Automixis: its distribution and status

Biologists have conclusively failed to arrive at a generally acceptable definition of sexual reproduction. Because of this, several reproductive processes are seen as sexual by some authors but as asexual by others. Included among these are automictic methods of reproduction. Automixis describes several reproductive processes whereby a new individual derives from a product or products of a single meiotically dividing cell. Several forms involve an episode of nuclear fusion and it is argued that, because of this, they should be seen as sexual processes irrespective of whether the fusing bodies are differentiated as gametes or are simply meiotic tetrad nuclei. Other forms involve no episode of nuclear fusion and it is argued that, because of this, they should be seen as asexual processes. These latter forms involve the generation of diploid eggs either by restitutional meioses, or by an endomitotic event preceding or following a reductional meiosis, or involve the generation of a diploid embryo by the fusion of cleavage division nuclei in a haploid embryo; in each case the egg develops parthenogenetically. In addition to the disagreement that exists over the reproductive status of automixis, considerable confusion exists over its taxonomic distribution. It is often described as being restricted to a few species of insects, where it is parthenogenetic, but in factde range of taxa, including both isogamous and anisogamous plants and fungi, where it may be either parthenogenetic or non-parthenogenetic. This confusion results both from a failure of many biologists writing on this subject to adequately consider the variation in life-cycles existing between major taxa and from a general failure by botanists and mycologists to distinguish between automixis and autogamous forms of self-fertilization (in which the fusing nuclei derive from different meioses). It is further compounded by a proliferation of synonyms for automictic processes. Thus in a number of publications automictic processes are variously described as being matromorphic, thelytokous, parthenogamic, autogamic or apomictic rather than as being automictic.