Impact of asymmetric male and female gamete dispersal on allelic diversity and spatial genetic structure in valley oak (Quercus lobata Née)

Abstract

The distribution and abundance of genetic diversity in plant populations is initiated by sexually asymmetric propagule dispersal through pollen and seeds. Because these processes occur serially, it is not transparent how each contributes to subsequent patterns of genetic diversity. Using combined seedling/seed coat assay for naturally distributed seedlings of Quercus lobata Nee, we extracted male and female gametic genotypes, and then assessed (wind-vectored) paternal and (gravity- and animal-vectored) maternal contributions to spatially distributed allelic diversity. We evaluated 200 naturally recruited seedlings from 4 open patches away from any adult canopies (denoted ‘open’), and 174 seedlings from 14 patches immediately beneath adult canopies (denoted ‘canopy’). The open patches included 19 % long distant dispersal events of >1 km while the canopy patches contained seedlings from one tree overhead. For each patch type, we partitioned average allelic diversity for six microsatellite loci for the whole study site (γ) into separate within-patch (α) and among-patch (β) components, translated into among-patch divergence (δ). We found that α-diversity resulting from seed dispersal was much less than that from pollen dispersal in both patch types, while total γ-diversity across the site was similar. Divergence (δ) among canopy patches was significantly greater than δ among open patches. We then evaluated spatial genetic autocorrelation (kinship) patterns for both open and canopy strata, separately for male and female gametes. Female gametes showed sharply declining kinship with increasing distance for canopy patches and modestly for open patches. In sharp contrast, male gametes from both patches showed only subtle decline of kinship, but seedlings still showed significant structure across patch types. On balance, sexual asymmetry in propagule dispersal shapes both the abundance and distribution of allelic diversity, with pollen dispersal promoting overall diversity but reducing spatial structure, but seed-dispersal reduces overall diversity and markedly increases spatial genetic structure.