Sympatric Sister Species of Californian Antirrhinum and Their Transiently Specialized Pollinators

We investigated pollinator response to a pair of sympatric sister taxa, Antirrhinum subcordatum and A. vexillo-calycalatum (Plantaginaceae). These two taxa, native to northern California, have overlapping geographic distributions, show specialization to different soil types and are morphologically similar except that they differ in flower color (white and purple respectively). We observed and identified floral visitors and performed array experiments. Our results show that similar suites of pollinators visit both plant taxa, which implies no fixed preference on the part of the pollinator species. However, when individual insects visit mixed arrays, they tend to restrict their visits to either the white or purple flowered taxon, likely resulting in assortative pollen transfer even in areas of fine-scale sympatry. The assortative visitation observed is noteworthy given that A. subcordatum and A. vexillo-calyculatum are best interpreted as incipient species. Thus, assortative mating caused by the specialization of individual pollinators towards a single flower color may be crucial for allowing the continued divergence of these lineages. These data contribute to our understanding of the relationship between angiosperm species and their animal pollinators and how these interactions may contribute to the diversification of flowering plants.