THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 26l 



have been made between Triticiim and these genera,^ and some new 

 polyploid hybrids have been synthesized which may have economic 

 importance (e.g the "octoploid" wheat-rye hybrid, derived from 

 doubling in a cross between hexaploid wheat and diploid rye).^ 



In other genera the analysis of the constitution of polyploids has been 

 made, with more or less certainty, purely on groimds of analogy. An 

 example is Spartina Townsendii, a species which appeared suddenly in 

 the latter half of the nineteenth century and is intermediate between 

 5. altemiflora (in = ioa: = 70) and 5. stricta (in = 8;c == 56). 

 Huskins^ showed that S. Townsendii has 2w = 126, and claimed that it 

 is an allo-octoendekaploid (i8;c) got by doubling in a hybrid between 

 these two species. 



There are many other cases in which the taxonomic relations of 

 species have been confirmed by the evidence of chromosome numbers 

 and plausible suggestions of polyploidy can be put forward. 



4. Secondary Polyploids 



Species which are not related as polyploids may differ in more 

 complex chromosomal changes. Perhaps the simplest type of change is 

 the reduplication of one or more chromosomes. Usually trisomic and 

 still more tetrasomic types are relatively inviable, but in higher-multiple 

 polyploids the reduplication of a few chromosomes does not cause such 

 unbalance, and a process of this kind seems in fact to have played some 

 part in plant evolution. Thus apples and pears belong to the family 

 Rosaceae, the greater part of which forms a polyploid assemblage with 

 a basic number of 7. In the Pomoideae section, however, the basic 

 number is 17, and it appears that this number may represent a trebly 

 trisomic diploid, i.e. a diploid with 3 reduplicated chromosomes 

 {2x + 3), this whole set being balanced and forming the basic number 

 of a new polyploid series.'* This phenomenon, which is known as secon- 

 dary polyploidy, is shown not only by the exceptional ratios obtained 

 with segregating factors, but also by the evidence of the secondary 

 pairing which happens to be very strong in Pomoideae. Secondary 

 pairing, which is also strongly developed in Diptera (p. 74), causes 

 the homologous split chromosomes to lie near each other at metaphase 

 in mitosis, or the two similar chromosome pairs to lie near together in 

 meiosis in a tetraploid. In an apple with 34 chromosomes (i.e diploid 

 in respect of its basic number 17) one finds three groups of three 



^ Cf. Kihara and Lilienfeld 1932. - Meister 1030. 



^ Huskins 1930. ^ Darlington and MofFett 1930, Moffett 1931. 



