68 AN INTRODUCTION TO MODERN GENETICS 



fair number of cases, however, there is an increase in the size and 

 general vigour of the organism parallel to the increase in chromosome 

 number. This is probably dependent on an increase in the size of 

 individual cells, which acts so as to keep a constant ratio between the 

 size of nucleus and cytoplasm. The phenomenon is not always found. 

 Wettstein^ showed that in mosses, in which polj^loids can be artificially 

 produced by regeneration of sporophytes, the relations were very 

 different in autopolyploids and allopolyploids. In the former, increase in 

 chromosome number led to a rapid increase in cell size, each species 

 having a characteristic rate of increase for a given change in number. 

 In allopolyploids made from the hybrids between pure species which 

 are not too nearly related, the cell size increased rapidly between the 

 haploid and diploid, but much less in the higher multiples, eventually 

 becoming more or less constant, so that very high polyploids (e.g. 

 hexadekaploids i6a:) could be prepared. The mechanism of this reac- 

 tion is not fully understood. 



In allopolj^loids, the appearance of the organism is usually inter- 

 mediate between that of the two ancestral species, the exact appearance 

 depending on the balance between the chromosome sets and the 

 dominance relations. Again the artificial moss polyploids provide a very 

 clear example. (Cf. p. 171). 



2. Meiotic Behaviour and Segregation in Polyploids 



The behaviour of the chromosomes of a polyploid at meiosis, and 

 therefore the segregation of genetic factors, depends on whether the 

 organism is a auto- or allo-polyploid. 



2a. Autopolyploids 



In an autopolyploid, all the homologous chromosomes of any one 

 kind may pair in a compound body at zygotene. The pairing, however, 

 is always strictly in twos at any one place, the compound body being 

 held together only by exchanges of partner among the threads, rather 

 as the four diakinesis threads in a diploid are held together at a later 

 stage. This pairing in twos can be directly observed in cytological 

 preparations, and is also indicated by the proportions of recurrent and 

 progressive cross-overs in triploid Drosophila (p. 107). 



The pairing seems to take place by a more or less random coming 

 together of the threads, but the randomness is modified by two factors. 



^ Wettstein 1926, 1927, 1937. 



