Somatic breakpoints,distribution of repetitive DNA and non-LTR retrotransposon insertion sites in the chromosomes of Chironomus piger Strenzke (Diptera,Chironomidae)

Structural aberrations, their frequency and distribution as well as distribution of the tandem repetitive minisatellite DNA clusters of Alu and Hinf elements and two retroelements, the LINE NLRCth1 and the SINE CTRT1, were analyzed in the genome of the chironomid C. piger Strenzke larvae from a Bulgarian population. A consistent somatic variability in the structure of the polytene chromosomes was detected, showing that the C. piger genome is more actively rearranging than supposed before. Breakpoints were concentrated in proximal parts of chromosomes significantly more often than in distal parts. By FISH analysis we could detect only one locus containing Alu elements and 38 Hinf cluster loci which appear to be dispersed equally all over the chromosomes. The retrotransposons NLRCth1 and CTRT1 are present only in a few loci, but highly variant among different individuals. The mean number of NLRCth1 sites per individual was 18.4 ± 2.09 and of CTRT1 was 54.8 ± 8.42. A third of breakpoint locations were close to or coincide with a locus occupied by a retroelement (either NLRCth1 or CTRT1). Nineteen percent of breakpoints coincided with Hinf repetitive DNA elements. Some breakpoints were identical in the two sibling species C. piger and C. riparius Meigen (syn.: C. thummi thummi) and are considered as conserved hot spots of chromosome breakage.