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 pollen flow. Dispersal of biparentally inherited (nuclear) and paternally inherited (conifer chloroplast) genes can be mediated by both seed and pollen, whereas for maternally inherited (angiosperm chloroplast and 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 population differentiation among them. This paper explores the effects of contrasting patterns of gene flow for different plant genes on their population structure under isolation by distance, on Nei's genetic distance measure, on divergence in nucleotide sequence between populations and on gene phylogenies. The possibilities are discussed of using data on population structure, genetic distance, sequence divergence and gene phylogenies as a basis for estimating the ratio of pollen to seed flow among subpopulations. One important general result from the isolation-by-distance model is that population differentiation for maternally inherited genes is greater than that for paternally inherited genes, which, in turn, is greater than that for biparentally inherited genes as long as the dispersal of seeds and pollen grains takes place. This is consistent with results obtained previously for the island and stepping-stone models in which populations are discretely distributed.