Linkage Studies in Gossypium. II. Altered Recombination Values in a Linkage Group of Allotetraploid G. Hirsutum L. as a Result of Transferred Diploid Species Genes.

ACCORDING to STEPHENS (1950) speciation in the genus Gossypium occurs both by cryptic structural differentiation of chromosomes and by major gene substitution and mutation. In the first paper of this series RHYNE (1958) reported a sizeable reduction of recombination values within linkage groups following a transfer of intact diploid Gossypium linkage groups into allotetraploid G. hirsutum. These results show that chromosomal structural differences are more important in the altering of genetic recombination than is the influence of genotypic background. GERSTEL (1956) and GERSTEL and PHILLIPS (1958) indicate that chromosomal structural differences were in part responsible for decreasing the segregation of marker genes in hexaploid hybrids of G. hirsutum x Gossypium diploid species. The segregation ratios were larger in the hexaploid hybrids 2AhDhD,, in which G. hirsutum L. 2AhDh and wild American D diploids species were involved, than the expected ratio of five dominant to one recessive for duplex genes in the autotetraploid D genome chromosomes. Each of these studies thus conforms to one of the criteria which STEPHENS (1950) proposed for indicating cryptic structural differentiation among chromosomes in Gossypium interspecific hybrids. The assumption is, therefore, that structural differences exist among chromosomes of diploid and allotetraploid Gossypium species, and that diploid structural arrangements can be incorporated into G. hirsutum chromosomes by an insertion of genes and segments of diploid linkage groups into G. hirsutum linkage groups. Possibly these inserted diploid structural arrangements could alter recombination values within the G. hirsutum linkage groups. One expected alteration should be a reduction of recombination values in certain regions of a linkage group. Such a reduction was reported by RHYNE (1958) for intact, diploid, linkage groups transferred into G. hirsutum. A second expected alteration could be an increase of recombination values in regions of a linkage group adjacent to structural arrangements. Such an increase of recombination values in regions adjacent to inversions in maize chromosomes was assembled by SWANSON (1957). The present paper reports on the insertion of genes and associated structural arrangements of diploid