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

Organisms reproducing via asexuality harbor a great diversity of lineages, morphotypes and ecotypes. However, classification of asexual taxa does not fit into contemporary species concepts, and hence the diversity of apomictic plant complexes is not well reflected in taxonomy. Plants reproducing via apomixis (i.e., asexual seed formation = agamospermy) exemplify the theoretical and practical problems of classification. Obligately asexual organisms do not form reproductive communities, but they do constitute ancestor-descendant lineages. From the conceptual side, evolutionary lineage concepts would fit best for species delimitation. Recent research showed that these lineages are not necessarily threatened by rapid extinction and do have persistence in time and space. Facultative sexuality and low levels of residual recombination counteract the accumulation of deleterious mutations due to the lack of recombination (Muller's ratchet). Apomictic lineages do have adaptive potential, which is demonstrated by the ability to occupy large distribution areas and to experience ecological niche shifts. The challenge for classification of asexual lineages, however, is to find operational criteria for species delimitation. Current practices of species delimitation can be grouped into four main principles: (1) the sexuals-first principle means that obligate sexual progenitor species are classified separately from their apomictic derivatives. (2) The all-in-one principle merges sexual progenitors and highly facultative apomictic derivatives into one species, whereby the apomicts often represent autopolyploids without differentiated phenotypes. (3) The cluster concept applies to allopolyploid complexes with facultative apomixis and a huge diversity of genotypes and morphotypes; here the main genetic clusters are treated as species. (4) Almost obligate apomictic lineages are being classified as agamospecies. The four principles reflect quite well evolutionary traits and diversity of lineages. Finally, a recommendation for a workflow is given, following this gradient from obligate sexuality to obligate apomixis.