Insect globin gene polymorphisms: intronic minisatellites and a retroposon interrupting exon 1 of homologous globin genes in Chironomus (Diptera)

Abstract

Exon 1 of globin gene ct-13RT in clone λgb2-1 from Chironomus thummi contains a 444 nt SINE ( CTRT1). Based on in situ hybridization to polytene salivary gland chromosomes, C. thummi ( ct), C. piger ( cp) and C. tentans ( ctn) contain copies of CTRT1 at multiple chromosomal loci. Genomic PCR amplifications reveal interrupted ( ct-13RT) and uninterrupted ( ct-13) alleles of the globin gene in the German population of C. thummi maintained in our laboratory, and only uninterrupted alleles or their homologs in different populations of C. thummi, C. piger and C. tentans. PCR amplification did generate different length fragments from cp-13 gene homologs in natural and laboratory C. piger populations that were due to variation in the length of minisatellite expansions of the central introns of the genes rather than a CTRT1-like SINE. Within minisatellite arrays, aligned homologs were more similar than paralogs in a single population, indicating that a tandem cluster of these repeats predated separation of the C. piger populations. The ct-13 genes of several C. thummi populations lack the minisatellites, suggesting their origin in C. piger only after the thummi/ piger split. CTRT1 transposition into a ct-13 allele is even more recent, occurring after separation of German and other European C. thummi populations. The nearly intact ct-13RT and comparison with its intact ct-13 allele support a very recent transposition of the CTRT1 SINE into one of at least two already diverged ct-13 globin gene alleles. PCR analysis of DNA from individual adults in C. thummi shows a 1:2:1 distribution of ct-13/ ct-13: ct-13/ ct-13RT: ct-13RT/ ct-13RT genotypes, consistent with a neutral spread of the ct-13RT allele since transposition, and indicating that the hemoglobin encoded by ct-13 is not necessary for survival, at least in a laboratory population of C. thummi.