Mega-introns in the dynein gene DhDhc7(Y) on the heterochromatic Y chromosome give rise to the giant Threads loops in primary spermatocytes of Drosophila hydei

The heterochromatic Y chromosomes of several Drosophila species harbor a sm all number of male fertility genes (fertility) factors) with several unusua l features. Expression of their megabase-sized loci is restricted to primar y spermatocytes and correlates with the unfolding of species-specific lampb rush looplike structures resulting from huge transcripts mainly derived fro m clusters of loop-specific Y chromosomal satellites. Otherwise, there is e vidence from genetic mapping and biochemical experiments that at least two of these loops, Threads in Drosophila hydei and kl-5 in D. melanogaster, co localize with the genes for the axonemal dynein beta heavy chain proteins D hDhc7(Y) and Dhc-Yh3, respectively. Here, we make use of particular Threads mutants with megabase-sized deletions for direct mapping of DhDhc7(Y)-spec ific exons among the large clusters of satellite DNA within the 5.1-Mb Thre ads transcription unit. PCR experiments with exon-specific primer pairs, in combination with hybridization experiments with exon- and satellite-specif ic probes on filters with large PFGE-generated DNA fragments, offer a simpl e solution for the long lasting paradox between megabase-sized loops and pr otein-encoding transcription units; the lampbrush loops Threads and the DhD hc7(I? gene are one and the same transcription unit, and the giant size of the DhDhc7(Y) gene as well as its appearance as a giant lampbrush loop are merely the result of transcription of huge clusters of satellite DNA within some of its 20 introns.