Asexual

 The term “asexual” means without sex; it is an adjective applied to multicellular organisms with missing or atrophied sexual organs and to forms of reproduction that do not involve the fusion of male and female gametes.

The initial concept of sex referred to diploid, multicellular species in which new individuals are produced by the fusion of two haploid gametes, a sperm and an ovum (sexual reproduction). Individuals of many of these species may be classified into two sexes, male and female, depending on whether they are able to produce sperm or ova (eggs), respectively. This is the situation in most animals and in dioecious plants. The sex of an individual may be determined genetically or by the environment. Individuals are asexual if the organs that produce gametes are missing or atrophied; intersexual if their sexual organs are intermediate or ambiguous; or hermaphrodite if they have both kinds of sexual organs, whether they are functional or not. Hermaphroditism is prevalent in many species of plants, whether both pollen and ova are produced in the same hermaphrodite flowers, or in separate flowers of the same plant (monoecious plants).

 These concepts cannot be transferred to unicellular organisms without some modifications. The haploid cells that predominate in the life cycles of many unicellular eukaryotes can often be classified into different mating types, sometimes more than two, with the criterion that only those of different mating type are able to fuse and form diploids. This situation is called heterothallism. The mating types of the lower eukaryotes are sometimes called sexes, even though cells of different mating types are not homologous to sperm and ova of the animals.

In the situation called homothallism, diploid cells may be formed by the fusion of two genetically identical haploid cells. This would happen if there were a single mating type or if haploid cells change their mating type when they find no partner of the opposite one, as occurs in the homothallic strains of the yeast Saccharomyces.

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