How apomictic taxa are treated in current taxonomy: A review

Abstract

Gametophytic 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.

The classification of asexual organisms: Old myths, new facts, and a novel pluralistic approach

“…Basically these categories follow a gradient from obligate sexuality to obligate asexuality. Other than Majeský & al. (2017) I will not provide detailed literature reviews of plant genera with gametophytic apomixis, but rather show how presently known apomictic plants would fit into these four approaches to species delimitation.…”

“…In the last few decades, the practice for taxonomists working with apomictic plants has been to follow the rather pragmatic views of genus-wise classifications, and quite often genus-specific concepts exist (Haveman, 2013;Majeský & al., 2017). The disadvantage of genus-specific concepts for assessment of overall biodiversity is that the resulting species are not readily comparable.…”

New chromosome counts and genome size estimates for 28 species of Taraxacum sect. Taraxacum

“…Rough estimates suggest the genus contains approximately 2,800 species in approximately 60 sections (Kirschner et al 2015), with the higher diversity in the mountains of Eurasia (Ge et al 2011); a total of 1,900 species in 35 sections are listed for Europe (Kirschner et al 2007). The complexity of Taraxacum taxonomy is caused by its combination of different reproduction strategies, including sexual reproduction (mainly outcrossing, less frequently selfing) and apomixis (meiotic diplospory; Richards 1973, Asker and Jerling 1992, Kirschner and Štěpánek 1994, Kirschner et al 1994, Majeský et al 2017). The vast majority of Taraxacum taxa are apomictic polyploid microspecies, only a few species are sexual diploids.…”

“…The diploid number (2n = 2x = 16) is confined to only sexually reproducing species, and sexual species are nearly all diploids, with only a few exceptions of sexual tetraploids known in section Piesis (Kirschner and Štěpánek 1994, 1998a, Trávníček et al 2013). In contrast, apomictic species are never diploids but always polyploids (Majeský et al 2017), having one of the genes involved in regulation of apomixis ( DIPLOSPOROUS ) located on the NOR chromosome (Vašut et al 2014). Most of the known chromosome numbers for apomictic Taraxacum species are at a triploid level (2n = 3x = 24), especially those of the widespread European sections Taraxacum sect.…”

An unexpected new diploid Hieracium from Europe: Integrative taxonomic approach with a phylogeny of diploid Hieracium taxa

“…This mechanism allowed to fix not only a huge number of morphologically well-differentiated polyploid taxa, but also a myriad of phenotypically very close morphotypes arisen from crosses of the same phenotypically variable parental taxa. As a consequence, roughly between 500 and 10,000 polyploid apomictic species/ microspecies (Zahn, 1921(Zahn, -1923Greuter, 2006-;IPNI, 2019), depending on the taxonomic concept (e.g., Zahn, 1921Zahn, -1923Schuhwerk, 2002;Tyler, 2006;Majeský & al., 2017), have been recognized. This holds also for the last two decades when some tens of new polyploid microspecies were described from different parts of the genus range (e.g., Chrtek & Marhold, 1998;Rich, 2011;Szeląg & Vladimirov, 2013;Di Gristina & al., 2014;Sennikov, 2014;Gottschlich & Pils, 2016;Tyler, 2017;Nilsson & Tyler, 2018;Szeląg, 2018).…”