CHAPTER VI 

 THE BEHAVIOUR OF POLYPLOIDS 



Autopolyploids and Allopolyploids — Origin in Experiment — Meiosis in 

 Allopolyploids — Differential Affinity — Autosyndesis — Classification of Polyploid 

 Species. 



The production of an indubitable sterile hybrid from completely fertile 

 parents which have arisen under critical observ^ation from a single common 

 origin is the event for which we wait. 



Bateson, 1922. 



I. THE ORIGIN OF POLYPLOIDS IN EXPERIMENT 



A POLYPLOID is an organism with more than two complete sets of 

 chromosomes (Ch. III). When the chromosome number is doubled 

 in a homozygous diploid such as Datura Stramonium, the new 

 organism has four identical sets of chromosomes. Such a form is 

 known as an Autopolyploid (Kihara and Ono, 1926). Its 

 chromosome complement can be represented as of the constitution 

 A ^A ^A ^A j^, B iB ^B ^B j^, and so on. Its chromosomes have a high 

 frequency of quadrivalent formation, subject to random chiasma 

 formation amongst the four homologues of each kind (Ch. IV). 

 Where the chiasma frequency is low, as in the short chromosomes 

 of Hyacinthus and in some chromosomes of Solanum and 

 Primula, bivalents are formed instead in a proportion of cells {v. 

 infra) . 



In contradistinction to the autopolyploid, the product of doubling 

 in hybrids is known as an Allopolyploid. The first of this type 

 to be discovered and understood was in the genus Pygcera, but 

 apart from this moth our knowledge of allopolyploids is derived 

 exclusively from the flowering plants. If we take the diploid 

 hybrid to be made up of pairs of chromosomes A^A^,, B^B^ — we 

 may represent the tetraploid as A^A^, A^A^, B-^B^, B^B2. Such 

 an allotetraploid is homozygous inasmuch as it may be derived 

 immediately from the union of two genetically identical gametes ; 



183 



