HETEROPLOIDY 347 



cyte; occasionally it is the mitotic figures themselves which unite. ^^ 

 The result in either case would be unreduced spore nuclei. The role 

 of the last mentioned aberrations, as well as that of abnormal nuclear 

 fusions occasionally seen at other stages, in the production of heteroploidy 

 is problematical. The fusion of two male gametes with the egg (dis- 

 permy) may possibly lead to triploidy in some cases. ^^ 



Although there are many ways in which the chromosome number may 

 be altered, it is probable that tetraploidy arises most often through a 

 doubhng of the number in a diploid zygote or young embryo, and that 

 triploidy is usually the result of a union between a monoploid and a 

 diploid gamete, the latter being formed either after meiosis in a tetraploid 

 plant or after ameiosis in a diploid one. Further increases in number may 

 follow. Doubling in a triploid gives a hexaploid; crossing a hexaploid 

 with a tetraploid may give a pentaploid ; doubling in a tetraploid, or cross- 

 ing a hexaploid with a diploid followed by doubling, may giveanoctoploid, 

 and so on. Thus it appears that the origin of tetraploidy is often the first 

 step in the development of higher euploid types. Aneuploid plants with 

 one or two chromosomes lacking or in excess are probably due chiefly 

 to non-disjunction; and since such plants produce spores and gametes 

 carrying various chromosome numbers, further unions may extend the 

 aneuploid series. As would be expected, many of the altered types fail 

 to meet the test of viability; or, if viable, they may fail to compete suc- 

 cessfully with normal types because of their reduced vigor or fertility. 

 Nevertheless, the fact that the monoploid chromosome numbers of so few 

 flowering plants, relatively speaking, are prime numbers indicates the 

 important role of polyploidy in the evolution of this great group (Winge, 

 1917). 



With regard to the evolution of new "basic" chromosome numbers, 

 from which new polyploid series may in turn be developed, it seems 

 improbable that simple additions or losses of whole chromosomes are very 

 significant in this connection, because of the physiological unbalance 

 which such changes introduce. If, however, they are accompanied by 

 compensatory alterations, they may become very important. Thus M. 

 Nawaschin (1932) cites evidence suggesting the following "dislocation 

 hypothesis": An extra chromosome is first added in one of the ways 

 mentioned above, so that the somatic cells of an individual contain three 



^ Gates (1915) on (Enothera, Rosenberg (1917) on Hieracium, Blackburn and 

 Harrison (1921) on Rosa, Ljungdahl (1922) on Papaver, McClintock (1929a) on trip- 

 loid Zea, Prywer (1931) on Beta. In Nereis oocytes treated with ultra-violet light 

 both meiotic mitoses occur near the center of the cell; the four resulting nuclei may 

 then unite with the sperm nucleus and a pentaploid embryo may begin development 

 (Just, 1933). 



" Dispermy has been seen in Gagea (Nemec, 1912), (Enothera (Ishikawa, 1918), 

 and other plants. It can easily be brought about in echinoderm eggs (Boveri). 



