Hybridization and the reproductive pathways mediating gene flow between nativeMalus coronariaand domestic apple,M. domestica
Gene flow from crops to wild populations is of increasing scientific and public interest, but the potential effects are not well understood when taxa differ in ploidy. We examined the potential for hybridization and gene flow between the introduced diploid domestic apple, Malus domestica Borkh., and the tetraploid crabapple, Malus coronaria (L.) Miller. Using ploidy and isozyme markers, we inferred the incidence of hybrids in natural populations (adults and open-pollinated seed) and in controlled crosses. Geographic range and flowering time overlapped sufficiently for cross-pollination between domestic apple and M. coronaria. Heterospecific crosses produced as many fruit and seeds as conspecific pollinations on M. coronaria mothers. Some seeds from heterospecific pollinations were hybrids, while others were apomictic in origin. In a natural population, all adult trees tested were tetraploid, but open-pollinated seeds varied in ploidy: 57% tetraploid (sexual or apomictic); 7.7% diploid (apomictic); 7.4% hexaploid or octaploid (sexual); and 27.5% triploid or pentaploid hybrids. Despite previous reports showing that the native and introduced gene pools remain distinct, the high proportion of hybrid seeds suggests there is significant potential for gene flow from domestic apples into native M. coronaria populations.