Elevation-dependent endopolyploid response suggests that plants with holocentric chromosomes are less stressed by UV-B

Abstract

Abstract Previous studies suggested that holocentric chromosomes may confer a selective advantage under high ionizing or UV-B radiation due to their tolerance of fragmentation, and that the first plant and animal colonizers of land in the Palaeozoic were or may have been holocentric. Holocentric chromosomes could have, therefore, aided terrestrialization of Earth’s biota half a billion years ago, because leaving water meant facing a sharp increase of UV-B. Because we cannot go back in time, the hypothesis needs to be tested with present-day species using an indicator of UV-B stress. We took advantage of the fact that UV-B intensity increases with elevation and tested whether holocentric plants (six species of Cyperaceae and Juncaceae) are less stressed with increasing elevation than monocentric plants (six species of Poaceae). Phylogenetically corrected regression showed that the proxy for UV-B stress (endopolyploidy index from 671 samples measured by flow cytometry) increased with elevation in holocentric and monocentric species, but the increase was more rapid in monocentric species. Although half a billion year elapsed since terrestrialization, holocentric Cyperaceae and Juncaceae still appear less stressed by UV-B than monocentric Poaceae, despite the other counter UV-B adaptations they both have evolved (graminoid morphology, silica bodies).