Genetic Analysis of Six Primary Monosomes and One Tertiary Monosome in Gossypium Hirsutum.

HE origin, fertility, and identifying vegetative characteristics associated with some monosomics of Upland cotton, Gossypium hirsutum L., have been described by BROWN and ENDRIZZI (1963). The monosomes were highly stable and n-1 gametes were transmitted with a high frequency through the egg, but very rarely through the pollen. ENDRIZZI and BROWN (1963) have obtained the cytogenetic data that identified monosomes for six of the 26 chromosomes of cotton. The six monosomics differ from each other and from normal disomics in specific syndromes of morphological features which are sufficiently distinct in a wide range of genetic backgrounds to permit accurate separation of monosomic from normal types. G. hirsutum is an amphidiploid, and consequently its genetic analysis for quantitative characters is somewhat difficult and less conclusive as compared to similar analyses in diploid species. To a great extent this difficulty is caused by the duplication of quantitative and qualitative factors located on several chromosomes which can interact additively and nonadditively and greatly influence the phenotypic expression of a trait. Since each variety or breeding stock may possess a different complement of alleles, segregating hybrid generations present difficulties in measuring and analyzing the genetic components determining a particular character. The establishment of the monosomics of cotton in a uniform genetic background provides a means of replacing any one chromosome by its homologue of another variety or breeding stock and evaluating it genetically. The substituted chromosome would be present intact in essentially a uniform genetic background; such substituted lines can be increased and performance tested in replicated field trials with the recipient line and the donor line, Furthermore, as applied to cotton breeding and the manipulation of genes that condition plant characters of economic value, the monosomic testing technique just described makes it possible to associate the effects of a gene (or group of genes) with a definitely identified or “marked” chromosome. Such studies are now in progress at this station. In addition to their value in associating genes affecting agronomic characters