| The spectrum of chlorophyll mutations show that some mutations like chlorina,
tipxantha,viridis and xantha are found relatively in sufficient number after
EMS treatment. It is known that in hexaploid wheat many traits are controlled
by triplicate genes. The fact that none of the nullisomics of hexaploid
wheat is deficient in chlorophyll suggests that the chlorophyll development
in this plant is controlled by several unlinked genes (SWAMINATHAN et
al. 1962 ; WASHINGTON & SEARS 1970). This means that the induction of
high rates of chlorophyll mutations might have been a multimutational event,
or simultaneous deletion of at least two different loci on homoeologous
chromosomes, or some special phenomenon is involved due to specific action
of the EMS. The experience in mutagenesis has shown the multimutations are
frequently induced but are usually those involving the linked genes, or
some cases pleiotropy may give an impression of multimutational event. Hence
the possibility of simultaneous mutation of many unlinked genes or simultaneous
deletion of different loci on homoeologous chromosomes though not totally
excluded is a very rare event. In this context, it seems that some other
phenomenon is involved in the induction of such mutations by EMS. One of
the possibility is based upon the specificity of EMS to certain region of
the chromosome i.e., those regions which are carriers of genes for
chlorophyll development. SWAMINATHAN et al. (1962) pointed out to
the randomness in the action of physical mutagens and the specificity of
EMS to some loci in barley and wheat. They suggested that in the evolution
of the gene placement along the chromosome arms, it is likely that linkage
groups constituted of genes without need for recombination are located near
centromere where linkage is tight and recombination is restricted. As such
genes for chlorophyll development are located near centromeres and also
in the proximal segments of the chromosomes. However, it is not practically
confirmed that the genes for chlorophyll production are concentrated near
centromeres and these blocks are more vulnerable to EMS action (WASHINGTON
& SEARS 1970). Therefore, the specificity of EMS to chromosome regions with
genes for chlorophyll development and the antimorphic action due to the
alteration in the function of the gene, are the main causes of high rate
and wide spectrum of chlorophyll mutations in EMS treatment. A broader induced viable mutation spectrum observed in present study, affected almost all parts of the plants. Some viable beneficial mutations observed at different frequency are (i) dwarf mutants (ii) increased spike length mutants (iii) increased spikelets and seeds per spike mutants (iv) increase grain weight mutants and (v) early mutants. These beneficial mutations transferred from one generation to the next show a direct quantitative improvement over parental cultivars. References AASTVEIT, K. 1968. Mutation in Plant breeding, 11, FAO/IAEA, Vienna, 5-14. LARIK, A.S. 1985. Wheat Infor. Serv. 61 (in press). LARIK, A.S. & Y.A. AL-SAHEAL. 1986. Cytologia 51 (in press). LARIK, A.S., H.M.I. HAFIZ & M.H.ARAIN. 1984. Genet. Agr. 38 : 35-42. NILAN, R.A., A. KLEINHOAS & C.F. KONZAK. 1977. Ann. N.Y. Acad. Sci. 287 : 367-384. SAHAMA RAO, H.K. & E.R. SEARS. 1964. Mutat. Res. 1 : 387-99. SWAMINATHAN, M.S., V.L. CHOPRA, & S.B. BHASKARAN. 1962. Ind. J. Genet. Plant Breed. 22 : 19-207. TIWARI, S.P. & N.S. SISODIA. 1981. J. Cytol. Genet. 16 : 141-150. TSUKUDA, H., S. RUMIHIKO, T. SUSUMU & O. YOSHIRO. 1977. Sci. Rep. Fac. Agric. Ibarki Univ. 23 : 1- 6. WASHINGTON, W.J. & E.R. SEARS. 1970. Can J Genet. Cytol. 12 : 851-859 |
| <-- Back |