200 



THE BEHAVIOUR OF POLYPLOIDS 



opposite parents in a hybrid (Ljungdahl, 1924). Both consist in 

 the pairing of dissimilar chromosomes, and the distinction does not 

 arise in a true-breeding plant where identical chromosomes pair. 

 The distinction between the two is merely relative to the immediate 

 parents, but the occurrence of autosyndesis is significant in an 

 allopolyploid's derivatives because it means that chromosomes 

 pair in the absence of identical mates, although they rarely or never 

 pair in the presence of such mates. 



The distinction may be shown by symbols as follows (the hyphen 

 joins the chromosomes that pass to opposite poles following pair- 

 ing) :— 



Hybrid 



Autosyndesis occurs in allopolyploids under the following 

 conditions : — 



1. In the normal polyploid species as an exceptional occurrence. 

 Thus quadrivalents are occasionally found in the tetraploid Primula 

 kewensis and in many allopolyploid species. In the hexaploids 

 Triticum vulgar e and Avena sativa quadrivalents are found (Huskins, 

 1927, 1928). In all these instances aberrant offspring appear which 

 evidently result from the segregation of dissimilar chromosomes 

 (i.e.f through exceptional autosyndesis), since they have in a more 

 marked degree the characters of one of the parental species (in 

 Primula kewensis) or of an ancestral species (in Triticum and 

 Avena). In aberrant plants of Avena sativa a genotypic abnormality 

 prevents the chromosomes, or some of them, from pairing. These 

 chromosomes or parts of chromosomes are therefore removed from 

 competition. Autosyndesis occurs and multivalents are more 

 frequently formed than in the regular and normal type (Ch. X). 



2. In the " haploid " derived partheno genetically from a polyploid. 

 Thus if Solanum nigrum (6x) consists of six sets, AABBCC, its 



