60 



AN INTRODUCTION TO MODERN GENETICS 



la. Diploid Parthenogenesis , Apospory and Apogamy 



In some animals diploid parthenogenesis occurs under the influence 

 of rather unspecific external conditions, e.g. diet, temperature, crowding, 

 etc.^ In other forms, e.g. Phylloxera, there is a regular alternation of 

 diploid females which produce parthogenetic diploid eggs and sexual 

 males and females which develop from these eggs. In still other forms 

 (some nematodes, some insects) diploid parthenogenesis is obligatory 

 and males are very rare or even unknown. Among plants, diploid par- 

 thenogenesis occurs in ferns and in many flowering plants. 



DIPLOID 



MEIOTIC 

 CHROMATIDS 



UNREDUCED 

 EGGS 



Fig. 23. Segregation in Diploid Parthenogenesis. — In a heterozygous diploid, 

 four types of chromatids must result from meiosis, since there must be crossing 

 over of factors far removed from the centromeres (p. 121). If diploid partheno- 

 genetic eggs are formed, there are six ways in which two of these chromatids 

 may come together in a single egg. Only one of these (1) gives a diploid of the 

 same constitution as its parent; in all the others segregation has occurred. Types 

 1, 3, 4 and 6 ar6 formed if the two daughter nuclei come together after the first 

 metaphase and form a restitution nucleus; there is segregation only of factors 

 distal to the crossing over. Types 2 and 5 occur when an egg unites with the 

 second polar body; segregation occurs for factors between the centromere and 

 the crossing over. 



Diploid parthenogenetic eggs may develop spontaneously or their 

 development may be stimulated by sperm or pollen of which the 

 nucleus degenerates and takes no part in the subsequent formation of 

 the embryo. This process is known as pseudogamy and is found as a 

 naturally occurring process in some nematodes and plants (Potentilla 

 and blackberry). 



Apospory may, in its genetic eff'ects, be considered as a variety of 

 diploid parthenogenesis, since it consists in the development of the 

 gametophyte generation from an unreduced (diploid) cell. It is known 

 in flowering plants and also in ferns. Apogamy is not so well authenti- 

 cated a phenomenon, but when it occurs it also gives rise to diploid 

 offspring by the development of two fused embryo-sac cells. Combina- 

 tions of apospory and apogamy also occur. 



All these forms of diploid apomixis give the same genetical results : 

 ^ Cf. Shull 1929, Mortimer 1935. 



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