GrainGenes
A Database for Triticeae and Avena
Barclay et al., pp. 22-24
II.2. Control of chromosome elimination in Hordeum vulgare - H. bulbosum hybrids.
I. R. Barclay, K. W. Shepherd and D. H. B. Sparrow. Waite Agricultural Research Institute, Glen Osmond, South Australia.
The production of large numbers of barley haploids is now possible through the selective elimination of H. bulbosum chromosomes following hybridization of 2x H. vulgare and 2x H. bulbosum (Kasha and Kao 1970).
This chromosome elimination in H. vulgare - H. bulbosum hybrids is known to be influenced by the ratio of vulgare (V) to bulbosum (B) genomes in the hybrids. Thus elimination occurs when this ratio is 1V:1B as with 2x X 2x and 4x X 4x crosses, but it does not normally occur in the triploid hybrid produced from the cross 2x H. vulgare X 4x H. bulbosum where the genome ratio is 1V:2B. However, in 4x H. vulgare X 4x H. bulbosum crosses although most progeny were 14-chromosome vulgare dihaploids, two stable 27-chromosome hybrids were also detected, suggesting that elimination may depend on the ratio of a specific vulgare and bulbosum chromosome in the hybrid, rather than the overall genomic ratio {Kao and Kasha, 1970).
The aim of our study was to find if specific chromosomes of H. vulgare have an effect on chromosome elimination in H. vulgare - H. bulbosum hybrids. Whereas it is known that stable 21-chromosome triploid hybrids with genomic constitution VBB become unstable and exhibit chromosome elimination when the genomic structure is changed to VVBB by adding an extra vulgare genome, we have attempted to determine whether this instability can also be induced by adding an extra dose of individual vulgare chromosomes, instead of the whole genome, to the triploid hybrid. Crosses between the trisomics of H. vulgare and 4x H. bulbosum were expected to provide suitable material for these tests. Thus when the trisomic vulgare chromosome has no effect on chromosome elimination, stable 21 and 22-chromosome hybrid progeny are expected. On the other hand, if 8-chromosome vulgare haploids occurred instead of the 22-chromosome hybrids, this would indicate that the trisomic chromosome is capable of inducing elimination of bulbosum chromosomes when present in extra dose in the original 22-chromosome zygote.
Trisomics of the cultivars Betzes and Shin Ebisu were identified at meiosis and then crossed as the female parent with 4x H. bulbosum (CPI 14804). After ten days the developing embryos were removed for cytological examination.
The results obtained with trisomics 2 and 3, of both cultivars, differ in two respects from those obtained with the other five trisomics. First, whereas the expected 21 and 22-chromosome hybrid embryos were detected with trisomics 1, 4, 5, 6 and 7, only 21-chromosome hybrids were detected with trisomics 2 and 3 (see Table 1). Second, micronuclei were rare or absent in the 83 embryos derived from five of the trisomics, but very large numbers of these micronuclei were observed in some of the embryos derived from trisomic 2 (6 out of 44) and trisomic 3 (12 out of 49) crosses.
Although chance sampling could account for the failure to observe 22-chromosome progeny with trisomic 2, it cannot explain their absence among the 18 identified progeny of trisomic 3, since, on the basis of a 25% transmission rate of the extra chromosome to female gametes with trisomic 3 (Tsuchiya, 1968), the probability of not observing at least one 22-chromosome embryo among 18 is (3/4)18 or 0.0056.
Instead, it is possible that with trisomics 2 and 3 the extra vulgare chromosome in 22-chromosome embryos induced chromosome elimination giving rise to the large number of micronuclei observed. In fact the frequency of these embryos with numerous micronuclei is consistent with the expected transmission rate of 20% and 25% for the extra chromosomes of trisomics 2 and 3, respectively. Unfortunately it has not been possible to obtain definite chromosome counts with any of the embryos containing large numbers of micronuclei, but nevertheless it has been observed that several of them possess many fewer than 21 chromosomes.
Although more embryos need to be examined to detect the critical 8-chromosome progeny, these preliminary results suggest the balances of chromosomes 2 and 3 of H. vulgare with H. bulbosum chromosomes may control chromosome elimination in H. vulgare - H. bulbosum hybrids.
Literature:
Kasha, Ko J. and Kao, Ko N. 1970. Nature, Lond. 225:874-876.
Kao, K N. and Kasha, Ko J. 1970. Proc. 2nd Int. Barley Gen. Symp.: 82-88.
Tsuchiya, To 1968. Barley Newsletter 12:8-9.
Acknowledgement:
Seed stocks of the trisomics of Betzes and Shin Ebisu were generously
supplied by Dr. R. T. Ramage and Dr. T. Tsuchiya, respectively.