MOLECULAR EVOLUTIONARY ANALYSIS OF THE YWVZ 7B GLOBIN GENE-CLUSTER OFTHE INSECT CHIRONOMUS-THUMMI/

We report the sequence of 8.1 kb of DNA containing the 3' end of one a nd seven other complete intronless globin genes from the YWVZ/7B locus of the dipteran Chironomus thummi thummi, One of these (ctt-v) appear s to be a pseudogene by virtue of a premature termination codon, where as the others encode apparently functional globin polypeptides. Taken together with previously published data, the C. th, thummi YWVZ/7B loc us codes for at least 11 globins, five of which differ from one anothe r by no more than two amino acids, In contrast, only nine globin genes are found in a comparable genomic clone isolated from C. th. piger. A s indicated by sequence alignment, this difference in copy number can be attributed to a loss of one gene (fusion of globin genes 7B8 and 7B 10) in the piger lines, coupled with a gain (globin gene 7B9) in the t hummi lineage. Comparisons between the thummi and piger sequences show ed that YWVZ/7B intergenic regions have maintained a level of 91% simi larity since the thummi/piger divergence: most differences are simply due to single base substitutions or insertion/deletion events in eithe r the thummi or the piger DNA, but three instances of partially overla pping deletions were also detected. A phylogenetic analysis of YWVZ/7B gene products was conducted, from which a plausible reconstruction of the evolutionary history of the locus was obtained. In addition, alig nment of globin 7B amino acid sequences suggested that globin genes 7B 2 and 7B3 (reported at the protein and cDNA level, respectively, but n ot contained on the C. th. thummi or C, th. piger genomic clones) are possibly chimeric genes. Given the trend toward expansion of the C, th ummi globin gene family in general and of the globin 7B sub-family in particular, we propose that increased copy number of these genes has b een positively selected as a mechanism to achieve a high Hb concentrat ion in the larval hemolymph.