Understanding Mediterranean-Californian disjunctions: molecular evidence from Chenopodiaceae-Betoideae
Chenopodiaceae subfam. Betoideae is distributed in both western Eurasia (four genera) and western North America (one genus). To understand the origin of this disjunction, the phylogeny of the subfamily was reconstructed and dated using ndhF, matK/trnK, trnL-trnF spacer, and ITS sequence variation, penalized likelihood and Langley-Fitch, and calibration with three different fossils. Maximum Parsimony and Maximum Likelihood analyses of the molecular data show that Betoideae are monophyletic, but that relationships of the Himalayan Acroglochin, traditionally included in Betoideae because of the shared possession of a circumscissile capsule, are uncertain. Among the betoidean genera, Beta (excl. sect. Procumbentes) is sister to a clade of Hablitzia, Patellifolia (= Beta sect. Procumbentes), Oreobliton, and Aphanisma. Apart from the strongly supported sister group relationship between the North African Oreobliton and the Californian Aphanisma interrelationships among these four genera are not unambiguously resolved. The crown group age of Betoideae was estimated to 38.4–27.5 my using different DNA sequences, and the age of the Oreobliton/Aphanisma split to 15.4–8.1 my. Considering all evidence available, we conclude that the western Eurasian-western North American disjunction of Oreobliton/Aphanisma is more likely to have resulted from the fragmentation of a Beringian than a North Atlantic ancestral range. Irrespective of the geographical location of this ancestral range we postulate that the evolution into dry habitats of Oreobliton and Aphanisma took place in parallel in western Eurasia and western North America. Evidence for this may be the very different life form and habitat of the two genera, of which Oreobliton is a subshrub of rocky ground at montane altitude, and Aphanisma an annual from coastal habitats. Hablitzia, a perennial vine of deciduous forests in the Caucasus area, is sister to Patellifolia/Oreobliton/Aphanisma in the ndhF and ITS data sets. The habitat requirements of Hablitzia may be similar to those of the ancestor of the subfamily. Comparing the age of the Oreobliton/Aphanisma disjunction with ages estimated for East Asian-eastern North American disjunctions, we conclude that in many cases these two types of disjunction represent different ecological trajectories of essentially the same historical phenomenon.
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