The genetic consequences of range expansion and its influence on diploidization in polyploids

Despite newly formed polyploids being subjected to myriad fitness
consequences, the relative prevalence of polyploidy both contemporarily
and in ancestral branches of the tree of life suggests alternative
advantages that outweigh these consequences. One proposed advantage is
that polyploids may more easily colonize novel habitats such as
deglaciated areas. However, previous research conducted in diploids
suggests that range expansion comes with a fitness cost as deleterious
mutations may fix rapidly on the expansion front. Here, we interrogate the
potential consequences of expansion in polyploids by conducting spatially
explicit forward-in-time simulations to investigate how ploidy and
inheritance patterns impact the relative ability of polyploids to expand
their range. We show that under realistic dominance models, autopolyploids
suffer greater fitness reductions than diploids as a result of range
expansion due to the fixation of increased mutational load that is masked
in the range core. Alternatively, the disomic inheritance of
allopolyploids provides a shield to this fixation resulting in minimal
fitness consequences. In light of this advantage provided by disomy, we
investigate how range expansion may influence cytogenetic diploidization
through the reversion to disomy in autotetraploids. We show that under a
wide range of parameters investigated for two models of diploidization,
disomy frequently evolves more rapidly on the expansion front than in the
range core, and that this dynamic inheritance model has additional effects
on fitness. Together our results point to a complex interaction between
dominance, ploidy, inheritance, and recombination on fitness as a
population spreads across a geographic range.