Establishment of polyploidy in natural populations of Mimulus guttatus

Whole-genome duplication (WGD) is a leading force of plant sympatric speciation. However, the evolutionary mechanisms promoting the establishment of neopolyploid mutants in nature remain elusive. We studied polyploid establishment in a unique natural system of Mimulus guttatus (Phrymaceae) in Shetland where a recently (< 100 years ago) locally formed autotetraploid still coexists with its diploid progenitor. We cytotyped 679 adults and 766 seedlings and scored relevant reproductive traits in the field and performed controlled crossings to infer differences in plant performance and get a first insight into the crossing barriers between ploidies. Tetraploids grew in 25% of Shetland populations, mostly in mixtures with diploids, but triploids were absent both among seedlings and adults. Seeds of both cytotypes sampled in areas of immediate sympatry exhibited similar, high germination rates (98% on average). In contrast, low (2%) germination of progeny from controlled interploidy crosses demonstrated strong postzygotic isolation. Yet, plants simultaneously pollinated by diploid and tetraploid pollen donors set viable progeny with ploidy levels identical to the seed parent. The abundant presence of fertile tetraploids in the field despite strong postzygotic isolation demonstrates the ability of novel autopolyploids to cope with both intrinsic and extrinsic challenges associated with WGD and to successfully establish in nature.