae (McEachern and Haber, 2006).

Figure 4. Clonal propagation of budding yeasts. Separation between mother cells (thick circles) and buds (thin circles) reflects a basic asymmetry in cell divisions, creating an immortal lineage made of successive buds while mother cells are gradually aging and eventually die. The bud lineages are equivalent to linear mycelium propagation in filamentous Ascomycetes (cartoon). In this case, successive mitoses occur at the apex of the mycelium, leaving behind the equivalent of “mother cells” that do not undergo subsequent mitosis, except in filament branching. In yeasts, contrary to the latter, mitosis of mother cells is not inhibited, converting an essentially linear mode of growth into an exponential mode of growth. In haploid clones, mating-type switching occurs (symbolized by colors) only in the aging lineages. Note that asymmetry in cell division is not specific to yeasts (Horvitz and Herskowitz, 1992). Adapted from Dujon, 2010.

Figure 5. Topological types of segmental duplications. In S. cerevisiae, long segments of chromosomes can duplicate spontaneously during mitotic cycles of either haploid or diploid cells, producing genomes with novel chromosomal structures of unequal stability during subsequent generations. Blue and orange rectangles symbolize, respectively, the copies of the green and yellow segments. Duplications encompassing centromeres (colored ovals) can form supernumerary chromosomes or circular episomes. Segmental duplications result from spontaneous replication accidents depending on a mechanism involving the subunit 32 of the δ polymerase and the presence of either dispersed repeated sequences or only regions of microhomology (Payen et al., 2008). Adapted from Dujon, 2010.

Figure 6. Possible mechanism of interspecies introgression. Left part: Recently observed interspecies introgressions reveal circular permutations of the same donor segment integrated at distinct locations within the recipient genome. (Adapted from Galeote et al., 2011.) Right part: the transfer of newly formed episomes (see Figure 5) from one nucleus to the other in heterokaryotic cells followed by nuclear segregation may explain the origin of introgression between distantly related yeasts after formation of transient hybrids.