The classification of reproduction types. The asexual reproduction

The ability to reproduce itself is one of the main features of life systems. On molecular level, reproduction process is determined by nucleic acids duplication ability. On ontogenetic level, self reproduction is performed in different forms: from simple division of protists to sexual reproduction of animals and plants, which is very complicate process in structural and functional aspects.

The reproduction - is ability of organisms to produce new organisms similar to them; and ability of organism to produce off springs. One’s being is supported by cell reproduction; and species being is supported by organism’s reproduction. The reproduction is necessary condition of species being and generation’s continuity in it. Although, the reproduction ways in worlds of plants and animals are very diverse, but they may be divided into two general types: asexual and sexual.

The asexual reproduction. The asexual reproduction is a reproduction in which only one parent organism takes part. As result of its division or budding one or several new organisms are formed. These organisms are identical in genotype to parent organism. During asexual reproduction, the somatic cell sets a new organism. There is no special reproductive cells formation.

Protists (ameba, flagellata, and infusoria), bacteria, cianobacteria and some green algae have asexual reproduction in form of binary fission, plasmodiums in form of shysogony, some fungi - in form of budding. A toxoplasm has an internal budding so called endogony. Between multicellular animals the sponges, hydras have asexual reproduction in form of budding; flatworms and annelids have asexual reproduction in form of fragmentation (body cleavage on several parts).

Multicellular plants have vegetative organs for asexual reproduction such as root, shoot, leaf, tuber, bulb and others.

As particular case of asexual reproduction, vertebrates have polyembryony. It was described by I. I. Mechnicov. He observed medusa blastula splitting and further development of entire organism from each cellular conjugate. Humans also have such way of reproduction. It leads to twins development (pic 4.1).

Pic. 4.1. The monozygous twins (by S. Sinnot, L. Dunn, Th. Dobzhansky, 1958)

The spore formation is a type of asexual reproduction, which is characterized by special cell formation. A spore (from a Greek, spora- a seed) - is one of life cycle stages using for reproduction. It is consist of cell, appeared from mitosis or meiosis. It is covered by coat, which defends it from harmful external conditions. Protists, algae, fungi, mosses, ferns and whisk ferns have a spore formation. Bacteria also produce spores, but their spores serve for surviving in inappropriate conditions, not for reproduction. Some higher plants have a spore formation as a way of reproduction.

The asexual reproduction is typical for animals with low level of structural and physiological organization. Among them, there are many human parasites. Their asexual reproduction serves not only for increasing organism’s number, but also facilitates to take a new areas for living and to survive in an inappropriate conditions.

The sexual reproduction. The sexual reproduction means a development of offspring from fertilized ovicell - zygota, i.e. fused male and female sex cells. While sexual reproduction the continuity between generations is performed by special sex cells - gametes (from Greek “gamos” - marriage). Such cells have a haploid chromosome set and they are formed in meiosis. These cells are spermatozoa and ovicells.

The sexual reproduction now dominates in animal and plant world. It has some advantages over asexual reproduction.

1. A higher reproduction coefficient is reached, i.e. it gives more new organism germs.
2. A full renewing of genome occurs. It happens because of mother and farther genetic information fusion. Such process is a permanent source of hereditary diversity. It extends an adaptation ability of species in abiotic and biotic conditions and provides a success in survivement competition.

The basement of sexual reproduction is sexual process. The essence of it is a fusion of genetic material of parents to genetic material of offspring.

The sexual reproduction of multicellular organisms, including a human, is characterized by three external morphological features.

1. A source for new organism formation is special cells of parent organism - gametes.
2. There are two types of sex cells: male and female. Commonly they are produced by different individuals, excluding genuine hermaphroditism cases. Sex cells differ from each other by morphological and physiological properties, i.e. they have sex differentiation (specialization).
3. The fusion of two gametes, male and female, is needed for new organism formation.

Eukaryotes have three main types of sexual reproduction. They have been formed during their evolution. They are conjugation, copulation and irregular types of sexual reproduction.

The conjugation (from Latin “conucatio” - fusion) - is a form of sexual process, providing increasing genetic variability, having no sex cell formation. It occurs in prokaryotes (enterobacteria, pseudomonads etc.), protists (infusoria), algae (ulotrix and others), and fungies (pic 4.2). During conjugation two organisms temporarily unites for genetic information exchange. As result of this, organisms genetically different from parents appear. Then they perform asexual reproduction.

Pic. 4.2. The electronic microscope photo of bacteria conjugation (by N.P. Dubinin, 1976)

The copulation (from Latin “copulatio” - joining) - is a process of sex cell fusion, in which cells are almost same (isogamy) or different (anisogamy). Isogamy occurs in monocellular algae, lower fungies, flagellatae. Anisogamy occurs in chlamidomonas and plasmodiums. If gametes are very different from each other (oogamy), the term fertilization may be used for pointing their fusion. Humans and higher vertebrates have an oogamic copulation.

The irregular types of sexual reproduction are parthenogenesis, ginogenesis and androgenesis.

Parthenogenesis (from Greek “parthenos” - virgin) - is a process of embryo development from unfertilized ovum (pic 4.3 b). A natural parthenogenesis was described by Sh. Bone in XVIII century. It may occur in lower Crustacea, some insects (bees) and birds (turkey). Parthenogenesis might be stimulated artificially, through ovicell activation by different factors. The artificial parthenogenesis was firstly described by A. A. Tichomirov.

Pic. 4.3. The types of sexual reproduction: A - usual fertilization; В - parthenogenesis; C - ginogenesis; D - androgenesis (by M. E. Lobanov, 1967)

It may be distinguished somatic (diploid) and generative (haploid) parthenogenesis. In somatic parthenogenesis, an ovicell is not subject to reducing division (meiosis). Moreover, even if it is subject to it, two newly formed haploid nucleuses are fused to one, restoring diploid chromosome set. In generative parthenogenesis, an embryo is developed from haploid ovicell. Plant parthenogenesis is called apomixes.

Ginogenesis (from Greek “ginos” - female) - is a type of sexual reproduction, when sperm serves only like activators of ovicell development, and fertilization doesn’t occur (pic 4.3c). Only female genetic material is used for embryo development. Ginogenesis occurs in some nematodes, amphibians and in some angiosperm plants.

Androgenesis (from Greek “andres” - male) - is a type of sexual reproduction when embryo development is performed from male nucleuses and female cytoplasm (pic 4.3d). It may take place in such cases when female nucleus has died before fertilization occurs. If only one sperm enters ovicell, there is development of haploid embryo. As usually, such embryos are weak. It may survive better if diploid chromosome set is restored. If there is a polyspermy, the fusion of two male nucleuses may occur. Androgenesis occurs in silkworm, some wasp species, corn and tobacco-plant.

Androgenesis and ginogenesis are used for studying heredity principles, nucleus- cytoplasm interactions, for getting strictly homozygote organisms and animals of same sex.