| Evidence of gene dosage effects in Triticum aestivum C. ZSCHEGE Institut fuer Pflanzenbau und Pflanzenzuechtung, Universitaet Goettingen, Deutschland One of the most striking gene effects in hexaploid wheat, the compactoidspeltoid characteristic, has been shown to be produced by a dosage effect of factor Q on chromosome IX (MAC KEY). Similar cases in wheat, however, are not frequently known as yet. Some new observations on such gene dosage effects therefore will be given, which grew out of cytogenetic investigations on chemically induced drastic mutants of spring variety "Heines Koga II"1). To begin with the experimentally induced compactoid mutants, the cytological analysis did not show any change from the original variety; heterozygotes of these mutants segregated in a monohybrid ratio of 214 normal types : 425 intermediate types (subcompactoid) : 201 compactoid mutants (P>0,05, chi 2 = 0,52). On the other side each of three speltoid mutants were characterized by a different degree of deficiency in a definite chromosome pair. In reciprocal crossing between these speltoids and compactoids the wellknown dosage effect of the Q factor appeared : The F1 of the combination of heterozygous subspeltoid (1Q) x homozygous compactoid (4Q) consisted of heterozygous subspeltoid (3Q) and normal phenotypes (2Q). Three other mutants of our sortiment displayed the very same manifestation for the sphaerococcoid factor (sp). All the subsphaerococcoid phenotypes, which were hardly discernible from the original variety, were trisomic. The more distinctive sphaerococcoids were tetrasomic. For the purpose of locating the factor in question the tetrasomic sphaerococcoids were reciprocally crossed to the monosomic XVI of "Chinese Spring". The resulting forms in F1 and F2 proved to be disomics when aestivum types, trisomics when subsphaerococcoid types, and tetrasomics when sphaerococcoid types. Numerous squarehead mutants were obtained in the mutation experiments cited above. Ten different forms could be distinguished cytologically : Two of those mutants were found to be tetrasomic for the squarehead chromosome. In a third progeny a deficiency - duplication (21II + 2 fragments of the squarehead chromosomes) was detected. In a fourth not segregating squarehead form 42 chromosomes were counted. In this case one quadrivalent had occasionally been formed in metaphase cells, while trivalents were found in the corresponding subsquarehead forms. The 4 or 3 chromosomes of these multivalents had equivalent lenght, and there is every reason to believe that this mutant originated from chromosome substitution. Consequently those squareheads are nulli - tetrasomic, and the subsquareheads mono - trisomic. No cytological difference from the original variety could be detected among the remaining 6 squarehead mutants investigated. In all the cases studied the squarehead character segregated with a monohybrid ratio dependent on the environment, however, there was a different amount of mutant deficiency. These observations are in contrast to all results on the inheritance of the squarehead characteristic known so far in the literature. |
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