Habitat Continuity and Geographic Distance Predict Population Genetic Differentiation in Giant Kelp

EcologyEcology, Volume 91, Issue 1, January 2010

Filipe Alberto, Peter T. Raimondi, Daniel C. Reed, Nelson C. Coelho, Raphael Leblois, Allison Whitmer, and Ester A. Serrão

“Isolation by distance (IBD) models are widely used to predict levels of genetic connectivity as a function of Euclidean distance, and although recent studies have used GIS-landscape ecological approaches to improve the predictability of spatial genetic structure, few if any have addressed the effect of habitat continuity on gene flow. Landscape effects on genetic connectivity are even less understood in marine populations, where habitat mapping is particularly challenging. In this study, we model spatial genetic structure of a habitat-structuring species, the giant kelp Macrocystis pyrifera, using highly variable microsatellite markers. GIS mapping was used to characterize habitat continuity and distance between sampling sites along the mainland coast of the Santa Barbara Channel, and their roles as predictors of genetic differentiation were evaluated. Mean dispersal distance (σ) and effective population size (Ne) were estimated by comparing our IBD slope with those from simulations incorporating habitat continuity and spore dispersal characteristics of the study area.

Synthetic map illustrating genetic differentiation of Macrocystis pyrifera  along the mainland coast of the Santa Barbara Channel.

Synthetic map illustrating genetic differentiation of Macrocystis pyrifera along the mainland coast of the Santa Barbara Channel. Colors represent the position of each spatial location along the first axis of a factorial correspondence analysis (AFC) of microsatellite alleles (shown below the map). Points indicate collection sites; sample codes are detailed in Appendix A. The surface canopy of giant kelp, as detected by aerial infrared photography in 2003, is shown to illustrate the level of habitat continuity of giant kelp forests in our study area.

“We found an allelic richness of 7–50 alleles/locus, which to our knowledge is the highest reported for macroalgae. The best regression model relating genetic distance to habitat variables included both geographic distance and habitat continuity, which were respectively, positively and negatively related to genetic distance. Our results provide strong support for a dependence of gene flow on both distance and habitat continuity and elucidate the combination of Ne and σ that explained genetic differentiation.”