Primary spermatocyte nuclei of Drosophila melanogaster contain three prominent lampbrush-like loops. The development of these structures has been associated with the transcription of three fertility factors located on the Y chromosome, named kl-5,
kl-3 and ks-1. These loci have huge physical dimensions and contain extremely long introns. In addition,
kl-3 and kl-5 were shown to encode two putative
dynein subunits required for the correct assembly of the sperm axoneme. Here, we show that both the kl-5 and
kl-3 loops are intensely decorated by
monoclonal antibodies recognizing triple-stranded
nucleic acids, and that each loop presents a peculiar molecular organization of triplex structures. Moreover, immunostaining of Drosophila hydei primary spermatocytes revealed that also in this species - which diverged from D. melanogaster 58 million years ago - Y-loops are decorated by anti-triplex
antibodies, strongly suggesting a conserved role of loop-associated triplexes. Finally, we showed that in D. melanogaster wild-type lines that are raised at the non-permissive temperature of 31+/-0.5 degrees C (which is known to induce
male sterility in flies) both the triplex immunostaining and the
axonemal dynein heavy chains encoded by
kl-3 and kl-5 are no longer detectable, which suggests a functional correlation between loop-associated triplexes, the presence of axonemal
proteins and male fertility in fly.