Near-Absent Levels of Segregational Variation Suggest Limited Opportunities for the Introduction of Genetic Variation Via Homeologous Chromosome Pairing in Synthetic Neoallotetraploid Mimulus

Abstract

Genetic variation is the fundamental medium of evolution. In allopolyploids, which are the product of hybridization and whole genome duplication, if homologous chromosomes always pair, then all descendants of a single diploid F ₁ hybrid lineage will be genetically identical. Contrarily, genetic variation among initially isogenic lineages is augmented when homeologous chromosomes pair; this added variation may contribute to phenotypic evolution. Mimulus sookensis is a naturally occurring, small-flowered allotetraploid derived from the large-flowered Mimulus guttatus and small-flowered Mimulus nasutus. Because diploid F ₁ hybrids between M. guttatus and M. nasutus have large flowers, phenotypic evolution post-polyploidization is implied in M. sookensis. Here, we present genetic and phenotypic analyses of synthetic neoallotetraploid Mimulus derived from a cross between M. guttatus and M. nasutus. Genetic marker data from S ₂ and BC _1N progeny suggest that chromosomes regularly pair with their homologous counterpart. By measuring the phenotype of synthetic neoallotetraploids, we demonstrate that polyploidization per se does not induce the small flowers of M. sookensis. Moreover, phenotypic measurements of synthetic allotetraploid F ₂s and S ₄ families suggest that rare homeologous recombination events have a negligible phenotypic effect in the first few generations. In total, the results are consistent with either exceedingly rare homeologous pairing and recombination or spontaneous fragment loss. The low levels of fragment loss and phenotypic variation in neoallotetraploids suggest that homeologous recombination after polyploidization is not a major mechanism of phenotypic evolution in M. sookensis. Rather, it may be that spontaneous mutations or epigenetic changes after allopolyploidization have driven phenotypic evolution in M. sookensis.