SEQUENCE AND EVOLUTION OF THE GENE FOR THE MONOMERIC GLOBIN-I AND ITSLINKAGE TO GENES-CODING FOR DIMERIC GLOBINS IN THE INSECT CHIRONOMUS-THUMMI

We isolated genomic clones containing sequences encoding globins I and IA from a Chironomus thummi thummi genomic library. Three clones cont ain globin IA (ctt-1A) genes, while one contains a globin I (ctt-1) ge ne. The coding regions of the four genes are identical except for the single base substitution accounting for the, globin I/IA polymorphism. The noncoding DNA flanking the coding region is more than 98% similar , confirming a previous hypothesis that the globin ctt-1 and ctt-1A ge nes are alleles. Hemoglobins I and IA are monomeric in the insect hemo lymph. Earlier in situ hybridization studies suggested that monomeric and dimeric globin genes are clustered at different chromosomal loci. In situ hybridization of ctt-1 DNA to polytene salivary gland chromoso mes places the ctt-1 gene on the same band as genes for the dimeric gl obins II beta and VIIB, forcing revision of the earlier hypothesis tha t genes for monomeric and dimeric globin genes are at different loci. The evolution of the ctt-1 and ctt-1A alleles and of the two globin ge ne loci are discussed.