INTERCHANGE IN POLYPLOIDS 157 



A number of triploids have more than the haploid number of 

 paired chromosomes at meiosis, although they are derived from 

 diploids showing no evidence of a polyploid origin (cf. Ch. VII), 

 e.g., in Festuca, Fragaria and Triticum with basic numbers of seven. 



This can be accounted for in one of two ways : the diploid from 

 which they have been derived may have been either (i) an inter- 

 change heterozygote or (ii) a reduplication homozygote. The 

 triploid would have a segmental constitution in the two cases as 

 follows : — 



Either type can form three pairs instead of the expected two 

 trivalents. 



Since all the triploids in which this behaviour is found are inter- 

 specific hybrids of ABC or A AB types {cf. Ch. VI), or, in Festuca, 

 (Enothera and Campanula, the offspring of interchange hetero- 

 zygotes, the heterozygote rather than the homozygote explanation 

 is favoured. Homozygous reduplication may also have contributed 

 to the result in some cases. 



A clearly defined heterozygote is the triploid derived from 

 (Enothera pycnocarpa. The parent has a ring of 14 chromosomes 

 which are presumably of the segmental constitution : 



AB CD EF GH KL MN OP 

 BC DE FG HK LM NO PA 



the upper row of chromosomes being derived from one gamete, the 

 lower from the other. 



The fertilisation of an unreduced gamete (with the constitution 

 of the zygote) by a normal gamete has given a triploid of the 

 constitution : — 



AB CD EF 

 AB CD EF 



BC DE FG, etc. 



