Tropical plant species complexes
Plant species complexes are good systems to study the origin and maintenance of the high species diversity in the tropics because this diversity is due in large part to the presence of many large genera with recent closely related species. The study of speciation requires evidence of genetic and phenotypic differentiation of an original population into independent units. In this context, I have worked with two examples of plant speciation using palm (Arecaceae) species complexes of Western Amazonia as a model system.
A. Sympatric speciation in the palm complex Geonoma macrostachys Mart.
There are very few documented examples of sympatric speciation in plants, or divergence of populations without geographical isolation. To test evidence for sympatric speciation one must demonstrate that populations co-occur in the same area (i.e. they are sympatric), their sister relationships, reproductive isolation, and phenotypic differentiation. Sympatric morphotypes of G. macrostachys were found to be reproductively isolated due to differences in pollinator guild and flowering times (Listabarth 1993, 1999). As part of my dissertation at Florida International University (USA), I analyzed the degree of habitat and genetic differentiation of the sympatric morphotypes of this complex in four Peruvian lowland forests. I first characterized the different morphologies of sympatric populations at each site and then established transects to quantify environmental variables and each morphotype’s densities. In all four sites, one morphotype occurred preferentially in floodplain habitat and the other in terra firme habitat with different soil characteristics. Second, using ISSRs as genetic markers, we found that morphologically and ecologically distinct sympatric forms were genetically closer to each other (sister relationship) than to morphologically similar forms from other sites. A phylogenetic analysis of tribe Geonomateae showed that G. macrostachys complex is monophyletic and that it originated ca. 6 million years ago. Thus, our ecological and genomic data suggested that parallel local diversification of G. macrostachys morphotypes is taking place or that phenotypic plasticity in this complex occurs in response to edaphic conditions.
Roncal, J., J. Francisco-Ortega, and C. E. Lewis. 2007. An evaluation of taxonomic distinctness of two Geonoma macrostachys (Arecaceae) varieties based on inter-simple sequence repeat (ISSR) variation. Botanical Journal of the Linnean Society 153: 381-392. Download pdf.
Roncal, J. 2006.Habitat differentiation of sympatric Geonoma macrostachys (Arecaceae) varieties in Peruvian lowland forests. Journal of Tropical Ecology 22: 483-486. Download pdf.
B. A test of allopatric speciation in the palm complex Astrocaryum murumuru Wallace.
The Andean uplift and subsequent formation of arches and ridges likely caused the vicariant diversification of lineages in Amazonia by isolation of populations into different inter-Andean valleys and Amazonian river basins. As part of my post-doctoral work at the IRD UMR DIADE (France) I studied the allopatric speciation, or divergence of species resulting from geographical isolation in a group of 16 Astrocaryum species. Through a collaboration with the Universidad Nacional Mayor de San Marcos in Peru, we have obtained detailed morphological and distribution data for each species. We conducted a phylogeographic study using 7 nuclear and plastid DNA regions to test this hypothesis, which we rejected in favor of a dispersal scenario followed by diversification in two areas of contrasting geological activity.
Roncal, J., M. Couderc, P. Baby, F. Kahn, B. Millan, Al. Meerow, and J.-C. Pintaud. 2015. Palm diversification in two geologically contrasting regions of western Amazonia. Journal of Biogeography 42: 1503-1513. Download pdf.
C. Evolution of the Coffea millotii (Rubiaceae) complex using transposable elements (TE).
I collaborated with a team of bioinformatitians and molecular biologists (IRD UMR EVODYN in France) to explore the possibility of using active tranposable elements (TE) to unravel the evolutionary history of the Coffea millotii species complex, a group of 6 species endemic to Madagascar. This project was built on the recently completed genome sequencing of the commercially important Coffea canephora which allowed us to identify 4 active TE. We found that two TE from the Gyspy superfamily successfully recovered some species boundaries and geographic structure among species, whereas one TE from the Copia superfamily did not. The peak of transpositional activity for each TE tested occurred before and after the speciation event. Thus, every TE has its own evolutionary history, and must therefore be experimentally tested.
Roncal, J., R. Guyot, P. Hamon, D. Crouzillat, M. Rigoreau, O. N’Guessan-Konan, J.-J. Rakotomalala, M. D. Nowak, A. P. Davis, and A. de Kochko. 2015. Active tranposable elements recover species boundaries and geographic structure in Madagascan coffea species. Molecular Genetics and Genomics Download pdf