Genetic and environmental influences on partial self-incompatibility in Lythrum salicaria (Lythraceae).
Premise of research. Angiosperms are commonly classified as self-incompatible or self-compatible. This dichotomy has influenced research on the ecological and demographic consequences of colonization because of the predicted benefits of self-compatibility for establishment at low density. Some individuals of self-incompatible species, however, exhibit partial self-incompatibility (PSI), meaning that they set variable amounts of seed following self-pollination. Here, we investigate genetic and environmental components of PSI in tristylous Lythrum salicaria as a context for understanding colonization and the floral morph structure of invasive populations. Methodology. We surveyed variation in PSI using experimental self- and cross-pollinations on plants (n=338) grown under glasshouse conditions. We compared the stability in expression of PSI over 2 yr (n=80), and by pollinating selected clones (n=12) grown under wet and dry conditions. We compared the compatibility status of mid-styled parents (n=14) and their selfed offspring to determine whether there was a genetic component to PSI and whether the expression of PSI, measured as fruit set after self-pollination, differed between segregating long- and mid-styled plants. Pivotal results. Approximately 34% of plants set seed following self-pollination. The mid-styled morph produced more fruit following self-pollination in comparison with the remaining morphs. An index of self-compatibility (ISC) exhibited significant repeatability for individual plants over two successive flowering seasons. There was no systematic influence of wet and dry growing conditions at flowering on PSI, but significant genetic differences among clones in overall expression of PSI were evident. A heritable component to PSI was confirmed by parent-offspring regression, and compatibility values were significantly higher in mid- versus long-styled progeny. Conclusions. Our study demonstrated a weak but significant genetic component to morph-specific variation in PSI. The capacity of the M-morph to set seed after selfing may enable individuals to found populations and contribute to the occurrence of dimorphic populations missing the short-styled morph in the invasive range of L. salicaria.