A postmeiotic route to stepwise polyploidy

Polyploidy, the presence of multiple complete chromosome sets, is a ubiquitous biological phenomenon that has played a major role in genome evolution in plants, fungi and animals. Yet the mechanisms by which autopolyploidization arises in natural populations remain incompletely understood. We identify a novel route to autopolyploidization, termed Sporulate Endoreplicate Mate ( SEM ), in which a properly reduced gamete undergoes postmeiotic endoreplication before mating with a sibling intact gamete, yielding a one-chromosome-set ploidy increase per cycle. In Saccharomyces cerevisiae , we experimentally demonstrate the transition from diploidy to triploidy and from triploidy to tetraploidy, positioning triploids as central intermediates in ploidy evolution rather than evolutionary dead-ends. We show that spores from intact asci can spontaneously undergo one or two successive SEM cycles, generating novel triploid and tetraploid strains without genetic manipulation. Natural yeast polyploids exhibit genomic signatures consistent with SEM , including the prevalence of triploidy, extensive aneuploidy, pervasive heterozygosity, and a strong association with heterothallism. Together, our findings establish stepwise polyploidization through iterative SEM cycles as the predominant natural route to polyploidy in yeast, offering a new framework for polyploid genome formation across eukaryotes.