Outcrossing rates in an experimentally admixed population of self-compatible and self-incompatible Arabidopsis lyrata

AbstractThe transition to self-compatibility from self-incompatibility is
often associated with high rates of self-fertilization, which can restrict
gene flow among populations and cause reproductive isolation of
self-compatible (SC) lineages. Secondary contact between SC and
self-incompatible (SI) lineages might re-establish gene flow if SC
lineages remain capable of outcrossing. By contrast, intrinsic features of
SC plants that reinforce high rates of self-fertilization could maintain
evolutionary divergence between lineages. Arabidopsis lyrata subsp. lyrata
is characterized by multiple origins of self-compatibility and high rates
of self-fertilization in SC-dominated populations. It is unclear whether
these high rates of selfing by SC plants have intrinsic or extrinsic
causes. We estimated outcrossing rates and examined patterns of pollinator
movement for 38 SC and 40 SI maternal parents sampled from an admixed
array of 1509 plants sourced from six SC and six SI populations grown
under uniform density. Although plants from SI populations had higher
outcrossing rates (mean tm = 0.78 ± 0.05 SE) than plants from SC
populations (mean tm = 0.56 ± 0.06 SE), outcrossing rates among SC plants
were substantially higher than previous estimates from natural
populations. Patterns of pollinator movement appeared to contribute to
lower outcrossing rates for SC plants; we estimated that 40% of floral
visits were geitonogamous (between flowers of the same plant). The
relatively high rates of outcrossing for SC plants under standardized
conditions indicate that selfing rates in natural SC populations of A.
lyrata are facultative and driven by extrinsic features of A. lyrata,
including patterns of pollinator movement.