The ecological limits and evolutionary consequences of cross-gradient pollen movement in a vernal pool annual plant

Environmental heterogeneity can influence the direction and magnitude of gene flow in natural plant populations as well as determine the fitness consequences of different patterns of gene flow. This study examines the ecological processes influencing gene flow by pollen transfer across a steep inundation gradient in Lasthenia fremontii, an annual plant endemic to California vernal pools, as well as the fitness consequences of cross-gradient and local pollen movement and seed dispersal. Flowering censuses in eight different vernal pools showed that the flowering phenology of L. fremontii is highly correlated with microelevation, with plants near the edge of pools flowering significantly earlier than those at greater depths. Pollinator observations and a pollen addition experiment revealed that the most abundant pollinators reinforce this phenological gradient by focusing their foraging behavior to narrow microelevation positions within flowering subsets of a population. A field experiment tested if the phenological and pollinator-mediated restrictions on pollen transfer were sufficiently strong to cause genetic differentiation between the center and edge of the population, and examined the consequences of local and center-to-edge pollen movement. Results indicated differentiation between center and edge lineages, with pure edge lineages having lower emergence and a trend towards lower overall fitness than pure center or center-to-edge lineages. The relatively low density of individuals at the edge of the population, coupled with restricted gene flow across the gradient, may have led to the accumulation of deleterious mutations at the population edge, rather than facilitating local adaptation and population expansion.