Extreme length and length variation in the first ribosomal internal transcribed spacer of ladybird beetles (Coleoptera : Coccinellidae)

DNA sequences of the first ribosomal internal transcribed spacer (ITS1) wer e isolated from 10 ladybird beetle species (Coleoptera: Coccinellidae) repr esenting four subfamilies (Coccinellinae, Chilocorinae, Scymninae, and Cocc idulinae). The spacers ranged in length from 791 to 2,572 bp, thereby inclu ding one of the longest ITS1s and exhibiting one of the most extreme cases of ITS1 size variation in eukaryotes recorded to date. The causes of length variation were therefore analyzed. Almost no putatively homologous sequenc e similarities were identified for the taxa included. The only exception wa s for the subfamily Coccinellinae, which yielded sequence similarities in s ix regions of approximately 550 nucleotide positions, primarily at the 5' a nd 3' ends of ITS1. The majority of differences in ITS1 length between taxa could be attributed to the presence of repetitive elements with comparativ ely long repeat units. Repetition arose several times independently and was confined to the middle of the spacer which, in contrast to the 5' and 3' e nds. had not been inferred in previous studies to be subject to functional constraints. These elements were characterized by high rates of evolutionar y change, most likely as a result of high substitution rates in combination with inefficient homogenization across repeats. The repeated origin and su bsequent divergence of "long" repetitive elements should thus be assumed to be an important factor in the evolution of coccinellid ITS1.