Fig. 4.

Model summarizing SAD2 involvement in floral isolation among O. sphegodes and O. exaltata. SAD2 activity is higher in O. sphegodes (blue arrows) than in O. exaltata (red arrows), due to expression (and possibly functional) differences. SAD2 reaction products are elongated and converted to 9- and 12-alkenes, the levels of which are higher in O. sphegodes than in O. exaltata. The exact source of high levels of 7-alkenes in O. exaltata is unknown. Floral alkenes are detected by pollinators, with 9- and 12-alkenes functioning as attractants to the bee Andrena nigroaenea (the pollinator of O. sphegodes). Conversely, the bee Colletes cunicularius (the pollinator of O. exaltata) is attracted by 7-alkenes, whereas 9-alkenes reduce this attraction. Overall, different alkene blends in the two species lead to differential pollinator attraction associated with reproductive isolation.