INCIPIENT GENOME DIFFERENTIATION IN GOSSYPIUM. I. CHROMOSOMES 14, 15, 16, 19 AND 20 ASSESSED IN G. HIRSUTUM, G. RAIMONDII AND G. LOBATUM BY MEANS OF SEVEN A-D TRANSLOCATIONS

The genus Gossypium is favorable for study of genome divergence at several levels. Early stages of divergence have been studied among four D genomes by comparing chiasma frequencies (reciprocal exchanges) between pairs of genomes and between individual counterpart chromosomes marked by heterozygous translocations. D5 (G. raimondii) shows barely detectable differentiation from from Dh (G. hirsutum), whereas D7 (G. lobatum) is considerably less closely related to Dh than is D5. Fragmentary data suggest that D2-2 (G. harknessii) falls between D5 and D7 in its relationship to Dh. Since chiasma frequencies in individual chromosomes and marked regions exhibit the same order of relationships as their corresponding whole genomes, it is concluded that the genome differentiation is generalized (i.e., nucleus-wide) rather than localized in specific chromosomes or chromosome regions. Estimates of relationships based on reciprocal exchange frequencies agree with those based upon preferential synapsis in allohexaploids reported previously. Since preferential synapsis and reciprocal exchange frequencies reveal the same order of relationships, it is concluded that to some extent they reflect common underlying changes in chromosome properties, despite recent evidence that synapsis and crossing over are under independent genetic control.