INTRON-CONTAINING GLOBIN GENES IN THE INSECT CHIRONOMUS-THUMMI

All previously reported chironomid globin genes are intronless, sugges ting that the ancestral chironomid globin gene was also intronless. In this study, the coding regions of the closely linked Chironomus thumm i globin (Gbs) II beta and IX genes are shown to be interrupted by non coding DNA bounded by a 5'-GT and a 3'-AG. Both genes have appropriate ly placed transcription and translation signals. Polymerase chain reac tions on genomic DNA with oligonucleotides flanking and within the put ative Gb II beta intron generated products the size predicted for a ge ne with a 64-nucleotide intron, and sequencing of a cloned PCR fragmen t also revealed the intron. A partial-length Gb II beta cDNA sequence exactly matches that of the Gb II beta coding regions. We conclude tha t the intron-containing chironomid globin genes are functional. Region s of the Gb II beta and IX genes spanning the introns are more similar (86%) than the exons themselves (72% similarity), possibly due to par tial gene correction. Surprisingly, Gb II beta and IX gene homologues in C. tentans are intronless. If the common ancestor of chironomid glo bin genes was not intronless, introns were lost in at least three C. t hummi globin-gene lineages, and more recently by Gb II beta and Gb IX genes in C. tentans. If, as previously believed, the ancestral chirono mid globin gene was intronless, then an intron was recently acquired i n only one C. thummi globin sublineage. These alternatives are discuss ed.