How apomictic taxa are treated in current taxonomy: A review

Abstract

AbstractGametophytic apomixis (asexual seed formation without syngamy of female and male gametes) is a highly interesting mechanism for researchers in plant biotechnology, genetics, evolutionary biology, and taxonomy. Apomixis evolved repeatedly and independently in the evolution of multiple genera. It is an effective reproduction barrier and, consequently, conserved apomictic genotypes may become overrepresented in nature. Apomictic plants may easily colonize free niches with only one or a few individuals and outcompete outcrossing plants. In spite of the indisputable pros of asexual reproduction, apomixis also has several cons. One of the most commonly mentioned is the accumulation of deleterious mutations in asexual lineages and decreased genetic variation. However, apomicts in general can be genetically highly diverse. The most common sources of this variation are the accumulation of mutations, hybridization with sexual plants, and facultative apomixis. Facultative apomicts are highly variable in their level of residual sexuality, which increases their genotypic and phenotypic variation. Even in the case of obligate apomicts, gene flow is possible due to functional male meiosis and the production of viable pollen grains by apomicts. Apomixis occurs in plant genera in which hybridization together with polyploidization play an important role in diversification and causes severe problems in taxonomy. How to accommodate apomictic taxa in taxonomic treatments, and understanding what should and what should not be referred to as a species are intriguing questions. This review aims to provide an overview of the main characteristics of “apomictic genera” and the approaches used to treat apomictic taxa within these genera. To achieve this aim, the review was divided into several parts. Firstly, the distinctive features of apomictic reproduction and apomictic taxa are described together with issues related to the taxonomic evaluation of apomictic taxa. The second part discusses approaches in the known apomictic genera, and the final part presents the authors’ view on important points, which need to be taken into account in the classification of apomictic taxa.