Among flowering plants, females often have little control over the genetic relatedness of pollen deposited on stigmas. Yet embryo sacs in many such species are rarely approached by more than one pollen tube, regardless of the original pollen load size. Thus, post-pollination/pre-fertilization events are critical to mate choice and reproductive isolation. We studied early post-pollination barriers in Betula papyrifera (2n = 2x = 28) and B. occidentalis (2n = 6x = 84). These wind-pollinated species form a broad hybrid zone in the northern Great Plains/Rocky Mountains, USA. A 3-fold difference in DNA content of gamete nuclei between these two species allowed us to determine the paternity of individual pollen tubes growing within female reproductive tissues. We tracked their developmental fate in conspecific, heterospecific, and mixed-species crosses. This enabled us to quantify the contribution of both male x male interactions and male x female interactions to reproductive isolation in a natural population. We found no evidence for strong male x male interactions. Instead, strong early post-pollination barriers such as pollen tube incompatibility, slower pollen tube growth, and aberrant cell cycle progression of the generative cell revealed the presence of favorable or unfavorable male x female interactions. In mixed-species pollinations, such barriers resulted in almost complete conspecific siring bias, as determined by an allozyme-based paternity analysis. This result was not due to differential embryo abortion. Strong selection on interspecific male x female interactions will act to restrict variation for mate recognition traits in F1 hybrids, and thus can be an important determinant of later-generation processes such as introgression and hybrid speciation.

Key words: Betula, hybrid zone, mate recognition, pollen competition, polyploidy, reproductive isolation