Differences in mating system and predicted parental conflict affect post-pollination reproductive isolation in a flowering plant

Mating system shifts from outcrossing to selfing are frequent in plant
evolution. Relative to outcrossing, selfing is associated with reduced
parental conflict over seed provisioning, which may result in postzygotic,
asymmetric, reproductive isolation in crosses between populations of
different mating systems. To test the hypothesis that post-pollination
reproductive isolation between populations increases with increasing
differences in mating system and predicted parental conflict, we performed
a crossing experiment involving all combinations of three self-compatible
populations (with low outcrossing rates), and three self-incompatible
populations (with high outcrossing rates) of the arctic-alpine herb Arabis
alpina, assessing fitness-related seed and plant traits of the progeny.
Predicted levels of parental conflict (“genome strength”) were quantified
based on strength of self-incompatibility and estimates of outcrossing
rates. Crosses between self-compatible and self-incompatible populations
yielded very small seeds of low viability, resulting in strong
reproductive isolation. In 14 of 15 reciprocal between-population crosses,
seeds were heavier when the paternal plant had the stronger genome, and
seed mass differences between cross directions increased with an increased
difference in parental conflict. Overall, our results suggest that, when
sufficiently large, differences in mating system and hence in expected
parental conflict may result in strong post-pollination reproductive
barriers contributing to speciation.