Specificity of these nucleo-cytoplasmic interactions was also indicated
by the following observations : An addition of chromosome 1D to the T.
durum genome was essential for viability of T. durum with the
cytoplasm of Ae. squarrosa, Ae. cylindrica, or Ae. ventricosa.
Therefore, chromosome 1D has critical nuclear genes controlling effects
conditioned by these cytoplasms. These genes were not essential for viability
of T. durum with the cytoplasms of Ae. uniaristata, Ae.
crassa or Ae. juvenalis. Apparently, chromosome 1D or another
D-genome chromosome has homoeoalleles which make T. aestivum fertile
in the cytoplasm of Ae. uniaristata, Ae crassa, or Ae. juvenalis,
even though alloplasmic plants of T. aestivum in these three cytoplasms
are less productive than those with cytoplasms of Ae. squarrosa, Ae.
cylindrica, or Ae. ventricosa. The critical gene(s) on an Ae.
uniaristata chromosome arm which restored plant vigor to T. durum
with Ae. uniaristata cytoplasm did not have male fertility restoring
genes (MAAN, 1977a). These plants were male sterile, and the effect of the
critical Ae. uniaristata chromosome remains to be examined in the
cytoplasms of Ae. crassa or Ae. juvenalis. These results further
indicate that polyploid nuclear genomes may be useful in the study of nucleo-cyioplasmic
interactions involving specific nuclear genes or chromosome segrnents and
alien cytoplasms.
In the studies of nucleo-cytoplasmic interactions between Triticum
genomes and alien cytoplasms, the cytoplasm donor species are crossed as
females, and Triticum species are used as the recurrent male parents.
Two types of plants appear in the backcross progenies. In the first type
of plant, all of the nuclear genes from the cytoplasm donor species are
eliminated, and the nuclear genome of the recurrent male parent is substituted
into the alien species cytoplasm. Plants so obtained may have cytoplasmically
inherited male sterility or other cytoplasmic effects such as reduced plant
vigor and enhanced sensitivity to environmental conditions. The male-sterile
plants may be maintained by repeated backcrossing with the recurrent male
parent. In the second type of plant, certain critical nuclear genes from
the genome of the female cytoplasm donor are transferred to the genome of
the recurrent male parent when parental chromosomes pair in the F1
or in the backcross plants. Alternatively, a chromosome or a chromosome
arm with critical nuclear gene(s) is transferred to the genome of the recurrent
male parent, if parental chromosomes do not pair in the F1. Plants
of the second type have noticeably improved male fertility and/or plant
vigor when compared to the first types and may be maintained by selfing,
if fertile. Usually most of the fertile alloplasmic plants are less vigorous,
and less productive than euplasmic controls used as the recurrent male parent
; either the critical nuclear gene(s) from the cytoplasm donor species are
not fully expressed when transferred to the wheat genome, or not all of
the genes necessary for complete control of cytoplasmic effects to produce
a normal phenotype are transferred to the wheat genome by the backcrossing
procedures. However, alloplasmic plants comparable to euplasmic controls
in plant vigor or productivity can be produced by repeated selection and
intercrossing of plants with maximum expression of the critical nuclear
genes controlling cytoplasmic effects. In plants having normal vigor and
fertility, certain genes with detrimental effects may be eliminated during
backcrossing and selection procedures, or certain genes in T. aestivum
may complement specific nuclear genes in improving nucleo-cytoplasmic interactions
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