Self- and intra-morph incompatibility and selection analysis of an inconspicuous distylous herb growing on the Tibetan plateau (Primula tibetica)

There is discussion over whether pollen limitation exerts selection on floral traits to increase floral display or selects for traits that promote autonomous self-fertilization. Some studies have indicated that pollen limitation does not mediate selection on traits associated with either pollinator attraction or self-fertilization. Primula tibetica is an inconspicuous cross-fertilized plant that may suffer from pollen limitation. We conducted a selection analysis on P. tibetica to investigate whether pollen limitation results in selection for an increased floral display in case the evolution of autonomous self-fertilization has been difficult for this plant. The self- and intra-morph incompatibility features, the capacity for autonomous self-fertilization, and the magnitude of pollen limitation were examined through hand-pollination experiments. In 2016, we applied selection analysis on the flowering time, corolla width, stalk height, flower tube length, and flower number in P. tibetica by tagging 76 open-pollinated plants and 37 hand-pollinated plants in the field. Our results demonstrated that P. tibetica was strictly self- and intra-morph incompatible. Moreover, the study population underwent severe pollen limitation during the 2016 flowering season. The selection gradients were found to be significantly positive for flowering time, flower number, and corolla width, and marginally significant for the stalk height. Pollinator-mediated selection was found to be significant on the flower number and corolla width, and marginally significant on stalk height. Our results indicate that the increased floral display may be a vital strategy for small distylous species that have faced difficulty in evolving autonomous self-fertilization.