Fine-scale genetic structure, gene flow and population genetics in neotropical forest Psychotria (Rubiaceae)

Despite the substantial contribution of understory trees and shrubs to plant diversity in tropical forests, little is known about the spatial scales of gene flow in such plants. If dispersal and gene flow are limited, genetic differentiation within species should occur at small spatial scales, leading perhaps ultimately to speciation. This study investigates spatial genetic structure and inferred gene flow in four Panamanian species of understory shrubs in Psychotria (Rubiaceae). Genetic variation was quantified using the AFLP (amplified fragment length polymorphism) technique of genetic fingerprinting; AFLP profiles were developed for 240 mapped individuals of each species. Dispersal and gene flow in these Psychotria is quite limited (RMS gene dispersal ca. 10--50m) and, as compared to other woody species, spatial genetic structure is remarkably strong (Sp 0.0163--0.0756). For three of the four species, populations on the east side of the Chagres River were more similar to one another (Nei's genetic distance 0.013--0.028; phiSC 0.0049--0.0312), despite the 16 kilometers separating them, as compared to geographically close populations separated by the river (Nei's genetic distance 0.060--0.091; phiCT 0.084--0.133). Among the four Psychotria species, relative fruiting rates and absolute density of fruiting individuals were positively correlated with estimated gene dispersal and genetic neighborhood size, with or without correction for phylogeny (P < 0.05). Fine-scale genetic differentiation is consistent with the hypothesis that seeds of fleshy-fruited trees in tropical understories are dispersed short distances by sedentary forest-interior birds. As evidenced by the study of populations flanking the Chagres River, even relatively small, natural landscape features may impede dispersal in tropical understory taxa. The tight positive relationship between fruiting and gene flow is consistent with models based on pollen dilution and seedling establishment. The data presented in this dissertation are consistent with a model invoking poor gene flow as a mechanism for high speciation in wet tropical understories. Given that Psychotria is a good representative of tropical understory shrubs in terms of floral and fruit characteristics, it is likely that the findings presented here may be used to extrapolate dispersal and gene flow in many understory taxa.