Results and Discussion
The hybrid between T. tetrgidum and the F1 amphidiploid
(Ae. sharonesis x T. monococcum) exhibited very low seed fertility
of 0.03% and only one viable seed was obtained out of 75 bagged heads. This
seed was germinated and its somatic chromosome number was found to be 2n=29.
Since the meiotic chromosome pairing of the hybrid was found to be irregular
(KUSHNIR & HALLORAN 1982a) with a mean of 8.11 bivalents per cell and with
maximum of 12 bivalents per cell, only rare chromosome segregation close
to normal gametic chromosome number of the tetraploid hybrid would be expected
to lead to the production of functional gametes and development of viable
seed. In these conditions the appearance of the extra chromosome in the
F2 progeny is not surprising. This F2 plant grew vigourously
with a phenotype intermediate between its parents with spikelets of intermediate
size and with the dominant blackish coloration from the F1 amphidiploid
parent (Fig. 1). It was examined cytologically (Table
1) and at first metaphase of meiosis it exhibited a mean bivalent frequency
of 13.48 per cell. The mean frequency of 2 univalents per cell and range
of 1-7 was caused mainly by the additional chromosome and, to lesser extent,
by some partial asynapsis in the chromosome set of the tetraploid hybrid.
The frequency of quadivalents was very low with only 0.01 per cell. T.
urartu is considered by SEARS (1981) to be a variety of T. monococcum
and hence the likely donor of the A genome of wheat. When it is crossed
with hexaploid common wheat, chromosome 4A is the only chromosome which
fails to pair with a chromosome from the diploid species (CHAPMAN et
al.1976; DVORAK 1976). It is therefore, assumed that the additional
chromosome in the F2 plant which appeared as a uinivalent in
all pollen mother cells in this study is either chromosome 4A of the T.
turgidum dicoccoides parent or chromosome 4Am from T. monococcum
of the F2 amphidiploid. This is the subject of a present study
and will be reported elsewhere. The F2 plant was of much higher
fertility than the F1 plant ; pollen fertility was high (92%)
and seed fertility was 54%. The seed was well-developed and viable. The
production of such seed in the hybrid between T. turgidum dicoccoides
and the F1 amphidiploid (Ae. sharonensis x T. monococcum)
is evidence for the possibility for gene flow between the genome of Ae.
sharonensis and the B genome of wheat. Although this event would probably
have been rare in nature it may have been of evolutionary significance.
|