Possibly, nuclear genomes of the polyploid species (i.e. T. aestivum)
have substantial inter-genomic variability because of the heterogeneity
(or multiplicity) of the homoeoalleles derived from the diploid parental
progenitor species. Therefore, polyploid species genomes may be less species-specific
in their interactions with certain alien cytoplasms than the genomes of
the diploid progenitors. Interactions between polyploid nuclear genomes
and the cytoplasms of two or more of the related species may result in apparently
similar plant phenotypes, or different genotypes of a polyploid species
may produce apparently different phenotypes in their interactions with the
same cytoplasm. For example, T. aestivum plants with the cytoplasms
of T. boeoticum (MAAN and LUCKEN 1970) or S. cereale (MAAN
and LUCKEN, 1971) had male sterility, greatly reduced vigor and reduced
seed viability ; the similar phenotype indicated similar interactions between
Triticum genomes and T. boeoticum or S. cereale cytoplasms.
Also, the difference in the extent of male sterility and delayed maturity
of alloplasmic T. aestivum cvs. Chris, Selkirk, and Chinese Spring
lines with Ae. ovata cytoplasms indicated that these nuclear genotypes
give different interactions with Ae. ovata cytoplasm (MAAN, unpublished).
These examples of apparent nonspecificity of nucleo-cytoplasmic interactions
between hexaploid T. aestivum genomes and T. boeoticum,
S. cereale or Ae. ovata cytoplasms can be explained as follows
: The Ae. ovata cytoplasm has similar effects on the expression of
various T. aestivum genotypes ; alloplasmic plants have reduced male
fertility and delayed maturity. The magnitude of the cytoplasmic effects
is modified by the nuclear genes in the three T. aestivum cultivars.
Similarly, alloplasmic T. aestivum with the cytoplasms of T. boeoticum
or S. cereale can be distinguished by a closer examination of anther
size and spore development. Alloplasmic plants with T. boeoticum
cytoplasm have smaller anthers with 0% stainable pollen grains and plants
with S. cereale cytoplasm have fairly well developed anthers with
some stainable pollen grains. Therefore, apparent non-specificity of the
interactions between T. aestivum genomes and the cytoplasms of T.
boeoticum, S. cereale, Ae. ovata or certain other species
does not distract from the other evidence for the specificity of interspecific
nucleo-cytoplasmic interactions.
Even though certain interactions between T. aestivum genomes and
the alien cytoplasma may appear to be less species-specific, the use of
the T. aestivum genome as a tester allows greater flexibility in
the choice of cytogenetic techniques that can be used, because aneuploid
stocks allow study of chromosomal location of nuclear genes controlling
cytoplasmic effects and relatively easy transfer of alien genes to wheat
chromosomes. Also, inherent heterogeniety of the parental genomes and the
homoeoalleles makes 6x T. aestivum more compatable with alien cytoplasms
than 4x T. durum. For example, T. aestivum with cytoplasms
of Ae. squarrosa, Ae. cylindrica, Ae. ventricosa, Ae.
crassa, Ae. juvenalis or Ae. uniaristata is fertile and
of near-normal vigor. T. durum is not viable in the cytoplasms of
Ae. squarrosa, Ae. cylindrica, or Ae. ventricosa. Plants of
T. durum in Ae. crassa, Ae. juvenalis or Ae. uniaristata
cytoplasms have greatly reduced vigor. Interactions with T. durum
genomes and the cytoplasms of these 6 Aegilops species indicate cytoplasmic
similarity between diploid Ae. squarrosa and tetraploid Ae. cylindrica
or Ae. ventricosa, and between diploid Ae. uniaristata and
polyploids Ae. crassa or Ae. juvenalis. Obviously, T. durum
is more species-specific than T. aestivum in its interactions with
alien cytoplasms (MAAN, 1978). |