Habitat isolation between sister species of neotropical Costus

Background/Question/Methods

Adaptation to different ecological conditions has been recognized as a major mechanism of speciation. We are investigating the role of habitat isolation in a pair of closely related Neotropical herbs, Costus allenii and C. villosissimus, which occupy distinct microhabitats. In central Panama, Costus allenii prefers shady, wet habitats along ravines in forest understory, while C. villosissimus inhabits drier, open sites along forest edges. To investigate the magnitude and mechanisms of local adaptation we are 1) comparing the distribution, precipitation, and soil moisture of natural populations, 2) conducting a reciprocal transplant experiment to determine whether there is evidence of local adaptation, and 3) performing a greenhouse drought tolerance experiment of the two parental species and their F1 hybrids.

Results/Conclusions

We observed strong microgeographic separation between species along a steep precipitation gradient. These distinct spatial distributions are correlated with rainfall differences during the dry season (December-April), with more precipitation in the northern region occupied by C. allenii (Frijolito: 481.97±54.08mm) than in the southern region occupied by C. villosissimus (Gamboa: 335.28±45.60mm). In the dry season of 2007, the average percentage soil moisture of C. allenii plants was about 2-fold higher than that of C. villosissimus. The reciprocal transplant study indicated that C. allenii has higher survivorship in their own habitats (77%) than in C. villosissimus habitats (35%), while C. villosissimus grew faster (relative growth rate) in their own habitats (0.65±0.16) than in the C. allenii habitats (-0.3±0.13). The greenhouse drought experiment indicated that C. allenii had lost all green leaves after 140 days without water, while F1 hybrids and C. villosissimus retained, respectively, 3% and 17% of their green leaves at this time period. This suggests that C. allenii has lower drought tolerance than C. villosissimus, which may result in the higher mortality rate of C. allenii in C. villosissimus habitats. All evidence suggests that drought is related to habitat specialization of C. allenii and C. villosissimus. To determine the importance of this abiotic factor, we will 1) estimate drought tolerance in a population of F2 hybrids grown in the greenhouse, 2) measure selection on this trait in an experimental population of the same F2 genotypes grown in the field, and 3) measure selection caused by other factors which also differ significantly between the two parental habitats, including light availability and herbivores. Taken together, these studies provide evidence that local adaptation and microhabitat specialization contribute to speciation in these species.