Asymmetric Origins of a Repeatedly Formed Neo-Allopolyploid Species (Mimulus peregrinus)

Hybridization between diploids and tetraploids can lead to new allopolyploid species, often via a triploid intermediate. Viable triploids are often produced asymmetrically, with greater success observed for “maternal-excess” crosses where the mother has a higher ploidy than the father. Here we investigate the evolutionary origins of Mimulus peregrinus, an allopolyploid thought to be derived from the triploid M. x robertsii, to determine whether reproductive asymmetry has shaped the formation of this new species. Using targeted, high-depth sequencing of 1200 genic regions, we confirm the parental origins of this new species from M. x robertsii, a sterile triploid hybrid between the two introduced species M. guttatus and M. luteus that are naturalized and widespread in the United Kingdom. We then used reciprocal crosses between the diploid (M. guttatus) and tetraploid (M. luteus) progenitors to determine the viability of triploid hybrids resulting from paternal- versus maternal-excess crosses. Organellar sequences obtained from pre-existing genomic data, supplemented with additional genotyping was used to establish the maternal ancestry of multiple M. peregrinus and M. x robertsii populations. We find strong evidence for multiple and asymmetric origins of M. peregrinus, but opposite to the common pattern, with paternal-excess cross significantly more successful than maternal-excess crosses.