EVOLUTION OF ORTHOLOGOUS INTRONLESS AND INTRON-BEARING GLOBIN GENES IN 2 INSECT SPECIES

While globin genes ctt-2 beta and ctt-9.1 in Chironomus thummi thummi each have a single intron, all of the other insect globin genes report ed so far are intronless. We analyzed four globin genes linked to the two intron-bearing genes in C. th. thummi. Three have a single intron at the same position as ctt-2 beta and ctt-9.1; the fourth is intronle ss and lies between intron bearing genes. Finally, in addition to its intron, one gene (ctt-13RT) was recently interrupted by retrotransposi tion. Phylogenetic analyses show that the six genes in C. th. thummi s hare common ancestry with five globin genes in the distantly related s pecies C. tentans, and that a 5-gene ancestral cluster predates the di vergence of the two species. One gene in the ancestral cluster gave ri se to ctn-ORFB in C. tentans, and duplicated in C. th. thummi to creat e ctt-11 and ctt-12. From parsimonious calculations of evolutionary di stances since speciation, ctt-11, ctt-12, and ctn-ORFB evolved rapidly , while ctn-ORFE in C. tentans evolved slowly compared to other globin genes in the clusters. While these four globins are under selective p ressure, we suggest that most chironomid globin genes were not selecte d for their unique function. Instead, we propose that high gene copy n umber itself was selected because conditions favored organisms that co uld synthesize more hemoglobin. High gene copy number selection to pro duce more of a useful product may be the basis of forming multigene fa milies, all of whose members initially accumulate neutral substitution s while retaining essential function. Maintenance of a large family of globin genes not only ensured high levels of hemoglobin production, b ut may have facilitated the extensive divergence of chironomids into a s many as 5000 species.