Relationship between herkogamy, incompatibility and reciprocity with pollen–ovule ratios in Melochia (Malvaceae)

Abstract

Heterostyly is a floral polymorphism that increases pollination efficiency by promoting cross-pollination and reducing pollen wastage. Efficiency in pollination has been related to plant investment in gamete production and to the pollen to ovule ratio (P/O), which has been proposed as an indication of the likelihood of enough pollen grains reaching the stigmas to result in maximum reproductive success. In heterostylous species, cross-pollination is promoted by the reciprocal position of sexual organs between morphs and a heteromorphic incompatibility system, which precludes selfing and fertilizations among plants of the same morph. Morphological features like reciprocity (between morphs) and herkogamy (within morph) together with the breeding system are thought to influence pollination quality. Therefore, a close relationship between the pollination efficiency, morphological characteristics, and incompatibility would be expected. Pollination treatments and morphological measurements were carried out to describe the breeding system, herkogamy, and reciprocity of six Melochia species. Afterward, the relation between the P/O (as a surrogate of the efficiency in pollination), and reciprocity, herkogamy and incompatibility was evaluated. Monomorphic M. nodiflora and distylous M. pyramidata are self-compatible species, whereas the rest of the species are self- and morph-incompatible. There was a positive relationship between the P/O value and the degree of herkogamy and incompatibility. However, P/O values appear to increase when higher reciprocity is found in the populations. As expected, the lower values of P/O are associated with lower levels of herkogamy and compatibility in the Melochia species studied. The relationship between the factors is discussed under different scenarios of the pollinators’ predictability.

Keywords:

• herkogamy
• heterostyly
• incompatibility system
• pollen limitation
• pollen/ovule ratio
• reciprocity

Acknowledgments

We would like to thank A. Vale, A. Perez-Obregón, T. Homar and E.M. Faife for their assistance in the fieldwork, as well as for all the helpful collaborators from communities around the work sites, especially, M. Centeno-Arias. We are also grateful to D. Rojas who contributed to the improvement of the early versions of the manuscript. Claire Teed reviewed the final version improving the English.