24 Mutations and Evolution. 



CHAPTER III. 

 FORMS HAVING AN EXTRA CHROMOSOME. 



For a long time it was generally supposed that (E. lata and 

 forms showing similar characters were the only ones containing an 

 extra chromosome. Fifteen chromosomes had been announced in 

 certain other forms by Miss Lutz (1908), but without convincing 

 evidence. Then a very different form, (E. incurvata, having 15 

 chromosomes, was described from the Swedish lamarckiana, 1 and 

 Miss Lutz (1916, 1917) has more recently published her studies, 

 which seem to show that the occurrence of mutations with an extra 

 chromosome is much more frequent than was formerly supposed. 

 Hance has also (1918) found 15 chromosomes in the mutant 

 scintillans. This raises problems with regard to the relationship 

 between these forms. 



It was natural to suggest that there might be seven distinct 

 types in this series, 2 with a different chromosome of the haploid 

 series figuring in each case. The number of types apparently with 

 15 chromosomes is now considerably in excess of seven, but as will 

 be observed from Table I, some of these appear to arise only from 

 lata x lamarkiana. This is undoubtedly a significant fact. 



Now if we call the haploid series of chromosomes in 

 (Enothera ABCDEFG and assume that A is the extra chromosome 

 in lata, then its chromosome formula would be AAABBCCDDEE- 

 FFGG ; and when the odd chromosome passes undivided to one 

 pole in the reduction division without further irregularities, the 

 gametes formed will be ABCDEFG and AABCDEFG. That is, 

 there are three members of the A chromosome, two of which enter 

 one gamete and one the other. It seems clear that such 

 15-chromosome forms as semilata Gates, exilis, exundans and Miss 

 Lutz's type 5365 are derived through a secondary modification 

 from lata. The evidence is strong that semilata Gates at any rate 

 arises only from lata x lamarckiana. It appears very probable, 

 on account of its characters intermediate between those of lata 

 and lamarckiana. and also from its origin only from lata, that the 

 extra chromosome is the same in both. 



In order to have a different chromosome become the extra one 

 in the sense in which it appears to be in lata, by selflng this 

 mutant (See Table 1), we must suppose that a gamete, which is, say, 



1 The Mutation Factor, p. 147. 

 * Gates and Thomas, 1914. 



