Cycadales, commonly known as cycads, represent the oldest living seed plants that have been in existence for more than 200 million years. Although Cycas are charismatic plants with a high profile for plant conservation, many phylogenetic relationships remain unresolved. Understanding the evolutionary relationships within this group is essential for inferring the putative causes and routes of diversification within a historic and geographic context.
Dr. XIAO Longqian of Xishuangbanna Tropical Botanical Garden (XTBG) and Dr. Michael Möller of Royal Botanic Garden Edinburgh conducted a study to seek the phylogenetic relationships of Cycas. They sampled 31 species (one individual per species) from cultivated plants at the Fairy Lake Botanical Garden and XTBG. They cloned and sequenced the nrDNA ITS regions from 31 species, covering all morphological and geographical diversity of Cycas. They further examined the patterns of non-concerted evolution of the nrDNA ITS arrays, identified functional copies and used these to clarify the phylogenetic relationships and trace the evolutionary history of the genus. They further linked these patterns with the different seed dispersal capabilities, and paleo-geography, and tried to elucidate the main forces behind the evolutionary processes in Cycas.
A total of 161 distinct genomic ITS sequences were obtained from the 31 Cycas species. The functional analyses to distinguish between functional ITS paralogs and pseudogenes / recombinants showed that among these, 94 were functional sequences and 66 were pseudogenes. The aligned functional ITS sequences matrix contained 1118 characters, of which 591 were constant and 527 were variable. The biogeographical analysis strongly favored a scenario of a South China origin for the extant Cycas species (93.95%), with an early dispersal to Indochina, and suggested a vicariant event between South China and Indochina (46.11%).
Within species of Cycas, a high diversity in paralogs of nrDNA ITS copies was revealed. This was attributed to the existence of putative pseudogenes, recombinants, and non-concerted evolution among functional paralogs. Although both pseudogenes and recombinants had lost their usefulness as phylogenetic signals, phylogenetic inference from functional paralogs provided insights into the evolutionary and biogeographic history of Cycas. Even though further support is still needed, a scenario can be rebuilt from the resulting phylogenetic patterns, the ancestral distribution reconstruction and the existence of different seed dispersal capabilities suggesting that both vicariant and long-distance transoceanic dispersal events are drivers for the late-Miocene rapid radiations of Cycas.
The study entitled “Nuclear ribosomal its functional paralogs resolve the phylogenetic relationships of a late-miocene radiation cycad cycas (cycadaceae)” has been published in PLoS ONE.
