HETEROPLOIDY 345 



the tissue concerned, giving one type of "somatic mutation," but it 

 affects the next generation only if it occurs in the hneage of the reproduc- 

 tive cells. -^ 



Very often, especially in meiocytes, one or more of the chromosomes 

 may fail to pass far enough toward either pole to be included in one of the 

 new nuclei. This is known as lagging, and it may play a very important 

 role in determining the make-up of the chromosome complements of 

 spores, gametes, and offspring, particularly in hybrids. The extra 

 chromosome in 2n + 1 types is very often lost through lagging, decreas- 

 ing the expected number of n + 1 gametes. 



Sometimes the entire complement of chromosomes behaves in an 

 aberrant manner. In the meiocytes the synaptic reaction may be defi- 

 cient, some of the chromosomes failing to conjugate (asynapsis) or losing 

 their synaptic association prematurely (desynapsis) . Irregularities in 

 chromosome distribution naturally follow. In case the whole complement 

 is involved, complete ameiosis may result; the chromosomes may all 

 divide equationally in the single mitosis which occurs, so that diploid 

 spores or gametes are formed.-'* A union of such a gamete with a normal 

 monoploid gamete results in a triploid individual.-^ When ameiosis 

 occurs in both the microsporocytes and the megasporocytes, two diploid 

 gametes may unite to form a tetraploid individual. 



Similar results may follow from 7ion-division in somatic cells. Here 

 the chromosomes split, thus doubling their number, but there is no divi- 

 sion of the nucleus as a whole. It is probable that such an aberration 

 occurs frequently in the zygote or young embryo, giving a tetraploid 

 individual directly. ^^ The formation of a tetraploid plant in this way has 

 been artificially induced by heating young ears of Zea soon after fertiliza- 



22 Non-disjunction, originally discovered by R. Gates (1908) in CEnothera, is respon- 

 sible for some of the 2n + 1 chromosome mutants of CEvothera and Datura. In 

 Datura, Belhng and Blakeslee (19246) saw eight cases of it in 1,137 sporocytes; in 

 Uvularia Belhng (1925a) saw 10 cases in three buds. Other examples are Nicotiana 

 (Goodspeed, 1923; Ruttle, 1927) and Zea. In Nicotiana tabacum the forms called 

 "enlarged" and "fluted" have 2n + 1 and 2n — 1 chromosomes, respectively 

 (Clausen and Goodspeed, 1924, 1926a). 



2* This may be the reason why in Zea, for example, tetraploid plants so rarely arise 

 directly from diploids with aberrant sporogenesis. Aberrations at this stage more 

 often lead to triploidy. 



-* Triploidy was first discovered in CEnothera by Stomps (1912) and Lutz (1912) 

 and was explained by them in the above manner. Triploids have since been produced 

 frequently by crossing diploids with forms producing occasional diploid pollen grains, 

 e.g., in Hyacinthus (de Mol, 1923). 



26 Gates (19096, 1913, 1924a) and Davis (1911) for CEnothera and other genera; 

 Strasburger (19106) for CEnothera and Wikstrannia; Tischler (1910) for Musa; Sax 

 (1921, 1922) for Triticum; Tahara (1921) for Chnjsanthemum; Afzelius (1924) for 

 Senecio; Clausen and Goodspeed (1925) for Nicotiana. Winge (1917) attributes such 

 doubling in hybrid zygotes to chromosomal incompatibility (see p. 369). 



