Adaptive Radiation in Mediterranean Cistus (Cistaceae)

Background

Adaptive radiation in Mediterranean plants is poorly understood.The white-flowered Cistus lineage consists of12 species primarily distributed in Mediterranean habitats andis herein subject to analysis.

Methodology/Principal Findings

We conducted a “total evidence” analysiscombining nuclear (ncpGS, ITS) and plastid(trnL-trnF, trnK-matK,trnS-trnG, rbcL) DNAsequences and using MP and BI to test the hypothesis ofradiation as suggested by previous phylogenetic results. One ofthe five well-supported lineages of theCistus-Halimium complex, the white-floweredCistus lineage, comprises the highernumber of species (12) and is monophyletic. Molecular datingestimates a Mid Pleistocene (1.04±0.25 Ma)diversification of the white-flowered lineage into two groups(C. clusii and C.salviifolius lineages), which display asymmetriccharacteristics: number of species (2 vs. 10), leaf morphologies(linear vs. linear to ovate), floral characteristics (small,three-sepalled vs. small to large, three- or five-sepalledflowers) and ecological attributes (low-land vs. low-land tomountain environments). A positive phenotype-environmentcorrelation has been detected by historical reconstructions ofmorphological traits (leaf shape, leaf labdanum content and leafpubescence). Ecological evidence indicates that modifications ofleaf shape and size, coupled with differences in labdanumsecretion and pubescence density, appear to be related tosuccess of new species in different Mediterranean habitats.

Conclusions/Significance

The observation that radiation in the Cistussalviifolius lineage has been accompanied by theemergence of divergent leaf traits (such as shape, pubescenceand labdanum secretion) in different environments suggets thatradiation in the group has been adaptive. Here we argued thatthe diverse ecological conditions of Mediterranean habitatsplayed a key role in directing the evolution of alternative leafstrategies in this plant group. Key innovation of morphologicalcharacteristics is supported by our dated phylogeny, in which aMediterranean climate establishment (2.8 Ma) predated theadaptive radiation of the white-floweredCistus.