Accumulation of Spock and Worf, two novel non-LTR retrotransposons, on the neo-Y chromosome of Drosophila miranda

Transposable elements constitute a major fraction of eukaryotic genomes. Here, I characterize two novel non-LTR retrotransposons, cloned from the neo-Y chromosome of Drosophila miranda. Worf is 4.1 kb in size and shows homology to the TI-2 non-LTR transposon characterized in Anopheles. Spock is 4.9 kb in size and shows similarity to the Doc element of D. melanogaster. Southern blot analysis of both elements yielded stronger signals for male DNA. In situ hybridization to polytene chromosomes revealed that both elements are accumulating on the neo-Y chromosome of D. miranda. PCR analysis was conducted to investigate the frequency of spock and worf and of the previously identified transposons, TRIM and TRAM, at individual chromosomal sites among 12 strains of D. miranda. Contrary to the observation that element frequencies are usually kept low at individual sites in Drosophila, the four transposons investigated are fixed at their genomic locations on the neo-Y chromosome. These results support the hypothesis that transposons accumulate in nonrecombining regions and may be one cause of the heteromorphism of sex chromosomes.