The satellited chromosomes or their homoeologous in T. aestivum
are reported to have gene(s) controlling cytoplasmic effects, even though
satellites are not known to have any mapped genes. KIHARA (1951) reported
that a satellited Ae. caudata chromosome, which was homoeologous
to group 1 of the wheat chromosomes, had a major gene or genes controlling
male fertility restoration to the alloplasmic wheat plants with Ae. caudata
cytoplasm. MAAN (1977b, 1978) reported that the long arm of chromosome 1D
had a gene or genes controlling the effects due to Ae. squarrosa, Ae.
ventricosa, or Ae. cylindrica cytoplasms. BAHL and MAAN (1972)
reported that several of the independently derived wheat lines from crosses
involving cytoplasm donor species had male fertility restoring gene(s) on
chromosome 1A and on other satellited wheat chromosomes or their homoeologues.
This evidence may indicate that genes controlling metabolic activity of
the nucleolar organizer may be directly or indirectly involved in coordinating
nucleo-cytoplasmic interactions. The relative expressivity of these genes
in alien cytoplasms may correspond to the genetic distance between related
species, if gene products of the nuclear organizer are species-specific.
Possibly, mutations in the satellited chromosome segment are viable only
if they are accompanied or followed by complementary cytoplasmic mutations.
This may explain relative stability of the nuclear and the cytoplasmic systems
and the lack of marker genes on the satellited chromosome segments.
Possibly different sets of species-specific recessive genes control nucleo-cytoplasmic
interactions resulting in normal fertility and normal plant growth in related
diploid species. Interactions among these genes result in sterility, inviability
of hybrid progenies even though meiotic abnormalities in F1 resulting
from chromosomal structural differences between the parental species are
reduced or eliminated in certain hybrid plants. The plasma-sensitive nuclear
genes are not directly transferable from one diploid to another diploid
species, even when the sterility of the interspecific hybrids is bypassed
via polyploidy or by bridging crosses. For example, KIHARA (1959) reported
that Ae. umbellulata genome could not be substituted into the Ae.
speltoides cytoplasm, because it was an inviable nuclear cytoplasmic
combination. Also, KIHARA (1963) reported that Ae. uniaristata (2n=
14; MuMu) genome could not be substituted into Ae.
comosa (2n=14; MM) cytoplasm, because all functional female gametes
of the Ae. comosa female x Ae. uniaristata male F1
had a complete set of the Mu-genome chromosomes. Female gametes
with Mu-genome chromosomes did not function in Ae. comosa
cytoplasm of F1 plants. All plants from a backcross with Ae.
uniaristata had MMu-genomes and resembled F1 plants.
All plants resembling Ae. comosa were obtained from F1
x Ae. comosa, because all viable seeds from this backcross had embryos
with MM-genome. Reciprocal F1 was not obtained because Ae.
uniaristata as a female was cross-incompatable with Ae. comosa.
Therefore, characteristics of Ae. uniaristata cytoplasm were not
examined from direct crosses with Ae. comosa. However, different
interactions were obtained between T. durum genome and cytoplasms
of Ae. uniaristata or Ae. comosa which indicated that these
diploids had different cytoplasms (MAAN, 1978). T. durum plants with
Ae. uniaristata cytoplasm were viable only if they retained a critical
Ae, uniaristata chromosome arm (1977a). These plants were male sterile
and produced a few plump and viable seeds, and a larger number of shrivelled
and inviable seeds when crossed with T. durum as a male parent. The
plump and viable seeds again produced male-sterile plants with maternal
chromosome number which included the critical Ae. uniaristata chromosome
(arm). These results indicated that seeds having embryos with ciritical
Ae. uniaristata chromosome arm were viable and embryos without the
critical telocentric were inviable in Ae. uniaristata cytoplasm.
However, alloplasmic T. durum plants with Ae. uniaristata cytoplasm
and without the critical Ae. uniaristata cytoplasm were obtained
when T. aestivum with Ae. uniaristata cytoplasm was used as
the cytoplasmic source to develop T. durum with Ae. uniaristata
cytoplasm. T. aestivum with Ae. uniaristata cytoplasm was
fertile and did not contain critical Ae. uniaristata telocentric.
These results indicated that : 1) the presence of the critical Ae. uniaristata
chromosome in T. durum genome having Ae. uniaristata cytoplasm
controlled the abortion of seeds whose embryos lacked that telocentric.
This telocentric had gene or genes which restored vigor to T. durum
plants with Ae. uniaristata cytoplasm, because weak alloplasmic T.
durum plants were obtained when alloplasmic T. aestivum was used
as the source of Ae. uniaristata cytoplasm, 2) hexaploid T. aestivum
genome was not cytoplasm-specific and had homoeolleles which c.compensated
for the critical cytoplasm-specific nuclear genes of Ae. uniaristata.
Therefore, hexaploid T. aestivum genome is more compatable and less
cytoplasm-specific than tetraploid T. durum genome, and T. durum
genome is less cytoplasm-specific than diploid genomes of Ae. uniaristata
or Ae. comosa. |