Self-incompatibility and pollen limitation in the rare tristylous endemic Hugonia serrata on La Réunion Island

Abstract

Surveys of the incompatibility status of island floras have shown a deficit in taxa possessing homomorphic or heteromorphic incompatibility. This observation provides strong evidence for Baker’s Rule (1967) in that self-incompatibility can impede these taxa of colonizing and establishing on islands. The woody liana Hugonia serrata Lam. (Linaceae) is an extremely rare plant species endemic to the Mascarene Islands of La Reunion and Mauritius in the southern Indian Ocean. Previous research has documented the existence of tristyly in this species, thereby raising the number of known tristylous families to six. However, data were based on a very small sampling size and currently no data on the self-incompatibility system of the species are available. In this study, we investigated in detail the breeding system of H. serrata. Calculation of reciprocity and precision indices based on measurements of reproductive organs in a much larger sample size confirmed tristyly in this species. This style–stamen polymorphism was accompanied by several ancillary polymorphisms in stigma size, stigma color, anther size and pollen size. The rigidity of the heteromorphic incompatibility system was tested using experimental hand-pollinations. Significantly fewer pollen tubes arrived at the ovary after self- and illegitimate pollination (7%) than after legitimate pollination (12%). Seed set after self-pollination (3.6%) was also much lower than after legitimate pollination (66.7%). Comparison of seed set after legitimate pollination with natural seed set showed evidence for outcross pollen limitation. Our results indicate that despite severe pollen limitation and limited mate availability the heteromorphic incompatibility system in H. serrata is still strict after colonization of La Reunion Island. Because H. serrata is extremely long-lived, these results suggest that establishment of heterostylous species on islands may have been favored by their long life span.