This paper contains a study of the lata and semilata series of mutations in Œnothera. Twenty-one such plants in cultures from various sources were examined and 15 chromosomes have been found in every case, though the mutants were derived from 14-chromosome races. The races in question were from such diverse sources as Sweden, Hungary, Madrid, Birkenhead, the cultures of de Vries, and the offspring of certain hybrids.
Especial interest attaches to a 15-chromosome mutant called (Œ. lata rubricalyx, which appeared in the F2 of Œ. mut. rubricalyx x Œ. grandiflora, and which combined the foliage and habit of lata with red pigmentation inherited from rubricalyx. Similarly the race of (Œ. biennis L. from Madrid contained a l a t a mutation, having lata-like foliage and the small flowers of biennis.
These cases, together with the previous work, prove that the peculiar characters of l a t a and semilata are constantly associated with the presence of 15 chromosomes, even when combined with other characters derived by inheritance from 14-chromosome individuals.
These mutants with 15 chromosomes have acquired the extra one by the occasional distribution of two chromosomes of a pair to the same daughter-nucleus in the reduction division, such occurrences having been discovered by one of us in 1908. The inconstancy of lata and semilata is explained by the behaviour of the extra chromosome, as is also the fact that lata x Lamarckiana gives both parent types in the F1, since lata produces some germ-cells having 7 and some having 8 chromosomes. The proportion of latas in the cross varies widely, from 4 per cent, to 45 per cent., the percentage being determined by the number of 8-chromosome germ-cells which mature. The fluctuation in this ratio is probably caused (1) by the environmental conditions at the time the germ-cells are being developed, and (2) by the physiological condition of the mother plant at this time. The various other hereditary peculiarities of lata and semilata are also explained by the presence of the extra chromosome.
The cause of the variability in the lata - semilata series of forms is at present obscure, but it may depend on the irregular distribution of portions of chromosomes during meiosis.
The extra chromosome is associated with the foliage and habit of lata or semilata in the same way that one of the sex chromosomes is associated with sex in such insects as Anasa tristis, with this difference, that in Œ. mut. lata or semilata the extra chromosome is a triplicate of a pair already present, while in these Hemiptera the presence of the accessory chromosome in duplicate is constantly associated with the female sex.
The extra chromosome in Œnothera resembles more closely in some respects the supernumerary chromosomes described by Wilson in Metapodius. The latter arise in a similar way, by the passing of botli idiochromosomes into the same nucleus in meiosis, but they are duplicates of the Y-element in sex determination, and as such apparently have no effect on the external characters of the organism.
The view is expressed that the initial nuclear difference, in having 15 instead of 14 chromosomes, determines the change in the external characters in the Œ. lata-semilata series of mutants. And the fact is emphasised that the chromosome number is a primary character of any organism, originating in the fertilised egg, while the external features are all secondary in development.
A number of different types of meiotic irregularity are described in the lata and semi la t a mutants. These are to be looked upon as germinal changes, though for the most part their products degenerate. The irregularities studied include the division of a chromosome (probably the extra one) on the heterotypic spindle, and various other types of fragmentation and degeneration of chromosomes, enumerated on p. 549.
The fact that lata-like mutations appear sporadically in various races, species and interspecific hybrids of (Enothera, combining in some cases the hybrid characters of their ancestors with those of lata, shows, as does the presence of the extra chromosome, that mutations and Mendelian hybrids should be contrasted; for they owe their origin to distinct causes, the former to a germinal change, the latter to a redistribution of the parental characters.
Memoirs: A Cytological Study of Œnothera mut. lata and Œmut. Semilata in Relation to Mutation