Interspecific pollen transfer and asymmetry in reproductive interference between Sagittaria trifolia L. and Sagittaria pygmaea L.

Closely-related sympatric species are often at risk of interspecific reproductive interference due to similar reproductive biological characteristics. Fruits can be formed in hand-pollination hybridization experiments of Sagittaria trifolia L. and S.pygmaea L., but the arrival of interspecific pollen tubes to the ovule is delayed due to different growth routes. Here, we investigated sympatrically growingS. trifolia and S. pygmaea populations with overlapping flowering periods and observed the interspecific movement patterns of shared pollinators, using pollen tracers to detect the existence and intensity of interspecific pollen transfer. The existence of reproductive interference and its effect on sexual reproductive fitness were measured using seed set in single-/mixed-plots of the two species. Results showed that: (1) The main shared pollinators visited flowers of both species, but the frequency of interspecific movements was significantly lower than that of intraspecific movements, and pollinators showed a preference for male flowers of S. trifolia. (2) Interspecific pollen transfer by shared pollinators was detected in both species under natural conditions, and the proportion of stigma with alien pollen was only about 1.4%. The pollen ofS. trifolia was carried further and in greater quantity to S. pygmaea flowers, while the pollen of S. pygmaea was mainly transferred in short distances. (3) When the two species coexisted in natural habitats, the proportion of undeveloped S. pygmaea seeds increased significantly, while the proportion of developed S. trifolia seeds remained above 80%, indicating asymmetry in interspecific reproductive interference. In this study, we explored the ecological mechanisms that mitigate reproductive interference, providing a new perspective for understanding the coexistence and reproductive strategies of related plant species in the wild.