| Young spikes of the F1 hybrids were fixed in Carnoy's 6:3:1
fixative for 24 hours, stored in 70 per cent alcohol in a refrigerator for
48 hours, then placed in 45 per cent acetic acid overnight, rinsed in water
and hydrolised in N HCl at 60C for 8 minutes. After staining the pollen
mother cells according to the Feulgen procedures they were squashed in 1%
Rosner aceto-carmine. The chromosome associations at the first meiotic metaphase
were studied in 200 pollen mother cells (PMC's) in the Nordum x A. distichum
hybrid. The results given in Table 2 show that
a mean of 13.88 chromosomes per PMC associate in bivalents and multivalents
by means of 9.17 chiasmata of which 9.08 are terminalised. As many as 55.5
per cent of the PMC's have 14 or more chromosomes associated in bivalents
and multivalents. This higher level of chromosome association than that
reported for the Inia 66 x A. distichum hybrid by PIENAAR (1979),
is mainly due to the higher frequency of multivalents in the Nordum x A.
distichum hybrid. The question which of the genomes of durum-wheat and A. distichum are sufficiently homologous to undergo synapsis and chiasma-formation in order to produce the large number of bivalents and multivalents, remains unsolved. Is it due to autosyndetic paring between the two A. distichum genomes or due to allosyndetic pairing of one of the Agropyron genomes with either the A or B genome of the durum-wheat? The fact that 16 or more chromosomes are associated in bivalents and multivalents at first meiotic metaphase in 31.5 per cent PMC's indicates that at least some A. distichum chromosomes synapse with wheat chromosomes and cross-over to produce chiasmata. It may then be possible to transfer useful genes from A. distichum to their wheat homoeologues by natural recombination. An analysis of 30 PMC's at first meiotic anaphase showed a mean distribution of 8.03 and 7.63 chromosomes to the two poles with 12.33 univalents congregating on the equator of the spindle. The univalents divide during late anaphase I and many of the daughter chromosomes fail to reach the poles in time, and produce micronuclei. The daughter chromosomes that do reach the poles, fail to undergo division during the second meiotic division, and lag on the spindle. Many micronuclei were observed in the microspore tetrads. These meiotic abnormalities are responsbile for the sterility of the hybrid. This investigation was financed from funds obtained from the Department of Agricultural Technical Services and the University of Stellenbosch. The technical assistance of Mr. H.S. ROUX, Mrs. M.H. LAMBRECHTS and Mrs. G.M. LOMBARD is gratefully acknowledged. Literature Cited PIENAAR, R. DE V., ROUX, H.S., VERMEULEN, E.C. & LOMBARD, M.G. 1977. Proc 6th Congr. S.A. Genet. Soc., pp. 76-83. PIENAAR, R. DE V. 1979. Wheat Inf. Serv. 49: 24-26. PIENAAR, R. DE V. 1980. Wheat Inf. Serv. 51: 13-14. |
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