IDENTIFICATION OF MONOSOMES FOR SIX CHROMOSOMES IN GOSSYPIUM HIRSUTUM

Six primary monosomes of the amphidiploid species Gossypium hirsutum have been identified as separate chromosomes of the complement. On the basis of cytogenetic tests involving translocations, a diploid species of the D genome group, and genetic markers, 4 of the 6 monosomes were designated chromosomes 1, 2, 4 and 6 of the A subgenome and 2 were designated chromosomes 17 and 18 of the D subgenome. Fourteen other monosomes were identified as duplicates of 3 of the above 6 primaries. THE CULTIVATED Upland cottonl, Gossypitm hirsutum L., is an amphidiploid with 26 pairs of chromosomes consisting of 13 pairs of large chromosomes (A subgenome) and 13 pairs of small chromosonmes (D subgenome). Due undoubtedly to the polyploid condition, this species can withstanid deficiencies of whole or parts of chromosomes (Menzel and Brown, 1952, 1954b; Brown and Endrizzi, 1964). Data on the origini, fertility, and the differentiatinig morphological characters associated with monosomics in G. hirsutum are being published separately (Brown and Endrizzi, 1964). This paper reports the cytological identification of monosomes for 6 of the 26 chromosomes of cotton. A total of 20 monosomes were studied of which 14 proved to he duplicates. MATERIALS AND METHODS-Cotton workers have adopted the system of designating chromosomes by arabic numiibers. For the amphidiploid species, numbers 1-13 are reserved for the A chromosomes and numbers 14-26 are reserved for the D chromosomes. Before a monosome can be designated as a specific chromosome of the complement, it must be determined whether the monosome is a member of the A or the D subgenome. Since the chromosomes of the A subgenome are larger than those of the D subgenome, the size of the monosome at MI is usually sufficient to place it in one or the other subgenome. However, foi confirmation, the procedure is to cross the nmonosomnic to a diploid species of the D genome group, or to translocations and genetic lines carrying marker loci whose genome identities are known. The genomic classification of 3 of the 6 different monosomes was determined by tests with 1 Received for publication July 2, 1963. Contribution No. 4468 from the Texas Agricultural Experiment Station, in cooperation with the Crops Research Division, Agricultural Research Service, U. S. Department of Agriculture. Part of the work was done under Regional Cotton Genetics Project S-I. interchanges in which the chromosomes have been identified, and the genomic classification of the remaining 3 was determined from crosses of the monosomes with the wild American diploid species G. raimondii Ulb. (2n = 26) of the D genomne group. In the critical F1 plants of a monosome X translocations, a modal pairing at MI of 23 II + I + IV shows that the monosome and the chromosomes in the interchange are different chromosomes. A modal pairing of 24 II + III, however, shows that the monosome is homologous to one of the chromosomes in the interchange. If 2 interchanges have 1 chromosome in common, then the 2 combinations above will identify the nmonosome. This procedure, using known translocations, permits the genomic placement of the monosome and serves to identify the monosome as a specific chromosomne of the complement. The triploid hybrids of a G. hirsutum monosome X the diploid D genome species G. raimondii are of 2 types, one with the normal nunmber of 39 chromosomes and the other wvith 38 chromosomes, the monosonlic. The normal 39-chromosonme hybrids exhibit a modal pairing at MI of 13 II + 13 I. The 38-chromosome hybrids, which are the critical F1's for genomic classification of the monosome, are of 2 classes, depending on whether the monosome belongs to the A or to the D subgenome. A modal association of 13 II + 12 I (13 small D bivalents + 12 large A univalents) shows the missing chromosome belongs to the A subgenome. The other class with a modal pairing of 12 II + 14 I (12 small D bivalents plus 13 large and 1 small univalent) shows that the missing chromosome belongs to the D subgenome. The morphological characteristics of the monosomes identified in the present study are presented by Brown and Endrizzi (1964); therefore, the over-all features of each monosomic will not be given in the present report. Although several plant characters are more or less distinctly af[The JOURNAL for January (51: 1-116) was issued January 28, 1964] AMERICAN JOURNAL OF BOTANY, Vol. 51: No. 2, 1964