MUTATIONS IN POLYPLOID CEREALS: III. THE CYTOGENETICS OF COMPACTOID WHEATS

Abstract

The compactoid mutants of wheat are, both phenotypically and cytogenetically, the reciprocals of the speltoid mutations. They are determined by excess dosages of the long arm of the C chromosome. This may be through primary polysomy involving the whole C chromosome, or through formation of isochromosomes comprising two long arms of C, or through fragmentation and the presence of excess telokinetic long arms of C. The short arm affects the phenotype slightly in the opposite direction. Other chromosomes present in excess may compensate for lack of C, especially in determining fertility. Most of the compactoid, subcompactoid, and subnormal types are of low fertility but the genetic data obtainable agree with cytological expectation. Very different progeny types and segregation ratios result from compactoid mutants that may be phenotypically and genically similar but different in the structure of their C chromosome constitution. There are three main types of subcompactoids and many subsidiary ones. Primary trisomic subcompactoids usually give only normal, subcompactoid, and compactoid progeny while a secondary type gives normals, subnormals, het speltoids, subcompactoids, and compactoids as regular segregates, besides other abnormal offspring. The data have significance for general problems of polyploid cytogenetics and the systematics of polyploid cereals.