Aridification as a driver of biodiversity: a case study for the cycad genus Dioon (Zamiaceae)

Abstract

<div class="section"> <a class="named-anchor" id="s1"> <!-- named anchor --> </a> <h5 class="section-title" id="d1020782e208">Background and Aims</h5> <p id="d1020782e210">Aridification is considered a selective pressure that might have influenced plant diversification. It is suggested that plants adapted to aridity diversified during the Miocene, an epoch of global aridification (≈15 million years ago). However, evidence supporting diversification being a direct response to aridity is scarce, and multidisciplinary evidence, besides just phylogenetic estimations, is necessary to support the idea that aridification has driven diversification. The cycad genus <i>Dioon</i> (Zamiaceae), a tropical group including species occurring from humid forests to arid zones, was investigated as a promising study system to understand the associations among habitat shifts, diversification times, the evolution of leaf epidermal adaptations, and aridification of Mexico. </p> </div><div class="section"> <a class="named-anchor" id="s2"> <!-- named anchor --> </a> <h5 class="section-title" id="d1020782e216">Methods</h5> <p id="d1020782e218">A phylogenetic tree was constructed from seven chloroplast DNA sequences and the ITS2 spacer to reveal the relationships among 14 <i>Dioon</i> species from habitats ranging from humid forests to deserts. Divergence times were estimated and the habitat shifts throughout <i>Dioon</i> phylogeny were detected. The epidermal anatomy among <i>Dioon</i> species was compared and correlation tests were performed to associate the epidermal variations with habitat parameters. </p> </div><div class="section"> <a class="named-anchor" id="s3"> <!-- named anchor --> </a> <h5 class="section-title" id="d1020782e230">Key Results</h5> <p id="d1020782e232">Events of habitat shifts towards arid zones happened exclusively in one of the two main clades of <i>Dioon</i>. Such habitat shifts happened during the species diversification of <i>Dioon</i>, mainly during the Miocene. Comparative anatomy showed epidermal differences between species from arid and mesic habitats. The variation of epidermal structures was found to be correlated with habitat parameters. Also, most of the analysed epidermal traits showed significant phylogenetic signals. </p> </div><div class="section"> <a class="named-anchor" id="s4"> <!-- named anchor --> </a> <h5 class="section-title" id="d1020782e241">Conclusions</h5> <p id="d1020782e243">The diversification of <i>Dioon</i> has been driven by the aridification of Mexico. The Miocene timing corresponds to the expansion of arid zones that embedded the ancestral <i>Dioon</i> populations. As response, species in arid zones evolved epidermal traits to counteract aridity stress. This case study provides a robust body of evidence supporting the idea that aridification is an important driver of biodiversity. </p> </div>