Molecular evolutionary analysis of the ywvz/7B globin gene cluster of the insect Chironomus thummi

Abstract

We report the sequence of 8.1 kb of DNA containing the 3' end of one and seven other complete intronless globin genes from the YWVZ/7B locus of the dipteran Chironomus thummi thummi. One of these (ctt-v) appears to be a pseudogene by virtue of a premature termination codon, whereas the others encode apparently functional globin polypeptides. taken together with previously published data, the C. th. thummi YWVZ/7B locus codes for at least 11 globins, five of which differ from one another 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. As indicated by sequence alignment, this difference in copy number can be attributed to a loss of one gene (fusion of globin genes 7B8 and 7B10) in the piger lines, coupled with a gain (globin gene 7B9) in the thummi lineage. Comparisons between the thummi and piger sequences showed that YWVZ/7B intergenic regions have maintained a level of 91% similarity since the thummi/piger divergence: most differences are simply due to single base substitutions or insertion/deletion events in either the thummi or the piger DNA, but three instances of partially overlapping 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, alignment of globin 7B amino acid sequences suggested that globin genes 7B2 and 7B3 (reported at the protein and cDNA level, respectively, but not contained on the C. th. thummi or C. th. piger genomic clones) are possibly chimeric genes. Given the trend toward expansion of the C. thummi globin gene family in general and of the globin 7B subfamily in particular, we propose that increased copy number of these genes has been positively selected as a mechanism to achieve a high Hb concentration in the larval hemolymph.

The amphibian globin gene repertoire as revealed by the <i>Xenopus </i>genome

“…Similar gene loss events by unequal or intra-chromosomal crossing-over have been reported e.g. in the globin gene family of the insect Chironomus (Trewitt et al, 1995), and recombination is known to produce mosaic sequences via heteroduplex formation and gene conversion (Bollag et al, 1992).…”

Evolution of Orthologous Intronless and Intron-Bearing Globin Genes in Two Insect Species

“…thummi and the ancestor to ctn-ORFB in C. tentans. We Trewitt et al 1995 conclude that the five globin genes in C. tentans and the six genes in C. th. thummi are descended from five genes (filled squares in Fig.…”

“…thummi globin amino acid sequences (Goodman et al 1988), three genes reported here are novel, encoding globins whose protein products have not yet been isolated. Building on a systematic approach to naming chironomid globin genes Trewitt et al 1995), these are called ctt-11, ctt-12, and ctt-13 (the numbers 1-10 being previously assigned). The historical use of Roman numerals to name globin polypeptides sequenced at the amino acid level is retained.…”

“…Further, comparative analyses of the two loci provide ample evidence that, over the short time separating these species (5-10 million years), many of the differences in the globin sequences within and between species are neutral (Trewitt et al 1995). Though C. th.…”

“…The first chironomid globin gene clone to be isolated has two copies each of genes ctt-3 and ctt-4 as well as ctt-E gene, a novel gene whose product has yet to be demonstrated (Antoine and Neissing 1984;Antoine et al 1987). Subsequently, ctt-1, ctt-1A, ctt-6, eight novel ctt-7B gene variants, and four more novel genes designated Y, W, V, and Z were characterized (Saffarini et al 1985;Trewitt et al 1987Trewitt et al , 1988Kao et al 1994;Trewitt et al 1995). Globin genes were also sequenced from C. th.…”