Reproductive isolation in five hybridizing species of western gooseberries (Ribes: Grossulariaceae)

Five species of western gooseberries (Ribes: Grossulariaceae) hybridize to varying degrees in northwestern California and southwestern Oregon. Pollinator surveys and hand-pollination experiments were conducted to identify reproductive barriers between the species pairs. I found little evidence of isolation due to pollinator behavior, since the five species tended to attract the same pollinators. My attempts to assess the level of post-pollination isolation were hindered by high rates of fruit abortion. Nevertheless, it appears that R. roezlii and R. menziesii are fully interfertile, and that unilateral barriers operate when R. lobbii, which has the longest styles of the set, is the maternal parent in crosses with the other species. Since the level of post-pollination isolation is not correlated with the observed frequency of hybridization in the field, ecological and/or geographical differences may play the most important role in isolating the species.