Sex determination.

  • Guido Bacci
Date:
[1965]
Catalogue details

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

Credit: Sex determination. Source: Wellcome Collection.

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    CHAPTER 2 REPRODUCTIVE CYCLES Meiosis and fertilization divide the life cycles of sexual organisms into two parts, through the alternation of nuclear phases. After meiosis and before fertilization organisms are said to belong to the haplophase; after fertihzation and before meiosis they belong to the diplophase. Asexual reproduction— as previously defined—may also take place either in the haplo, or in the diplophase or in both phases. Meiosis takes place in some species immediately after the formation of the zygote, which is then the only diploid cell of the whole cycle. The organisms that undergo such zygotic reduction show the highest possible extension of the haplophase and therefore they are called haplonts. Zygotic reduction takes place in Phytomonadina, in Coniugatae and in most Chlorophyceae. Phycomycetes, the most primitive among Fungi are also generally pure haplonts and among animals some Hypermastigina and apparently all Sporozoa undergo zygotic meiosis in their life cycles. The intermediate reduction of haplo-diploid species (or haplo-diplonts) takes place after the diploid generation, represented among plants by the sporophyte, has developed from the zygote. Meiosis gives rise to spores or gonia and to a haploid generation (the gametophyte when plants are con¬ cerned) where gametes are formed. The relative extent of the haplo and diplophase varies in different groups and species. Intermediate reduction is represented in some groups of Chlorophyceae and Ulva is a well-known example of equal development of the sporophytic and gametophytic phase. Chaetomorpha and Cladophora show also examples of this type of cycle. The brown algae Ectocarpus and Dictyota are haplo- diplonts as well as Allomyces, a member of the aquatic order of phycomy¬ cetes. It is well known that a progressive reduction of the gametophyte stage is observed in the life cycles of higher plants from Hepaticae to Angiospermae through mosses and ferns, a trend that is observed also in some genera of Phaeophyceae, the brown algae, and it appears to have some general evo¬ lutionary significance. Examples of haplo-diploid organisms among animals are found in the Foraminifera. All pluricellular animals, some flagellates, some amoebae, Hehozoa and ciliates among the protozoa are pure diplonts, that is meiosis takes place at gamete formation (gametic reduction). A number of yeasts are also pure 24
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