Novel brood-site pollination mutualism between sympetalous Heterosmilax (Smilacaceae, Liliales) and a cecidomyiid gall midge (Cecidomyiidae, Diptera) breeding in fallen male flowers.

Heterosmilax is a unique dioecious genus of Smilacaceae (Liliales, Monocotyledon) in that both male and female flowers are sympetalous, ellipsoid, and almost closed. Our field observations in the Ryukyu Islands of Japan showed that H.japonica flowers are visited and pollinated exclusively by females of one cecidomyiid gall midge species, whose larvae breed in fallen male flowers and feed initially on pollen and later on floral tissue. This is the first example of obligate gall midge-associated brood-site pollination mutualism in which the pollinator brood site is fallen male flowers. The pollinator gall midge is described as Dasineuraheterosmilacicola sp. nov. (tribe Dasineurini, supertribe Lasiopteridi). A molecular phylogenetic analysis reveals that it derived from a flower parasite or flower-bud galler. The sympetalous ellipsoid male flowers are thought to have adapted to allow pollen dusting on the post-abdomen of the pollinator midge, in addition to protecting and incubating internal pollinator larvae in the fallen flowers.