| Meiotic pairing in the addition lines is rarely so regular as in Holdfast,
but regularity differs markedly between different additions. The irregularity,
which is usually asynapsis of the alien pair, is perhaps due to the lack
of balance between the chromosomes of the donor and the receptor species.
Thus the genetical control and synchronisation of meiosis, for which selection
has long operated in wheat through the mutual adjustment of the chromosomes,
is not effective for the alien pair. Such irregularities are likely to lead
to reduced fertility and to the instability of the addition condition, but
it may be that balanced and integrated combinations could be produced by
selection. However, only one variety of wheat was used in making the present
additions so as to preserve an agriculturally tested genotype. Further,
by the methods by which the disomic additions were produced the added alien
chromosomcs are completely homozygous. Heterozygosity must therefore be
introduced if selective adjustment is to be made in the addition lines.
Crosses are therefore being made between disomic additions and Triticale
derived from the same wheat variety, Holdfast, but a different rye gametes.
In the F2 of these, crosses it should be possible to extract
44 chromosome disomic additions in which some factors of the alien pair
are heterozygous, but those of the wheat chromosomes are homozygous. Selection
may be practised in subsequent generations for fertility and genetical stability,
and so for arrangements of the alien pair which are genetically integrated
with the receptor genotype. It is possible that the stability of alien chromosome
substitution lines may be improved if the alien pair has been previously
adjusted in this way. |