434 Haploids and Autopolyploids 



one genome that there is anything abnormal about the haploid 

 state. 



In Chapter 4, we pointed out that the gametophyte generation 

 of plants is normally haploid. In some animals, such as the 

 bees and related insects, the females are always diploid whereas 

 the males are haploid. Since this is a normal condition, and 

 since the males produce normal, viable sperm, the meiotic di- 

 visions which occur in the production of germ cells in animals 

 must be modified in such a way that the haploid number already 

 present is not further reduced. In the adjustment which this 

 organism makes the first meiotic division is an abortive affair, 

 resulting in a normal haploid secondary spermatocyte and a 

 small mass of cytoplasm without a nucleus. The second meiotic 

 division is of the equational type and produces cells which mature 

 into haploid spermatozoa. 



In organisms in which a generation that is normally diploid is 

 produced with the haploid number of chromosomes, there is no 

 such adjustment to the haploid condition, and no modification 

 of the meiotic mechanism has been developed of such nature 

 that large numbers of haploid products of meiosis will result. A 

 haploid plant or animal can merely be regarded, so far as its 

 meiotic behavior is concerned, as an organism which is mono- 

 somic for all its chromosome pairs. Each chromosome behaves 

 like a monosomic and is normally independent of all the others. 

 There is no zygotene pairing, as no chromosome has a partner, 

 so that each chromosome moves on to the equator as a univalent 

 and passes to either pole at random and completely independently 

 of the others. Theoretically, the chance that any given chro- 

 mosome will pass to a certain pole is % and that all the chromo- 

 somes will pass to the same pole is ^ X H X ■!^2 • • • to n 

 terms, where n represents the number of univalent chromosomes. 

 In other words, the frequency of spores or gametes with n chro- 

 mosomes will be 1/2''. Quite obviously, the greater the number 

 of chromosomes, the smaller the chance of including all in one 

 cell. Since gametes or gametophytes that have fewer than n 

 chromosomes are frequently inviable, such haploids are highly 

 sterile. 



Although theoretically all the chromosomes at the first meiotic 

 division of haploids should be univalents, they are not always 



