COS 98-9
Presence of a close relative forces a shift in flowering time in Dudleya brittonii (Crassulaceae)
Can presence of a close relative cause a shift in flowering time within a plant population? We investigated patterns of flowering phenology in two easily-distinguished interfertile forms of a perennial succulent rosette plant in northwestern Baja California, Mexico. The silvery glaucous form is restricted to a bluff-forming volcanic formation along an approximately 13-mile (21-km) stretch of coastline. Within this range, the more widespread green form grows either (1) intermingled with the glaucous form on steep slopes (talus and soil) below outcrops occupied exclusively by the glaucous form or (2) separate from the glaucous form on nearby terraces and slopes. The green form extends both to the north and south of the area of overlap with the glaucous form. We estimated flowering initiation and termination dates from population samples of both forms using a design that eliminates physical factors such as temperature or moisture gradients as causal factors in flowering time differences. We quantified flowering periods of green populations at 16 locations, including locations in the area of overlap (green form growing either intermingled with or separate from the glaucous form) as well as locations to the north and to the south of the area of overlap. We also transplanted young individuals from different locations into a common garden and observed flowering times when they matured.
Results/Conclusions
At locations with intermingled green and glaucous plants, the mean dates of initiation and termination of flowering by green plants were delayed by a minimum of six weeks compared to locations where green plants grew separately from glaucous plants. Where the two forms grow intermingled, the mean initiation of flowering by green plants is generally after the mean termination of flowering by the intermingled glaucous plants, despite some overlap of late flowering glaucous individuals with early flowering green individuals. These location to location differences in flowering time are maintained when young plants transplanted to a common environment mature and flower. Thus it appears that that presence or absence of a close interfertile relative can force a temporal shift in flowering within a plant population, creating pre-zygotic reproductive barriers where the two come into contact and providing an example of reproductive character displacement. The differentiation of flowering times at such a local level is remarkable given the well-known propensity of Dudleya species to hybridize and is likely to be a factor in the relative infrequency of hybrids observed at these locations.