Autonomous apomixis in Praxelis clematidea (Asteraceae: Eupatorieae), an invasive alien plant. Issue 2 (29th January 2021) - Zhang, Yuhuan
Abstract: Understanding the reproductive mechanisms of invasive alien species can lay the foundation for effective control measures. Praxelis clematidea is a triploid neotropical Asteraceae species that is invasive in China and other countries. However, few studies have focused on its reproductive biology. In this study, flow cytometric seed screening (FCSS) was used to identify and confirm the reproductive mode of the species. The development of ovules, anthers, and mega- and microgametophytes was observed using a clearing method and differential interference contrast microscopy. Pollen viability was measured using the Benzidine test and Alexander's stain. Pollen morphology was observed via fluorescence microscopy after sectioning the disk florets and staining with water-soluble aniline blue or 4′6-diamidino-2-phenylindole nuclei dyes. Controlled pollination experiments were conducted on four populations in China to examine the breeding system and to confirm autonomous apomixis. The reproductive mode was found to be autonomous apomixis without pseudogamy, according to FCSS. Megaspore mother cells developed directly into eight-nucleate megagametophytes without meiosis, conforming to Antennaria -type diplospory. The unreduced egg cells developed into embryos through parthenogenesis, while the endosperm was formed by the fusion of two unreduced polar nuclei. Pollen viability was very low (0.82 ± 0.57 % and 0.36 ± 0.44 %) as measured by the Benzidine test and Alexander's stain, respectively. The majority of the pollen grains were empty and had neither cytoplasm nor nuclei. The seed set was >90 % for all treatments of open pollination, bagging and emasculated capitula. Mature cypselae developed in capitula that were emasculated before flowering, which confirmed that the breeding system of P. clematidea was autonomous apomixis. The present study is the first report of autonomous apomixis in P. clematidea in China. Antennaria -type autonomous apomixis in P. clematidea greatly increases the probability of successful colonisation and dispersal of P. clematidea into new areas, which likely contributes to its high invasion potential. Effective control measures should be implemented to prevent autonomous (pollen-independent) seed production. Abstract : Praxelis ( Praxelis clematidea ) belongs to Asteraceae like sunflower and originated in South America. It has become invasive in China and other countries. Its mode of reproduction was previously unknown. A team from China, Germany and Brazil explored the process of embryo sac and seed formation of Praxelis . They found that the seeds of Praxelis can develop by apomixis, a process of asexual seed formation. The plant embryo develops from an unreduced egg cell, and seed formation does not depend on fertilization with sperm, like in the majority of sexually reproducing plants. This special way of pollen-independent seed production is called autonomous apomixis and likely fosters the spread of invasive plants.