When two flowering plants share the pollination niche, interspecific pollen transfer caused reproductive interference and make the coexistence difficult. Especially between the related species, heterospecific pollen deposition (HPD) via interspecific pollen transfer greatly decrease seed production by pollen tube growth competition in the style and/or ovule discounting. Then, HPD was thought to strongly promotes competitive exclusion and niche partitioning (e.g. floral trait replacement and spatiotemporal flowering segregation). For the last decade, however, a few studies have found that sympatric distribution of two native close-relatives in the presence of reproductive interference via HPD. Here, we report the field study of HPD by using native and closely related plant species Commelina communis (Cc) and C. communis f. ciliata (Ccfc), which have very large pollination niche overlap (sympatric and simultaneously flowering and use same pollinator) but seldom produce hybrid. We examined: (1) the frequency (relative abundance) dependencies of HPD negative effect on a natural seed set (2) the extent to which autonomous self-pollination mitigates HPD negative effects by combing hand-pollination experiments and field surveys, and (3) how the reproductive interference via HPD influences the spatial distribution of the two species. We then considered the mechanisms promoting the coexistence of species sharing pollination without niche-partitioning.
Results/Conclusions
We revealed the negative effect of HPD between Cc and Ccfc is mutual but asymmetric. The potential opportunity (i.e. interspecific pollinator movement) occurred dependent on relative flower abundance equally, but Cc flower produced more seeds than Ccfc flower when intraspecific flower is relative minority. This advantage of Cc flower is explainable by higher rate of prior autonomously selfing of Cc. In addition, we found that Cc dominates in meso-scale distribution and this fact is also consistent to these results. However, it does not mean Cc competitive exclude Ccfc in the meso-scale completely because competitive superior species can interchange reflected local-scale relative floral frequency. We conclude that prior autonomously self-pollination function as reproductive assurance to HPD and might promote the coexistence. This is the one of few studies to examine the HPD effect in natural condition and our results give the new and important insight for coexistence of plant species without pollination niche partitioning.