WILLIAMS, JOSEPH H., JR.*, WILLIAM E. FRIEDMAN, AND MICHAEL L. ARNOLD. 1, 3: Department of Genetics, University of Georgia, Athens, GA 30602; 2: Department of EPO Biology, University of Colorado, Boulder, CO 80309. - Interspecific pollen competition in a diploid-polyploid hybrid zone in birch (Betula).
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