title: Strongly asymmetric hybridization barriers shape the origin of a new polyploid species and its hybrid ancestor.
abstract: 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 of Mimulus peregrinus, an allohexaploid recently derived from the triploid M. ×robertsii, to determine whether reproductive asymmetry has shaped the formation of this new species.
  
  
  
  We used reciprocal crosses between the diploid (M. guttatus) and tetraploid (M. luteus) progenitors to determine the viability of triploid M. ×robertsii hybrids 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 of M. peregrinus to 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 multiple M. peregrinus and M. ×robertsii populations.
  
  
  
  We found strong evidence for asymmetric origins of M. 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 114 M. ×robertsii individuals, and 27 of 27 M. peregrinus, had an M. guttatus maternal haplotype.
  
  
  
  This study, which includes the first Mimulus chloroplast 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.
authors.affiliation: Biological and Environmental Science, School of Natural Sciences, University of Stirling, Stirling, Scotland, FK9 4LA UK.
authors.forename: Mario
authors.initials: M
authors.lastname: Vallejo-Marín
authors.affiliation: Biology Department, Whitman College, Walla Walla, Washington 99362 USA.
authors.forename: Arielle M
authors.initials: AM
authors.lastname: Cooley
authors.affiliation: Biological and Environmental Science, School of Natural Sciences, University of Stirling, Stirling, Scotland, FK9 4LA UK.
authors.forename: Michelle Yuequi
authors.initials: MY
authors.lastname: Lee
authors.affiliation: Department of Biology, College of William and Mary, Williamsburg, Virginia 23185 USA.
authors.forename: Madison
authors.initials: M
authors.lastname: Folmer
authors.affiliation: Donald Danforth Plant Science Center, St. Louis, Missouri 63132 USA.
authors.forename: Michael R
authors.initials: MR
authors.lastname: McKain
authors.affiliation: Department of Biology, College of William and Mary, Williamsburg, Virginia 23185 USA jrpuzey@wm.edu.
authors.forename: Joshua R
authors.initials: JR
authors.lastname: Puzey
mesh_terms: D005075:Biological Evolution; D004171:Diploidy; D054628:Genome, Chloroplast / Q000235:genetics; D054629:Genome, Mitochondrial / Q000235:genetics; D018745:Genome, Plant / Q000235:genetics*; D005838:Genotype; D006824:Hybridization, Genetic / Q000235:genetics*; D032241:Mimulus / Q000235:genetics*; D011003:Ploidies; D012098:Reproduction / Q000235:genetics; D060047:Reproductive Isolation; D057885:Triploidy
keywords: Mimulus peregrinus; allopolyploidy; asexual reproduction; chloroplast genome; genome skimming; hybridization; introduced species; mitochondrial genome; sterile hybrid; triploid block