Extremely low genetic diversity and extensive genetic admixture at the northern range margins of Bombax ceiba

Understanding the evolutionary processes in species at the margins of their range is of great significance, because marginal populations may harbor local adaptations and will initiate further expansion in response to changes in the environment. Here we examined genetic variation in two nuclear genes and one chloroplast intergenic spacer in 13 northern marginal populations and one geographically central population of Bombax ceiba, a tree distributed mainly in tropical regions. Our results revealed an extremely low level of genetic diversity in each population at the northern margin of its range and strong genetic differentiation between southern China and South Asia. Cultivated and natural populations showed no significant differences in genetic variability. Genetic admixture in a nuclear gene was detected in 10 of the 13 populations at the northern margin of their range. The founder effect, in which a small number of individuals colonize the northern margins of its range, may explain the extremely low genetic diversity. During the establishment of new populations, different source populations may mix and undergo further genetic drift and differentiation. This study indicates that patterns of genetic diversity in tropical species at the margin of their range may also be severely influenced by founder effects.