Developmental selection within the angiosperm style: Using gamete DNA to visualize interspecific pollen competition

Among flowering plants, females often have little control over the genetic relatedness of pollen deposited on stigmas. Thus, postpollination processes are of primary importance for mate discrimination. The ability to screen a nd select among male gametes during pollen tube growth within the female ti ssues of the stigma, style, and ovary is critical to the process of mate ch oice and reproductive isolation. However, direct evidence of the mechanisti c/developmental processes associated with mate choice in flowering plants i s sparse. We studied the contribution of postpollination, prefertilization processes to reproductive isolation in two wind-pollinated species of birch (Betula) that commonly hybridize in nature. We exploited the 3-fold differ ence in ploidy level between these two species to determine the paternity o f individual pollen tubes growing within female reproductive tissues. We th en tracked their developmental fate in conspecific, heterospecific, and mix ed-species crosses. This design allowed a direct comparison of developmenta l processes responsible for reproductive isolation in both single-species a nd mixed-species pollinations. Our results have important implications for the population genetic outcome of hybridization dynamics in natural populat ions. Paternity analysis of progeny from mixed-species pollinations reveale d that conspecific pollen tubes sired more than 98% of seedlings. Biased si ring success was not the result of differential embryo abortion. We detecte d strong, early postpollination barriers such as pollen tube incompatibilit y, slower pollen tube growth, and delayed generative cell mitosis, Conspeci fic fertilization precedence was mediated by favorable or unfavorable male- female interactions, but there was no evidence for antagonistic male-male i nteractions.