Island and mainland model systems have been used to compare genetic divergence between isolated populations. In general, island species tend to show decreased genetic variation resulting from founder effects and small population size. Along the Southern California coast, the Channel Islands represent ideal sites to compare mainland and island conspecifics. Of special interest is a dominant chaparral species, Ceanothus megacarpus, which is a non-sprouter after wildfire, and thus undergoes complete generational turnover after each fire event. The species also has limited seed dispersal. In 2007, fires occurred on both Santa Catalina Island (Catalina) and the adjacent coastal mainland in the Santa Monica Mountains (SMM) near Malibu, California. Entirely new generations of C. megacarpus germinated in the post-fire environment at both sites. We hypothesized that the genetic diversity of C. megacarpus on Catalina would be lower than in the SMM due to lower fire frequency and the smaller island population size. We used Amplified Fragment Length Polymorphisms (AFLPs) to analyze genomic diversity between two Catalina and one SMM populations. We assessed ninety-seven alleles in 18-20 individuals per population and performed an Analysis of Molecular Variance (AMOVA) using GenAlEx. We also performed a STRUCTURE software analysis.
Results/Conclusions
Our analyses revealed three distinct genetic populations (P <0.001). These segregated as one population comprised entirely of SMM specimens with two additional, distinct populations on Catalina. Interestingly, the Catalina island geographic populations did not correlate with the two genetic populations. That is, genetic populations were intermixed at both island sites. These two genetic populations on Catalina may represent a recent divergence in island subspecies, C. megacarpus megacarpus and C. megacarpus insularis. Among all seedlings sampled, two appeared to be hybrids between Catalina and the SMM. This suggests possible pollen migration between the island and mainland populations. Currently, we are examining morphological differences among individuals to pursue the possibility of subspecies divergence and between population hybridization.
