TRIMORPHIC INCOMPATIBILITY IN MEXICAN POPULATIONS OF PONTEDERIA SAGITTATA PRESL. (PONTEDERIACEAE)

SUMMARY Populations of Pontederia sagittata from the northern coastal plain of Mexico possess a tristylous breeding system. Styles and stamens are reciprocally arranged in the three floral morphs. Differences in anther size, pollen size and pollen production are associated with the three anther levels. Controlled pollinations performed in the glasshouse indicate that physiological self‐incompatibility accompanies floral trimorphism. Pollinations between morphs with anthers and stigmas at equivalent heights are highly productive of seed in comparison with self‐ or intra‐morph pollinations. In each floral morph, substantial differences in seed production result from self‐pollination with alternate anther levels. Overall, the mid‐styled morph exhibits weak self‐incompatibility whereas the long‐ and short‐styled morphs are more strongly self‐incompatible. In all pollen‐stigma combinations pollen germinated readily and pollen tubes penetrated stigmatic tissue. Inhibition of pollen tubes occurs in the style and possibly the ovary. The rate of pollen tube growth differs between anther levels following self‐pollination. The three floral morphs were represented in eight populations examined although there was a tendency for the long‐styled morph to be under‐represented. Strong pollen trimorphism allows the origin of pollen deposited on naturally pollinated stigmas to be determined. Considerable variation in the amount and composition of pollen deposited on stigmas by syrphid flies was recorded. The likelihood of legitimate pollination appears to be highest in the long‐styled morph, intermediate in the mid‐styled morph and lowest in the short‐styled morph. Tristyly in P. sagittata is compared with the closely related P. cor data and P. rotundifolia. Implications of weak self‐incompatibility in the mid‐styled morph for the breakdown of tristylous breeding systems in the Pontederiaceae are discussed.