The origin and morphological character evolution of the paleotropical woody bamboos

The woody bamboos exhibit distinctive biological traits within Poaceae, such as highly lignified culms, several distinct caryopsis types, rapid growth and synchronous, usually monocarpic with flowering every ~30-60 years. Much of the remarkable morphological diversity within Bambusoideae exists within a single hexaploid clade, the paleotropical woody bamboos (PWB). The complex evolutionary history and morphology of this clade makes it ideal to investigate the factors underlying morphological evolution in bamboos, however, the origin and biogeographical history of PWB remain elusive, as does the effect of environmental factors on the evolution of morphological characters within PWB. We generated a robust and time-calibrated phylogeny of PWB using SNPs retrieved from MiddRAD-seq, and explored the evolutionary trends of habit, inflorescence, and fruit type in relation to environmental factors including climate, soil, and topography. We inferred that the PWB started to diversify across the Oligocene/Miocene boundary and formed four major clades, i.e. Melocanninae, Racemobambosinae s.l. (comprising Dinochloinae, Greslanlinae, Racemobambosinae s.str. and Temburongiinae), Hickeliinae and Bambusinae s.l. (comprising Bambusinae s.str. plus Holttumochloinae). The ancestor of PWB was reconstructed as having erect habit, indeterminate inflorescence and basic caryopsis, but all these characters have since undergone multiple changes and reversals among PWB. The evolution of all three traits was correlated with, and hence likely influenced by, aspects of climate, topography, and soil, with climate factors most strongly correlated with morphological traits, and soil factors least so. However, topography had more influence than climate or soil on the evolution of erect habit, whereas both traits had greater effect on the evolution of bacoid caryopsis than did soil. Our results provide novel insights on morphological diversity and adaptive evolution in bamboos for future ecological and evolutionary research.