The Floral Biology and Pollination Ecology of Two Desert Lupines

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Abstract
Lupinus arizonicus (Wats.) Wats. and L. sparsiflorus Benth., 2 closely related desert annuals, are taxonomically distinguished by trichome sculpturing, floral coloration and leaflet width-length ratios. Insect vectors [Xenoglossodes sp., Anthophora sp., Apis mellifera, Bombus sp., Centris rodopus, Andrena sp., Anthidium sp., Ashmeadiella sp., Megachile fulcata and Osmia sp.] are attracted by and display a distinct behavior response towards the yellow white banner spot present on both species prior to pollination. Vector behavior changes with a distinct post-pollination spot color change. Insect vectors actively avoid all flowers with the new deep purplish red banner spot. These lupines appear to have a high potential for interspecific hybridization as indicated by their occasional sympatric occurrence; their overlapping flowering periods; their identical protandrous floral development; the occurrence of several pollinators readily visiting both lupine species on the same foraging trips; their high compatibility at the pollen-stigma level; and the observed ability of the pollen tube to grow normally on the foreign pistil. The absence, nevertheless, of hybrid formation in the field and in the greenhouse, is the result of a definite reproductive isolating mechanism, and is here attributed to an incompatibility at the gametophyte level.