Evolution of Ceratozamia cycads: A proximate-ultimate approach.

Evidence suggests that past climatic fluctuations affected speciation of extant cycads. However, empirical genetic and morphological evidence explaining patterns and processes of species diversification are scarce. There are some explanations for the origin and evolution of the genus Ceratozamia, but with inconclusive results. To elucidate the evolution of Ceratozamia, we used genetic and phenotypic sources as empirical data, which were applied in a 'proximate-ultimate' framework (ecological and evolutionary scale, respectively). Our results suggested that the evolutionary mechanisms of speciation were shaped by deterministic (natural selection-adaptation) driven by climatic conditions associated to water stress, and probably enhanced by stochastic processes (gene drift and inbreeding). In general terms, punctuated evolution models were those that best explained the patterns of speciation throughout the phylogenetic history of the lineages encompassed in the genus Ceratozamia. Finally, we provide empirical evidence on the tempo and mode of the evolution of a 'living plant fossil'.

The phylogeography of the cycad genus Dioon (Zamiaceae) clarifies its Cenozoic expansion and diversification in the Mexican transition zone.

Background and Aims: Biogeographic transition zones are promising areas to study processes of biogeographic evolution and its influence on biological groups. The Mexican transition zone originated due to the overlap of Nearctic and Neotropical biota, which promoted great biological diversification. However, since most previous studies in this area were focused on revealing the phylogeography of Nearctic plants, how historical biogeographic configuration influenced the expansion and diversification of the Neotropical flora remains almost unknown. Using the cycad genus Dioon (Zamiaceae), this study aimed to test whether the biogeographic provinciality of the Mexican transition zone reflects the history of diversification of Neotropical plants. Methods: Two chloroplast DNA (cpDNA) regions were analysed from 101 specimens of 15 Dioon species to reveal the distribution of haplogroups. In addition, genome-wide single nucleotide polymorphisms (SNPs) from 84 specimens were used to test the concordance between phylogenetic clusters and the biogeographic provinces. An ultrametric tree was constructed from the sequences containing SNPs to reconstruct the biogeographic events of vicariance and dispersal of Dioon across the Neotropical biogeographic provinces. Key Results: Four Dioon lineages with strong phylogeographic structures were recognized using both cpDNA and SNP data. The lineages correspond to two clades that originated from a common ancestor in Eastern Mexico. One clade expanded and diversified in South-east Mexico and Central America. Another clade diversified into three lineages that dispersed to North-east, South and North-west Mexico. Each lineage was biogeographically delimitated. Biogeographic provinces might have provided disparate ecological conditions that facilitated speciation in Dioon since the Miocene. Conclusions: The current genetic structure and species diversity of Dioon depict the history of expansion and diversification of the northernmost Neotropical provinces. Past biogeographic connectivities were favoured by elevated topographies, since mountain systems served as corridors for the migration of Dioon and as refugia of tropical communities that diversified during the formation of modern Neotropical forests.

Geographic variation in cone volatiles and pollinators in the thermogenic African cycad Encephalartos ghellinckii Lem.

Heat and odour production can have profound effects on pollination in cycads. It is therefore expected that these traits would co-vary geographically with pollinator assemblages. Such intraspecific variation, may lead to the evolution of pollination ecotypes, which can be an early stage of pollinator-mediated speciation. We measured cone temperatures using miniature temperature data loggers and examined the composition of cone volatile odours using headspace sampling and analysis with gas chromatography-mass spectrometry in four populations spanning the range of the African cycad Encephalartos ghellinckii. Pollinator assemblages were also investigated in three populations. Male and female cones were thermogenic at pollen shed and receptive stages, respectively, but patterns of thermogenesis did not vary among populations. Scent emissions from cones in populations in the Drakensberg Mountains were characterised by cis-ß-ocimene, ß-myrcene and (3E)-1,3-octadiene, while camphene and α-pinene were characteristic of scent emissions from cones in populations closer to the coast. These differences in volatile blends corresponded with differences in insect assemblages. These results confirm intraspecific variation in volatile emissions of E. ghellinckii and support the predictions that intraspecific variation in volatile emissions will be associated with shifts in pollinator assemblages. While further work needs to be done to test for local adaptation in this system, this preliminary evidence is consistent with the formation of pollination ecotypes in the E. ghellinckii species complex.

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

Background and Aims: 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 Dioon (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. Methods: A phylogenetic tree was constructed from seven chloroplast DNA sequences and the ITS2 spacer to reveal the relationships among 14 Dioon species from habitats ranging from humid forests to deserts. Divergence times were estimated and the habitat shifts throughout Dioon phylogeny were detected. The epidermal anatomy among Dioon species was compared and correlation tests were performed to associate the epidermal variations with habitat parameters. Key Results: Events of habitat shifts towards arid zones happened exclusively in one of the two main clades of Dioon. Such habitat shifts happened during the species diversification of Dioon, 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. Conclusions: The diversification of Dioon has been driven by the aridification of Mexico. The Miocene timing corresponds to the expansion of arid zones that embedded the ancestral Dioon 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.

Anatomical basis of variation in mesophyll resistance in eastern Australian sclerophylls: news of a long and winding path.

In sclerophylls, photosynthesis is particularly strongly limited by mesophyll diffusion resistance from substomatal cavities to chloroplasts (r(m)), but the controls on diffusion limits by integral leaf variables such as leaf thickness, density, and dry mass per unit area and by the individual steps along the diffusion pathway are imperfectly understood. To gain insight into the determinants of r(m) in leaves with varying structure, the full CO(2) physical diffusion pathway was analysed in 32 Australian species sampled from sites contrasting in soil nutrients and rainfall, and having leaf structures from mesophytic to strongly sclerophyllous. r(m) was estimated based on combined measurements of gas exchange and chlorophyll fluorescence. In addition, r(m) was modelled on the basis of detailed anatomical measurements to separate the importance of different serial resistances affecting CO(2) diffusion into chloroplasts. The strongest sources of variation in r(m) were S(c)/S, the exposed surface area of chloroplasts per unit leaf area, and mesophyll cell wall thickness, t(cw). The strong correlation of r(m) with t(cw) could not be explained by cell wall thickness alone, and most likely arose from a further effect of cell wall porosity. The CO(2) drawdown from intercellular spaces to chloroplasts was positively correlated with t(cw), suggesting enhanced diffusional limitations in leaves with thicker cell walls. Leaf thickness and density were poorly correlated with S(c)/S, indicating that widely varying combinations of leaf anatomical traits occur at given values of leaf integrated traits, and suggesting that detailed anatomical studies are needed to predict r(m) for any given species.