Natural hybridization between species of Calceolaria with different pollination syndromes in southern Patagonia, Argentina

 Calceolaria uniflora and C. polyrhiza differ markedly in flower structure, color, and in their pollination mechanisms. Flowers of C. polyrhiza have a hidden glandular oil-secreting appendage that in the genus typically attracts bee pollinators, while in C. uniflora this is replaced by an external, white and fleshy structure that functions as food body and attracts a bird pollinator. Populations with intermediate flower morphology, presumably hybrids, were found in several sites over a large area of southern Patagonia, Argentina. The intermediate forms have so far been found always in contact or close proximity with typical populations of both presumed parent species. The distribution pattern and phenetic intermediacy in several independent flower characters strongly suggest widespread interspecific hybridization in the range of geographic overlap of the two species. The pattern of variation of intermediate forms in some sites is indicative of a hybrid swarm and possible introgression. Of particular interest is the range of intermediate structures and positions of the flower appendage, between oil gland and food tissue, that can be found in hybrid populations. The observed phenomenon raises several questions concerning the biological mechanisms generating it and its evolutionary consequences. Received May 31, 2001 Accepted September 4, 2001