| (50') |
The theoretical frequency of F2-plants
with various chromosome numbers is in good accord with the observed. |
| (51)T |
How will the chromosome numbers change
in successive generations ? |
| (52) |
In the offspring of 28- and 42-chromosome
plants these parental chromosome numbers remain constant. Among the
29- to 41-chromosome plants the 35-chromosome ones behave like the
F1-hybrid. The others can be divided into 2 groups, one
with 29 to 34, the other with 36 to 41 chromosomes. The former decreases
its chromosome number generation by generation until 28. It is called
"decreasing group". The latter increases the number of chromosomes
in successive generations until 42 and is called "increasing
group". |
| (53) |
In both groups fertility and morphological
characters approach those of the respective parent hand in hand with
the decrease or increase of chromosome numbers. |
| (53') |
Constant plants with intermediate chromosome
numbers are completely or nearly sterile and their viability is strongly
reduced.
It became evident that to keep reproductive and physiological functions
on a normal level, the 7 chromosome-sets have to be complete. Such
sets are called "genomes". |
| (54) |
From this study it was for the first time
made clear that the behavior of hybrids between species with different
chromosome numbers follows certain definite rules. |
| (55) |
In a pentaploid wheat hybrid 2 homologous
pairs of genomes, AA and BB, form 14 bivalents. 7 univalents are derived
from the Dinkel parent. If these are lacking, the chromosome number
reverts to 28, the number of the Emmer parent. On the other hand,
if all 7 univalents have been doubled, a 42- chromosome plant is the
result, having the chromosome number of the Dinkel parent. |
| (56) |
The third chromosome-set, typical of Dinkel,
was designated, by Kihara as the D-genome. |
| (57) |
Therefore, the Dinkel group has the genome
formula AABBDD. |
| (58) |
From the pentaploid hybrids it has become
evident that one of the ancestors must have been tetraploid AABB wheat.
But which related species contributed the D-genome? A tetraploid grass,
Aegilops cylindrica, related to wheat contains this genome,
besides another genome, C, known from Aegilops caudata. Therefore,
Aegilops cylindrica itself could not have contributed directly
to the make-up of bread wheat. Kihara concluded that only a diploid
grass of the genome constitution DD could be the long sought after
other ancestor. |
| (59) |
Now, what morphological characteristics
should have a DD-plant, supposed to be the ancestor of bread wheat
as well as Aegilaps cylindrica? Let us follow Kihara's train
of thoughts. |