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Sex determination.

  • Bacci, Guido.
Date:
[1965]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Sex determination. Source: Wellcome Collection.

  • Front Cover
  • Title Page
  • Table of Contents
  • Back Cover
    30/328 (page 14)
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    14 SEX DETERMINATION Exclusively asexual reproduction was formerly considered as primitive in consideration of the complexity of the sexual cycle and of the low taxonomic position of most asexual organisms. Now that the existence of quasi sexual processes has been demonstrated in various strains of bacteria (see Chapter 3) there is a tendency to attach a fundamental importance to the sex cycle and to assume that sex has been secondarily lost in asexually reproducing species. As a matter of fact no definite position can be held at present in this respect, considering how little is known about the life cycles of the so called primitive organisms. Only in metazoa and in angiosperms can agamic reproduction be regarded as the result of a secondary evolution. Asexual reproduction may take place with the development of a new individual from a single cell, and it has been called agamogony (Hartmann, 1904), or from a group of cells. It is called vegetative reproduction sensu stricto. This distinction is based upon the germ plasma theory of Weismann (1886a) as in agamogony the single asexual cells either belong to unicellular organisms, where a somatic and a germinal line cannot be distinguished, or they belong to the germ line. In vegetative reproduction the new organism develops from either undifferentiated or dedifferentiated cells, that in most cases cannot be said to belong to a specialized germ line. Although the germ plasma theory has lost much of its former interest, the concept is useful because it is a well established fact that agamogony takes place with peculiar and uniform characters in the most varied organisms while processes of vegetative reproduction differ very much in the different organisms where they appear as ex novo adaptation. Agamic reproduction in both its aspect of agamogony and vegetative reproduction may represent the only reproductive process of a species or it may take place within the sexual cycle of an organism as an essential or inessential part of its life cycle. 2. Agamogony Agamogony of the unicellular organisms, where a germ line is not separa¬ ted, takes place essentially in three ways: (1) equal binary fission as in some Chlamydomonas (Fig. 2.1), in Amoeba or in Paramecium; (2) budding, as in Suctoria; (3) multiple fission, in which the nucleus divides several times before the cytoplasm falls into as many parts as there are nuclei. Multiple fission is well known especially in Sporozoa (Fig. 2.2). Agamogony is the only way of reproduction in some Amoebina and it represents simply a step of the comphcated sexual cycle of the Sporozoa. Highly differentiated processes of agamogony lead to the production of specialized germinal cells that are called spores and develop into a new organism without fertilization. They may be obtained as the immediate result of meiosis as in Ectocarpus (Fig. 2.3), in mosses (Fig. 2.4), in angio¬ sperms or in Basidiomycetes (Fig. 2.9) and give rise to haploid organisms
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