Was southern French Guiana always forested, a refugium for forest species, or was it dominated by more open vegetation during drier, glacial times? A recent paper in Molecular Ecology tries to decide between these competing hypothesis, and the interesting thing for us at the Agricultural Biodiversity Weblog is that it does so through a genetic study of a crop wild relative. 1
Manihot esculenta ssp. flabellifolia is the closest wild relative of cassava. It is “distributed on an arc partly encircling the Amazon basin, from eastern Bolivia and Peru eastward to northeastern Brazil, northward to the Guianas and then westward to Venezuela.” Its habitat is the transition zone between forest and cerrado in the south, and open environments such as savannas and rocky outcrops in the north. In French Guiana, which was the focus of the study, it is found both in the coastal strip, and on isolated granitic outcrops (inselbergs) in the forested south, with a large gap in between.
Seven microsatellite loci were used to investigate the genetic relationships among 14 populations, 4 from inselbergs and the rest from the coast. The results are pretty easily summarized. First, the inselberg populations were very similar to each other. Second, they were quite different as a group from the coastal populations. Finally, the coastal populations were highly differentiated among themselves.
So, what do these results tell us about the past vegetation history of the region? One conclusion was that the coastal populations (which incidentally, in contrast to the inselberg populations, showed some evidence of introgression from the crop) are relatively recent, and arrived from savannas to the west through a series of bottlenecks, rather than from the south. As for the southern inselberg populations, given the limited range of pollen and seed flow, they seem to be the remnants of a formerly more extensive, fairly homogeneous population. 2 That suggests that southern French Guiana was drier and had a more open vegetation before the Late Glacial Maximum 10,000 years ago. There was probably a forest refugium in the central part of the country, but not in the south.
Assuming, of course, that the adaptation of the species hasn’t changed much along the way. It remains to be seen whether the same pattern will be found in other taxa. Perhaps other species of agrobiodiversity interest will be investigated in the same way.