ON ESTIMATION OF THE RATIO OF POLLEN TO SEED FLOW AMONG PLANT-POPULATIONS

Gene flow occurs in two ways for hermaphrodite plants; seed flow and p ollen flow. Dispersal of biparentally inherited (nuclear) and paternal ly inherited (conifer chloroplast) genes can be mediated by both seed and pollen, whereas for maternally inherited (angiosperm chloroplast a nd most mitochondrial) genes only seed flow contributes to dispersal. This produces asymmetrical migration for biparentally, paternally and maternally inherited genes and may lead to different levels of populat ion differentiation among them. This paper explores the effects of con trasting patterns of gene flow for different plant genes on their popu lation structure under isolation by distance, on Nei's genetic distanc e measure, on divergence in nucleotide sequence between populations an d on gene phylogenies. The possibilities are discussed of using data o n population structure, genetic distance, sequence divergence and gene phylogenies as a basis for estimating the ratio of pollen to seed flo w among subpopulations. One important general result from the isolatio n-by-distance model is that population differentiation for maternally inherited genes is greater than that for paternally inherited genes, w hich, in turn, is greater than that for biparentally inherited genes a s long as the dispersal of seeds and pollen grains takes place. This i s consistent with results obtained previously for the island and stepp ing-stone models in which populations are discretely distributed.