R. Kurek et al., Molecular aspects of intron evolution in dynein encoding mega-genes on theheterochromatic Y chromosome of Drosophila sp., GENETICA, 109(1-2), 2000, pp. 113-123
Fertility genes on the heterochromatic Y chromosome of various Drosophila s
pecies are unique for several reasons. Most of them are megabase-sized. The
ir expression is restricted to premeiotic spermatocytes and often associate
d with unfolding of huge species-specific lampbrush loops. Molecular analys
is of the orthologous dynein genes Dhc-Yh3, DhDhc7(Y) and DeDhc7(Y) on the
Y chromosome of the three species D. melanogaster, D. hydei and D. eohydei,
respectively, revealed that the megabase gene size as well as the species-
specific morphology of the corresponding lampbrush loops kl-5, Threads and
diffuse loops result from huge introns and their specific sequence composit
ion, whereas the majority of all 20 introns in each of the three genes is i
n a size of 45-72 bp. The loop-specifying introns are extreme exceptions du
e to extended assemblies of degenerated transposable elements and/or large
clusters of satellite DNAs. Here we use sequence information from the compl
ete intron sets of three orthologous Y chromosomal dynein genes to deduce a
scenario for an evolutionary pathway leading to the megabase-sized genes o
n the heterochromatic Y chromosome of Drosophila. The obvious bias between
very small and species-specific mega introns is explained as the result of
an autocatalytic mode of intron growth. An initial coincidental hit by a si
ngle transposable element extends the size of a 50 bp intron for about two
orders of magnitude and determines it for preferential extension by similar
insertion events. This phase of continuous moderate growth is followed by
rapid size enlargements by repeating amplifications generating extended clu
sters of satellite DNA. Size control by recombination, on the other hand, i
s suppressed in Drosophila males by achiasmatic meiosis.