Usually, a large number of the plant traits are modified as a result of
interaction between a Triticum genome and alien cytoplasms. The magnitude
of cytoplsmic effects depends on the nuclear genotype as well as the environmental
conditions in which plants are grown. Possibly, several plasma-sensitive
nuclear genes in the Triticum genome may produce observed phenotypic
alterations, and a similar number of the critical nuclear genes from the
respective cytoplasm donor species control the effects on the expression
of the plasma-sensitive genes. A majority of the critical nuclear genes
remain closely linked because of their location on an alien chromosome segment
which may not (or may less often) pair with a homeologous wheat chromosome
segment. For example, the 43-chromosome (monosomic-addition) wheat plants
with T. boeoticum cytoplasm and a critical T. boeoticum chromosome
have near-normal fertility and plant vigor (MAAN, unpublished). They produce
a few plump and viable seeds having embryos with the critical T. boeoticum
chromosome, but a majority of the seeds lack the critical chromosome and
are shrivelled. Some of the shrivelled seeds are viable and they produce
male-sterile plants of greatly reduced vigor. After one or two additional
backcrosses, only inviable seeds are produced. Similar results are obtained
from the 43-chromosome (monosomic-addition) wheat plants with a critical
Secale cereale chromosome and S. cereale cytoplasm (MAAN and
LUCKEN, 1972). The T. boeoticum and S. cereale cytoplasms
induce male sterility and reduction of plant vigor in wheat plants. The
plants with T. boeoticum cytoplasm have sterile pollen grains and
smaller anthers than those with S. cereale cytoplasm. Therefore.
T. boeoticum and S. cereale cytoplasms affect different plasma-sensitive
genes of T. aestivum. Also, nuclear genes on the critical T. boeoticum
chromosome do not control effects due to S. cereale cytoplasm and
genes on critical S. cereale chromosome do not control effects due
to T. boeoticum cytoplasm. Therefore, the species-specific plasma-sensitive
genes of T. aestivum which interact with T. boeoticum cytoplasm
may be different from those which interact with S. cereale cytoplasm,
and specific nuclear genes on critical T. boeoticum or S. cereale
chromosomes control effects due to these cytoplasms.
Nucleo-cytoplasmic interactions involving Triticum genomes and Aegilops
cytoplasms indicate that several species-specific plasma-senstive genes
from the cytoplasm donor species control these interactions. Since interspecific
hybrid sterility and loss of plant vigor of hybrid progenies are among the
major genetic isolating mechanisms in related plant species, the interspecific
nucleocytoplasmic differentiation may be intimately involved in the evolutionary
processes separating species. Therefore, interspecific and intergeneric
nucleocytoplasmic interactions may indicate relative genetic relationships
among the species of Triticum and Aegilops. The inviability
of T. aestivum in the cytoplasms of T. boeoticum or S.
cereale in the absence of specific nuclear genes from the respective
cytoplasm donors and specificity of the cirtical nuclear genes controlling
cytoplasmic effects indicates a remote genetic distance between T. boeoticum
and S. cereale and between each of these two species and T. aestivum.
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