| Results and Discussion As outlined by KUSPIRA & UNRAU (1959), two methods might be employed to establish gene chromosome relationships in tetraploid wheat using common wheat aneuploids. One method is to produce a hexaploid by crossing a tetraploid variety with Aegilops squarrosa (D-genome), polyploidise the hybrid and analyse the F1, F2 and F3 generations of crosses between the artificial hexaploid and a series of hexaploid monosomics or nullisomics. An alternative method would be to cross the tetraploid with the first fourteen lines of the hexaploid monosomics and analyse the F1 generation genetically or cytogenetically. If tetraploids are crossed with hexaploid monosomics, the pentaploid hybrids will all be monosomic for chromosomes 1D to 7D as well as being either monosomic or disomic for one of the first fourteen chromosomes (KUSPIRA & MILLIS 1967, BOZZINI & GIORGI 1971). One of the difficulties of a cytogenetic analysis of tetraploids by crossing these with hexaploid monosomics is that, most of the monopentaploid hybrid lines are completely or nearly completely sterile (HANCHINAL & GOUD 1981), thus causing the difiiculty in studying further segregating generations. Therefore only recessive or partially dominant alleles of the variety to be tested can be identified and attributed to a specific chromosome in the F1 generation. All this is based on the assumption that differential expression between disomics and monosomics is caused by the lack of genes on the missing chromosome. Genetic investigations of plant height in wheat are numerous but not conclusive. Plant height has been reported to be under polygenic control with innumerable modifiers (ALLAN & VOGEL 1963, ALLAN et al. 1968). Cytogenetic investigations have led to the location of genes affecting plant height on almost all chromosomes in one or the other variety. In the present investigation, four critical monopentaploid lines viz., 2A, 4A, 6A and 6B were shorter than the eupentaploid, indicating the presence of dwarfing genes on these chromosomes (Table 1). The other two critical lines i.e. 3B and 4B were taller than the eupentaploid, indicating the presence of genes for tallness on these chromosomes. SEARS (1954) and BHOWAL (1970), identified chromosomes of homoeologous group, I, II, III, IV and 6A possessing genes for plant height expression in common wheat. But BOZZINI & GIORGI (1971) identified 1A, 2A, 6A, 2B, 3B, 4B and 5B to be critical for this character. As in the case of plant height, the nature of dominance with respect to days to heading is an unsettled issue. Dominance or partial dominance of earliness is reported by several workers (CRUMPACKER & ALLARD 1960, JOHNSON et al. 1966, BOZZINI & GIORGI 1971, AHMAD and AKSEL 1972, BHAT & GOUD 1979). But few other reports indicated lateness to be dominant over earliness (PARODA et al. 1972, CRUMPACKER & ALLARD 1972). In the present investigation partial dominance type of gene action for delay in heading was observed. The critical lines 5A, 2B and 7B delayed heading indicating that the chromosomes promoting lateness and the chromosomes 1A, 7A and 5B showed the presence of genes for earliness located on them in Bijaga Yellow (Table 1). |
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