报告题目：Evolutionary biology of the most enigmatic group of bryophytes: the hornworts

报告人：Dr. Juan Carlos Villarreal, Post-doctoral researcher, Institute of Systematic Botany and Mycology, University of Munich (LMU)

主持人： 朱瑞良   教授

报告时间：4月28日 11:00

报告地点：闵行生科院534小会议室

报告摘要： The talk has two main parts revolving around hornworts, a phylogenetically critical group of species that appears to be the sister group to the vascular plants. The first part of the talk will focus on the evolution of pyrenoids, proteinaceous bodies consisting of up to 90% RuBisCO, the second on a study case of a Southern Appalachian clonal hornwort.

RuBisCO has a crucial role in carbon fixation but a slow catalytic rate, a problem overcome in some plant lineages by physiological and anatomical traits that elevate carbon concentrations around the enzyme. Such carbon-concentrating mechanisms (CCMs) are hypothesized to have evolved during periods of low atmospheric CO2. Hornworts have a CCM that relies on pyrenoids, structures easily seen in living cells at magnifications of 20x but not in dried herbarium material. A molecular clock-dated phylogeny for 36% of the species (Villarreal and Renner, PNAS, 2012) implies at least 5 - 6 origins and an equal number of subsequent losses of pyrenoids in hornworts, with the oldest pyrenoid origin at ca. 100 mya, and others at <35 mya. The non-synchronous appearance of pyrenoid-containing clades, the successful diversification of pyrenoid-lacking clades during periods with low CO2, and the maintenance of pyrenoids during episodes of high [CO2] all argue against the previously proposed relationship between pyrenoid origin and low [CO2]. The selective advantages, and costs, of hornwort pyrenoids thus must relate to additional factors besides atmospheric CO2 and yet remain to be explored.

The impact of glaciation on rivers courses has affected gene flow among populations, and the signatures of these processes are sometimes still detectable in the current genetic architecture of organisms, particularly in those that are dispersal limited. I studied possible links between Pleistocene landscape evolution and present-day genetic diversity in Appalachian populations of Nothoceros aenigmaticus, a species that presents an enigmatic case of sex allopatry, with 2 watersheds consisting of male gametophytes and all 4 watersheds of female gametophytes. Using microsatellites, I found a lack of gene flow between the unisexual North American populations and the closest populations of Nothoceros aenigmaticus with sexually reproducing plants in Mexico. Within the Southern Appalachians, there are populations that appear to have reproduced asexually for at least 50,000 years (and thus are entirely clonal), with little downstream dispersal. Genotype sharing between several catchments may be explained by stream capture. For example, I found shared chloroplast, mitochondrial and nuclear genotypes between two unconnected drainages, the Coosa River (Alabama drainage) and the Ocoee River (Tennessee drainage), fitting with some geological evidence and genetic data from salamanders and fishes. These results raise questions about the eco-physiology of Nothoceros aenigmaticus, including possible habitat preferences of the sexes or nutrient levels in the watersheds. Additional studies using climatic envelope tools are now required. The research has impacts on habitat conservation of the sole clonally reproducing US Nothoceros species.