ALLOPOLYPLOIDY 



parental species, for example in regard to the mealiness of the leaves. 

 Secondly, one or two quadrivalents are seen at meiosis in each mother 

 cell. These quadrivalents behave like those in autopolyploids ; by 

 their unequal three-and-one separation they give rise to gametes and 

 to progeny with unbalanced chromosome sets. Some of these 3 5- 

 and 37-chromosome plants live and are seen to be deformed, while 

 others evidently die and thereby reduce the fertility (Table 16). 



TABLE 16 



DISTRIBUTION OF THE CHROMOSOMES AS SHOWN BY 

 THE FREQUENCIES OF POLLEN GRAINS WITH DIF- 

 FERENT NUMBERS IN THE TETRAPLOID PRIMULA 

 KEWENSIS; x = 9 AND n = 18, THE MEAN EXPECTED 

 WITHOUT LOSS (UPCOTT, 1939) 



Apart from the occasional quadrivalents of the Primula the same 

 principle applies to both these allopolyploids. The differences which 

 cause sterility in the diploid, whether with good pairing or with bad 

 pairing, in the tetraploid cause a preferential pairing of identical 

 partners, and this favours regular pairing (without quadrivalents or 

 univalents), regular segregation, and in consequence a high fertility. 

 The association of more than two chromosomes is the danger 

 to the fertility of the polyploid. The association of less than two is 

 the danger to the diploid. For this reason the more sterile is a 

 diploid, the more fertile will be the tetraploid to which it gives 

 rise. In fact completely fertile diploids like Primula sinensis, Datura 

 stramonium or the tomato give rise to tetraploids of reduced fertility. 

 And for this reason, too, when we find autopolyploid species of 

 flowering plants it is because, as with the pentaploid Tulipa clusiana, 

 seed production has ceased to matter or, as with the tetraploid 

 T. turkestanica or octoploid Dahlia variabilis, no chromosome forms 

 more than one chiasma and so quadrivalents are evaded (Fig. 35). 



Between the two types of hybrid, in which structural hybridity 

 is too slight or too great to be defined, lie a host of crosses between 

 species, like those in Lilium or Drosophila, in which the chromosomes 

 of the parents differ recognizably by inversions, interchanges, dupli- 



139 



