|
The chromosome number in root tip cells of the hybrid is
2n=35. Since the chromosome number of the hexaploid wheats
is 2n=42, and that of A. distichum 2n=28, the hybrid
nature of the plant obtained from their cross was
proved.
Young ears of the hybrid were fixed in Carnoy's acetic
alcohol fixative. The pollen mother cells (PMC's) were
stained according to the Feulgen procedure and squashed in
acetocarmine. The chromosome associations at the first
meiotic metaphase were studied in 300 PMC's. The results
given in Table 1 show that a mean
of 11 , 2 chromosomes per PMC associate in bivalents,
trivalents and quadrivalents, and these associations have a
mean chiasma frequency of 6,59. As much as 31, 6 percent of
the PMC's have 14 or more chromosomes associated in
bivalents, trivalents and quadrivalents.
These results are similar to the levels of chromosome
association that have been reported in other wheat x
polyploid Agropyron hybrids by MATSUMURA &
MURAMATSU (1956) and POPE & LOVE (1952). When these high
levels of chromosome association in the lnia 66 x A.
distichum hybrid are compared with the meiotic
associations found in polyhaploid wheat by KIMBER &
RILEY (1963). MARTIN & CHAPMAN (1977), and MILLAR &
RILEY (1972), it is evident that in the hybrid only one or
two bivalents may result from the autosyndetic associations
of the homoeologous chromosomes in the three wheat genomes.
The high incidence of associated chromosomes in the hybrid
must therefore be due to one or more of the following :
a. India 66 and A. distichum have one genome in
common, or the homoeology of many chromosomes of these two
species are so close that allosyndetic associations occur
readily.
b. A. distichum is an autopolyploid or
segmental-allopolyploid and as such its two genomes in the
hybrid are able to produce autosyndetic associations. It is
known from the investigations of CAUDERON (1966), and
SCHULZ-SCHAEFFER & ALLERDICE (1966), that many
Agropyron species are indeed auto-allopolyploids or
segmental-allopolyploids.
d. The A. distichum genotype like those of T.
speltoides (Ae. speltoides) and T.
tripsacoides (Ae. mutica) (RILEY, 1966; RILEY
& KIMBER, 1966) suppresses the diploidising activity of
the Ph gene of hexaploid wheat, with the result that
in its hybrids with wheat a number of allo- and autosyndetic
associations can take place between the homoeologues of the
five genomes. Investigations to determine which of the above
alternatives are operative, are continued.
The technical assistance of Mr. H.S. ROUX, Mrs. E.C.
VERMEULEN and Mrs. M.G LOMBARD is gratefully acknowledged.
This investigation was financed from funds obtained from the
Department of Agricultural Technical Services and the
University of Stellenbosch.
|