Asexual Reproduction

ASEXUAL REPRODUCTION, occurs without the participation of egg cells, through the uniform or uneven division of an entire organism into two or several parts. Asexual reproduction is never the sole method of reproduction of an organism, but is always merely concomitant with sexual reproduction. Asexual reproduction is frequently 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 agency of a single individual. In many animals, asexual reproduction regularly alternates with sexual reproduction, the result of which is the alternation of generations. In parasitic Protozoa, asexual reproduction often serves to enhance auto-infection of the host animal.

Asexual reproduction is encountered predominantly in lower, more simply organized animals. It is very widespread in protozoa, sponges, and coelenterates; it is observed more rarely in worms and echinoderms, and is completely absent in mollusks and arthropods. Asexual reproduction is also present in lower chordates (tunicates) and is never encountered in acraniates and vertebrates. The main types of asexual reproduction are the following: 1) division, when the body of the animal breaks up into two approximately equal parts, which regenerate the organs lacking in the other half. In protozoa, there is also a special method of multiple division, or schizogony, when the organism immediately breaks up into a large number of daughter individuals (malarial plasmodium and others); 2) budding consists in the formation of a new individual at the expense of only a small region of the maternal individual, which retains its individuality. In Metazoa, 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), appear to be incorrect: trematode agamocytes represent parthenogenetic eggs. At present, it is believed that special interstitial cells play a major role in bud formation, retaining their embryonic character and broad potentials for development. In some animals (sponges, bryozoans), the formation of incompletely formed buds covered by a dense membrane (gemmules, statoblasts) sometimes occurs inside the body, which 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 lacking 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 those cases where the buds resulting from asexual reproduction permanently retain a connection with the maternal organism, asexual reproduction leads to the formation of colonies (corals, bryozoans, and many others).