Background
Adaptive radiation in Mediterranean plants is poorly understood. The
white-flowered Cistus lineage consists of 12 species
primarily distributed in Mediterranean habitats and is herein subject to
analysis.
Methodology/Principal Findings
We conducted a “total evidence” analysis combining
nuclear (ncpGS, ITS) and plastid
(trnL-trnF, trnK-matK,
trnS-trnG, rbcL) DNA sequences and using
MP and BI to test the hypothesis of radiation as suggested by previous
phylogenetic results. One of the five well-supported lineages of the
Cistus-Halimium complex, the white-flowered
Cistus lineage, comprises the higher number of species
(12) and is monophyletic. Molecular dating estimates 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 asymmetric characteristics: 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-sepalled flowers) and ecological attributes (low-land vs. low-land to
mountain environments). A positive phenotype-environment correlation has
been detected by historical reconstructions of morphological traits (leaf
shape, leaf labdanum content and leaf pubescence). Ecological evidence
indicates that modifications of leaf shape and size, coupled with
differences in labdanum secretion and pubescence density, appear to be
related to success of new species in different Mediterranean habitats.
Conclusions/Significance
The observation that radiation in the Cistus salviifolius
lineage has been accompanied by the emergence of divergent leaf traits (such
as shape, pubescence and labdanum secretion) in different environments
suggets that radiation in the group has been adaptive. Here we argued that
the diverse ecological conditions of Mediterranean habitats played a key
role in directing the evolution of alternative leaf strategies in this plant
group. Key innovation of morphological characteristics is supported by our
dated phylogeny, in which a Mediterranean climate establishment (2.8 Ma)
predated the adaptive radiation of the white-flowered
Cistus.