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.

    29/328 (page 13)
    SEXUAL AND ASEXUAL REPRODUCTION 13 Self fertilization, which is a type of automixis, is on the contrary rather uncommon and it is prevented in many instances by a number of genetic and physiological devices. It is nevertheless widespread in a number of organisms, for instance in Rhizopoda, where the case of Actinophrys sol represents an outstanding example (Belar, 1922). Examples of self fertilizing Metazoa are frequently being discovered (Cognetti, 1956a; Delavault, 1958; Parenti, 1960) and several normally autogamous plants are also well known. Automictic parthenogenesis (see page 216) is also essentially a kind of self fertilization. It is possible therefore that the prevalence of amphimixis over automixis has been overestimated under the influence of theoretical considerations. The most important deviation from the normal sequence of events in fertilization is perhaps shown by certain Fungi where two haploid nuclei from different mycelia migrate in the same cell without undergoing caryogamy and divide several times in the same mycelium by conjugated mitotic division until caryogamy takes place. Thus plasmogamy and caryogamy appear as quite distinct phases in the fertilization process. Other minor variations in fertilization are represented by the fact that gametes may or may not show diff'erences in size or may be represented by nuclei that are not surrounded by a cellular body having a definite boundary. Reproduction and Cellular Differentiation The alternation of nuclear diploid and haploid phases in sexually repro¬ ducing organisms is accompanied by a series of cytoplasmic modifications of the reproductive cells which is more or less evident in different organisms. This is true not only for most of the cellular elements that take part in fertili¬ zation processes but also for most of the reproductive elements that give rise to new individuals in the absence of fertilization and are said therefore to reproduce agamically or asexually. /. Diffusion and Types of Asexual Reproduction Organisms that reproduce in the absence of any sexual process are not uncommon although recent investigation has shown the existence of a sexual cycle in many groups where sexual processes were unknown. Neither fertili¬ zation nor meiosis have ever been observed among Chloromonadina, Silicoñagellates, Cryptomonadina and in most dinoflagellates. Sexual processes are also unknown in the successful and widespread genus Chlorella and in most species of Chlamydomonas. Most zooflagellates and amoebina seem also to reproduce only asexually without any trace of sexuaHty. Very few asexual species are known, however, among higher plants and ani¬ mals. AsexuaUy reproducing individuals are also known to occur in regular or irregular alternation with sexually reproducing individuals in many species.