Data from: Asymmetric hybridization and gene flow between Joshua trees (Agavaceae: Yucca) reflects differences in pollinator host specificity.

The angiosperms are by far the largest group of terrestrial plants. Their
spectacular diversity is often attributed to specialized pollination.
Obligate pollination mutualisms where both a plant and its pollinator are
dependent upon one another for reproduction are thought to be prone to
rapid diversification through co-evolution and pollinator isolation.
However, few studies have evaluated the degree to which pollinators
actually mediate reproductive isolation in these systems. Here, we examine
evidence for hybridization and gene flow between two subspecies of Joshua
tree (Yucca brevifolia brevifolia and Yucca brevifolia jaegeriana)
pollinated by two sister species of yucca moth. Previous work indicated
that the pollinators differ in host specificity, and DNA sequence data
suggested asymmetric introgression between the tree subspecies. Through
intensive sampling in a zone of sympatry, a large number of
morphologically intermediate trees were identified. These included trees
with floral characters typical of Y. b. jaegeriana, but vegetative
features typical of Y. b. brevifolia. The opposite combination—Y. b.
brevifolia flowers with Y. b. jaegeriana vegetative morphology—never
occurred. Microsatellite genotyping revealed a high frequency of
genetically admixed, hybrid trees. Coalescent-based estimates of migration
indicated significant gene flow between the subspecies and that the
direction of gene flow matches differences in pollinator host fidelity.
The data suggest that pollinator behaviour determines the magnitude and
direction of gene flow between the two subspecies, but that specialized
pollination alone is not sufficient to maintain species boundaries.
Natural selection may be required to maintain phenotypic differences in
the face of ongoing gene flow.