Strongly asymmetric hybridization barriers shape the origin of a new polyploid species and its hybrid ancestor

Abstract

PREMISE OF THE STUDY: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 investigated the evolutionary origins ofMimulus peregrinus, an allohexaploid recently derived from the triploidM. ×robertsii, to determine whether reproductive asymmetry has shaped the formation of this new species.  METHODS:We used reciprocal crosses between the diploid (M. guttatus) and tetraploid (M. luteus) progenitors to determine the viability of triploidM. ×robertsiihybrids resulting from paternal- vs. maternal-excess crosses. To investigate whether experimental results predict patterns seen in the field, we performed parentage analyses comparing natural populations ofM. peregrinusto its diploid, tetraploid, and triploid progenitors. Organellar sequences obtained from pre-existing genomic data, supplemented with additional genotyping was used to establish the maternal ancestry of multipleM. peregrinusandM. ×robertsiipopulations.  KEY RESULTS:We found strong evidence for asymmetric origins ofM. peregrinus, but opposite to the common pattern, with paternal-excess crosses significantly more successful than maternal-excess crosses. These results successfully predicted hybrid formation in nature: 111 of 114M. ×robertsiiindividuals, and 27 of 27M. peregrinus, had anM. guttatusmaternal haplotype.  CONCLUSION:This study, which includes the firstMimuluschloroplast genome assembly, demonstrates the utility of parentage analysis through genome skimming. We highlight the benefits of complementing genomic analyses with experimental approaches to understand asymmetry in allopolyploid speciation.