Asexual Reproduction

ASEXUAL REPRODUCTION,

occurs without the participation of egg cells, by means of the uniform or non-uniform division of the whole organism into two or several parts. Asexual reproduction is never the sole method of reproduction for an organism, but always merely accompanies sexual reproduction. Asexual reproduction is often designated as monogony, i.e., reproduction originating from a single individual, in contrast to amphigony, or sexual reproduction, in which two individuals of a given species usually participate. However, in autogamy and parthenogenesis, sexual reproduction also proceeds through the mediation of a single individual. In many animals, asexual reproduction regularly alternates with sexual reproduction, the result of which is an alternation of generations. In parasitic Protozoa, asexual reproduction often serves to intensify the autoinfection of the host animal. Asexual reproduction is encountered, predominantly, in lower, more simply organized animals. It is very common in protozoa, sponges, and coelenterates; it is observed less frequently in worms and echinoderms and is completely absent in mollusks and arthropods. Asexual reproduction also exists in lower chordates (tunicates) and is never encountered in acraniates and vertebrates. The main types of asexual reproduction are as follows: 1) fission, when the body of the animal breaks up into two approximately equal parts, which regenerate the organs missing from the other half. In protozoa, there is also a special method of multiple fission, or schizogony, when the organism immediately breaks up into a large number of daughter individuals (malaria plasmodium, etc.); 2) budding consists of the formation of a new individual at the expense of only a small section of the maternal individual, which retains its individuality. In Metazoa, at least several cells always participate in the formation of buds. Data on the origin of rediae and cercariae in trematodes from individual asexual cells, agamocytes (Hartmann's term), are apparently incorrect: the agamocytes of trematodes are parthenogenetic eggs. At the present time, it is believed that special interstitial cells, which retain their embryonic character and broad potencies for development, play a major role in the formation of buds. In some animals (sponges, bryozoans), the formation of not fully formed buds, covered by a dense membrane (gemmules, statoblasts), sometimes occurs inside the body; these develop only after a certain period of rest. These are resting buds, and their formation is called internal budding. Asexual reproduction is always accompanied by the regeneration of those organs and parts of the body that are missing in the bud or in the separated half of the individual. During budding, the main organs of the animal are laid down in the bud, usually even before separation from the maternal organism. In cases where the buds resulting from asexual reproduction retain a permanent connection with the maternal individual, asexual reproduction leads to the formation of colonies (corals, bryozoans, and many others).

Asexual reproduction of plants, without the participation of sex cells, is encountered in almost all plants. A distinction is made between: 1) asexual reproduction proper, by means of special cells (spores) in algae and fungi; 2) vegetative reproduction, by means of a vegetative body that separates from the maternal plant and grows into a new one. Only this type of asexual reproduction is characteristic of higher plants. Examples of this include reproduction by tubers (potatoes), parts of rhizomes (in the majority of herbaceous perennial plants), and others. Furthermore, a distinction is made between natural and artificial reproduction. Artificial reproduction is spoken of when a known part of a plant is artificially separated for the purposes of propagation (reproduction by layering, cuttings, grafting); asexual reproduction, in particular artificial reproduction, is widely used, especially in horticulture, and has the goal, on the one hand, of propagating a given plant variety, and on the other, of preserving it in an unchanged state, which for many horticultural varieties that are hybrids, is not achieved by reproduction by seeds (i.e., by sexual means). Thus, for example, almost all varieties of our fruit trees are propagated exclusively by grafting.