Gates .— The Trisomic Mutations of Oenothera . 561 
ratios of laeta and velutina obtained by pollination from, e.g., Oe. Hookeri 
or Oe. strigosa , according to whether the extra chromosome in the mutant 
was carrying laeta or velutina. 
Summary. 
This paper records the occurrence in the F\ of Oenothera rubricalyx x Oe. 
Hezvettii of two mutants having 15 chromosomes, but of a type different 
from Oe. lata and having viable pollen. It probably most nearly resembles 
some of the semilata mutations, which also have 15 chromosomes. The 
occurrence of a pair in such cases is significant, indicating that non-disjunc¬ 
tion of a pair of chromosomes in the heterotypic division of a pollen mother¬ 
cell of the male parent (in this case Oe. Hezvettii) led to the formation of two 
pollen grains having eight chromosomes, both of which afterwards func¬ 
tioned in fertilizing eggs. 
In diakinesis in the pollen mother-cells of this mutant as many as five 
ring-pairs of chromosomes are found. One of these ring-pairs may perhaps 
be descended from the original pair of chromosomes which underwent non¬ 
disjunction in a pollen mother-cell of Oe. Hezvettii. The others must be 
formed by the union of homologous chromosomes from rubricalyx and 
Hewettii. The presence of chromosome rings, while an indication of strong 
attraction between synaptic mates, cannot therefore be regarded as evidence 
that the species is homozygous. 
Some of the chromosome-rings persist on the heterotypic spindle even 
until metaphase. The later irregularities in the chromosome behaviour of 
this mutant are not so great as in Oe. lata , but chromosomes are frequently 
left behind in the cytoplasm. 
This is followed by a discussion of all the trisomic forms of Oenothera 
(having 15 or 16 chromosomes). The origin of mutations with 14 chromo¬ 
somes such as nanella could be accounted for by double non-disjunction as 
well as through crossing-over, but neither theory explains the later genetic 
behaviour of these forms. 
The evidence now indicates that forms with aberrant chromosome 
numbers make up the great majority of Oenothera mutations. There is 
an older group of 15-chromosome mutants from Oenothera Lamarckiana 
which includes lata , scintillans , alb id a, oblonga , subovata , and probably 
several others. More recently a series of others more like Oe. Lamarckiana 
has been recognized, including cana> pallescens , Lactuca , and liquida. 
Apparently lata x Lamarckiana can give rise to any of these. 
* Trisomic mutants appear much more frequently in the offspring of lata . 
than in the offspring of Lajnarckiana. Again, several at least of the 
simple trisomic mutants are interchangeable. Thus lata gives rise to scin¬ 
tillans and scintillans to lata. The view is developed that this results from 
secondary irregular chromosome distributions. 
P p 2 
