Cacho, N. Ivalu , Baum, David A. .
Testing a ring-species hypothesis in a Caribbean Euphorbia species-complex.
The Pedilanthus clade is remarkable within Euphorbia for its zygomorphic cyathium bearing a spur in which the nectar-glands are concealed. The species Euphorbia tithymaloides is notable in the group for: a) its unusually broad distribution which spans from Florida to northern South America, with populations throughout Mexico, Central America, and most islands in the Caribbean, and b) its degree of subspecific morphological differentiation, with eight subspecies being recognized.
We analyze two nuclear (one locus of G3PDH and a potential homolog of SGN-Unigene 237757) and one plastid (mat-K) markers at a population level scale (up to 12 individuals for each of 67 populations spanning the range of Euphorbia tithymaloides) to explore the colonization of the Caribbean basin by the E. tithymaloides species-complex. Alleles present in a population were isolated through a combination of SSCP (single-stranded conformational polymorphism), PCR, and cloning.
We test the following biogeographic scenarios: (1) the Caribbean subspecies are the result of multiple colonization events; (2) there was a single colonization front, either from Mexico, Central America, or South America, and; (3) there were two colonization fronts that converge in the Greater Antilles, as suggested by Robert Dressler (1957): “while [subspecies] angustifolia and parasitica are broadly sympatric and appear to behave as distinct species […], each is closely tied in to the mainland population by a different chain of taxa.”. This latter scenario is of evolutionary interest because it raises the question of whether a ring-species could have been formed (or could be in the process of forming). We find evidence of geographic structure along the range of E. tithymaloides in support of the one-front or two-front colonization hypotheses; evidence of genetic cohesion among subspecies, and; evidence of gene flow between subspecies at least in one case, where populations of ssp. parasitica and ssp. angustifolia grow in close proximity.
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1 - University of Wisconsin-Madison, Department of Botany, Birge Hall, 430 Lincoln Drive, Madison, WI, 53706, USA
2 - University of Wisconsin Madison, Department of Botany, Birge Hall, 430 Lincoln Drive, Madison, Wisconsin, 53706-1381, USA
Presentation Type: Oral Paper:Papers for Topics
Location: Cottonwood B/Snowbird Center
Date: Monday, July 27th, 2009
Time: 4:15 PM