11.15. Chromosome elimination in interspecific hybrids involving Hordeum arizonicum (6 x) x Hordeum vulgare (2 x) and Hordeum arizonicum (6 x) x Hordeum bulbosum (2 x).
Rafiq Islam and D. H. B. Sparrow, Waite Agricultural Research Institute, University of Adelaide, South Australia, Australia.
Interspecific Hordeum hybrids in which chromosome elimination has been reported are H. vulgare (2 x) x H. bulbosum (2 x) [also H. vulgare (4 x) x H. bulbosum (4 x)], H. lechleri (6 x) x H. vulgare (2 x) and H. bulbosum (4 x) x H. secalinum (4 x). In the first case, bulbosum chromosomes are completely eliminated leading to the production of haploid (or dihaploid) vulgare plants; in the second case two types of plants were produced, either lechleri haploids with 21 chromosomes or true hybrids with 28 chromosomes; in the third case hybrids were obtained with sectors subsequently becoming haploid for H. bulbosum.
Chromosome elimination has been observed by the present authors in new interspecific combinations, not so far reported. These are H. arizonicum (6 x) x H. vulgare (2 x) and H. arizonicum (6 x) x H. bulbosum (2 x). In the first combination cytological examinations of 4-9 day old embryos showed cells with varying chromosome numbers (21-28). Cells with 21 chromosomes were however, more frequent. The process of elimination may be the same as in H. vulgare x H. bulbosum hybrids. Anaphasic bridges, micro-nuclei and fragments of chromosomes were observed in the embryonic cells. In the majority of the embryos analysed, 21-chromosome cells predominate, but in a few 28-chromosome cells were more frequent. Cells with 21 chromosomes can be reasonably expected to be the haploid arizonicum complement. This will be confirmed from plants obtained through embryo culture. But because of the presence of 28 chromosome cells in particular embryos (8-9 days old) we feel that the same system of elimination which is operative in H. lechleri (6 x) x H. vulgare (2 x) hybrids may be operative. That is, two types of plants could result, either true hybrids with 28 chromosomes or arizonicum haploids.
In the second combination, H. arizonicum (6 x) x H. bulbosum (2 x) elimination seems to be more complete. In the few embryos scored so far only 21 chromosomes have been observed.
Rajhathy and Morrison (1961) have reported that the three hexaploid species, H. lechleri, H. arizonicum and H. hexaploidum have the genomes of H. jubatum (4 x) in common. If some of the chromosomes of these jubatum genomes are responsible for the elimination of vulgare and bulbosum chromosomes, elimination would be expected to occur in hybrids involving all the three hexaploid species, and also in H. jubatum x H. vulgare hybrids. Rajhathy and Morrison (1959) reported the somatic chromosome numbers of the latter hybrid to be 21, but they observed a variable number of chromosomes in the PMC's of the F1 hybrids. The other possibility is that the genomic ratio between the hexaploids and vulgare is responsible for the elimination. Work is in progress to cross H. arizonicum (6 x) with tetraploid vulgare and bulbosum lines, to test this possibility.
Acknowledgement:
The authors wish to express their sincere thanks to Professor R. Takahashi for the seed of H. arizonicum Covas (B.562) which was used in this study.
References:
Cauderon, Y. and A. Cauderon. 1956. Etude de l'hybride F1 entre Hordeum bulbosum L. and H. secalinum Schreb. Ann. l'Amel. Plantes 3:307-317.
Rajhathy, T. and J. W. Morrison. 1959. Cytogenetic studies in the genus Hordeum IV. Hybrids of H. jubatum, H. brachyantherum, H. vulgare and a hexaploid Hordeum sp. Can. J. Genet. Cytol. 1:124-132.
Rajhathy, T. and J. W. Morrison. 1961. Cytogenetic studies in the genus Hordeum. V. H. jubatum and the New World species. Can. J. Genet. Cytol. 3:378-390.