Jhg. Von Der Schulenburg et al., Extreme length and length variation in the first ribosomal internal transcribed spacer of ladybird beetles (Coleoptera : Coccinellidae), MOL BIOL EV, 18(4), 2001, pp. 648-660
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.