General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
Catalogue details

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

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

    327/384 (page 309)
    §7.1 Deviant cytogenetic systems 309 details see Rutishauser (1967), who also mentions the earlier, clas¬ sical reviews by Battaglia (1963) and Gustafsson (1946, 1947, 1948). 7.1.2. Animals In animals too apomixis occurs; in higher forms (usually not higher than insects and Crustaceae), almost exclusively as parthe¬ nogenesis. The entire order of Hymenoptera and many other in¬ sect families have haploid parthenogenesis as their sex determina¬ tion system: the unfertilized, haploid eggs develop into males, the fertilized eggs into females (arrhenotoky ). Diploid parthenogenesis (thelotoky) occurs also; unfertilized, unreduced eggs develop into females that are genetically identical to the mother. Obligate par¬ thenogenesis without any possibility for sexual reproduction must be distinguished from cyclical parthenogenesis, where with the season diploid parthenogenesis alternates with sexual reproduc¬ tion. Obligate parthenogenesis is often combined with polyploidy (compare section 5.6.3). It can revert to sexual reproduction, for instance after hybridization, and then lead to sexually reproducing polyploids (Astaurov, 1969). Cyclic parthenogenesis permits an efficient adaptation to the en¬ vironment but requires a special system of sex determination. In several species of aphids the fertilized eggs winter over and in spring each egg produces one wingless female (the fundatrix). This female produces parthenogenetically a number of winged daugh¬ ters (emigrants) that migrate to other food plants and in their turn produce parthenogenetically numerous generations of female pro¬ geny. At the end of the season, the aphids start producing two types of oocyte. From one type arises, again parthenogenetically, a female with the normal diploid number of chromosomes ( 2«= 14). She produces haploid eggs after a normal meiosis. In the second type of oocyte a rudimentary meiosis takes place: only the X-chro- mosomes pair and segregate. One of the two X-chromosomes goes to one pole and remains single. At the other pole the other X- chromosome and all the remaining chromosomes come together: 27/-1 = 13. These egg cells develop (parthenogenetically) into XO males. The males have normal meiosis, but only the X-bearing