Heterostyly promotes compatible pollination in buckwheats: comparisons of intraflower, intraplant, and interplant pollen flow in distylous and homostylous Fagopyrum

Premise of the study: Heterostyly, the reciprocal positioning of stigmas and anthers in different floral morphs, has long been thought to promote inter-morph pollination. However, extensive intra-morph pollination occurs commonly in heterostylous species, leading to recurrent questions about the functional and evolutionary significance of heterostyly.

Methods: To identify the sources of stigmatic pollen [autogamous (intra-flower), geitonogamous (intra-plant), vs. inter-plant], we conducted one-flower and whole-plant emasculation treatments in experimental populations of the two closely related buckwheat species, distylous Fagopyrum esculentum and homostylous F. tataricum. Differences in pollen size allowed unambiguous identification of pollen on stigmas.

Results: Only 2.4% of F. tataricum pollen and 1.5% of F. esculentum pollen arrived successfully on compatible stigmas of other plants. In the former (homostylous) species, 71.3% of the pollen load on stigmas was autogamous, 10.8% was geitonogamous, and 17.9% was inter-plant. In the latter (distylous) species, 37.45% of the pollen on stigmas was autogamous, 13.8% was geitonogamous, 17.0% was intra-morph, and 31.75% was inter-morph. The amount of incompatible pollen arriving on stigmas was greatly decreased by both one-flower and whole-plant emasculations, and thus the proportion of compatible pollen deposited increased with one-flower emasculation and increased even more with whole-plant emasculation.

Conclusions: Our quantification of pollen-donor sources in these two species indicated that heterostyly in Fagopyrum esculentum provided a nearly two-fold fitness advantage (in terms of compatible pollination) over expected (random) pollen transfers between morphs. Because of reduced herkogamy, the homostylous F. tataricum was highly autogamous.