Simulation code for: Clones on the run - the genomics of a recently expanded partially clonal species

Why species that in their core areas mainly reproduce sexually become
enriched with clones in marginal populations ("geographic
parthenogenesis") remains unclear. Earlier hypotheses have emphasised
that selection might promote clonality because it protects locally adapted
genotypes. On the other hand, it also hampers recombination and adaptation
to changing conditions. The aim of the present study was to investigate
the early stages of range expansion in a partially clonal species and what
drives an increase in cloning during such expansion. We used genome-wide
sequencing to investigate the origin and evolution of large clones formed
in a macroalgal species (Fucus vesiculosus) during a recent expansion into
the postglacial Baltic Sea. We found low but persistent clonality in core
populations, while at range margins, large dominant clonal lineages had
evolved repeatedly from different sexual populations. A range expansion
model showed that even when asexual recruitment is less favourable than
sexual recruitment in core populations, repeated bottlenecks at the
expansion front can establish a genetically eroded clonal wave that
spreads ahead of a sexual wave into the new area. Genetic variation
decreased by drift following repeated bottlenecks at the expansion front.
This resulted in the emerging clones having low expected heterozygosity,
which corroborated our empirical observations. We conclude that
Baker's Law (clones being favoured by uniparental reproductive
assurance in new areas) can play an important role during range expansions
in partially clonal species, resulting in a complex spatiotemporal mosaic
of clonal and sexual lineages that might persist during thousands of
generations.