A Third Case of Compensatory Recombination in Interspecific Hybrids of Gossypium.

HERE are now several cases reported in the literature in which the transfer a chromosomal segment from one species of Gossypium to another results in altered recombination frequency in the chromosome to which the “foreign” segment has been transferred (SILOW 1944; RHYNE 1958; PHILLIPS 1961; STEPHENS 1961). Further, it is known that the altered recombination may take the form of either increased or decreased map distance (RHYNE 1958) and that a decrease in the region including the inserted segment may be more or less exactly compensated by an increase proximal to the inserted segment ( STEPHENS 1961). This type of compensation is not confined to the chromosomes of species known to be cytologically differentiated, but also occurs between chromosomes which exhibit apparently normal pairing behavior during the meiotic cycle, and whose products of recombination appear to be completely viable (i.e. complete fertility of the F, hybrid). The present case is concerned with recombination in the leaf shape-crinkle-green lint linkage group of Gossypium hirsutum L. into which has been inserted by backcrossing a segment including the leaf shape locus from the related wild American species, G. thurberi Tod. The linkage group has been described previously by STEPHENS (1955). He found that leaf shape ( L ) and crinkle (cr) are 41.2 map units apart, and that the map distance between crinkle (cr) and green lint ( L g ) is 6.6. These map distances were based on recombinations obtained in intervarietal crosses of hirsutum. The diploid species, G. thurberi, carries a LLhomologous” chromosome which exhibits preferential pairing with its hirsutum counterpart in appropriate synthetic polyploid combinations (GERSTEL and PHILLIPS 1958). The narrow leaf of thurberi is known to be an allele of the leaf shape series in hirsutum (GREEN 1953), so that one might expect that the thurberi and hirsutum chromosomes would be homologous in the L region. GILES (in press) found that the thurberi chromosome does not carry a normal allele (Cr) of hirsutum crinkle (cr) which may indicate that the thurberi and hirsutum chromosomes are nonhomologous in this region. The seeds of G. thurberi bear no true lint; seeds of the synthetic amphiploid, G. arboreum x G. thurberi, bear a greyish-white fiber if the arboreum parent is white-linted. In crosses between G. hirsutum types with green lint (Lg) and the synthetic amphiploid, arboreum x thurberi, with grey-