Invasive alien clones, autonomous apomixis in Praxelis clematidea

According to Baker’s law, self-propagating or apomictic plant species can more easily establish populations after long-distance dispersal than outcrossers that rely on conspecific male gametes from another individual and pollinators to reproduce. Apomixis is defined as a natural process that allows clonal reproduction through seeds, avoiding meiosis and fertilization via parthenogenetic development of an unreduced egg or a somatic cell. Apomictic species are also more likely to naturalize in introduced areas and become invasive than species reliant on sexual reproduction.

Praxelis clematidea is an annual or short-lived perennial plant that belongs to the Asteraceae family. The species originated in South America but has been become invasive in China and other countries. Although highly “successful” as an invasive species, its mode of reproduction is little understood. It has been hypothesized that apomixis contributes to the invasion potential of this alien Asteraceae species.

In their new study published in AoBP, Zhang et al. explored the process of embryo sac and seed formation of Praxelis clematidea. In their work, a global team with scientists from China, Germany and Brazil used flow cytometric seed screening (FCSS) to identify and confirm the reproductive mode of the species. They also investigated floral development, pollen viability and pollen morphology of this elusive species.

Zhang et al. found, for the first time, that seeds of P. clematidea can develop by apomixis. The plant embryo develops from an unreduced egg cell, and seed formation does not depend on fertilization with sperm. Pollen viability was very low, and most pollen grains were found to be empty with neither cytoplasm nor nuclei present. Yet, in their experiments, the authors found seed set was over 90 % for all treatments. Zhang et al. conclude that autonomous apomixis greatly increases the probability of successful colonisation and dispersal of P. clematidea into new areas, contributing to its high invasion potential. From a management standpoint, they suggest that efforts should be taken to remove above-ground plant parts and to prevent the formation of seeds.