P-ELEMENT REGULATION AND X-CHROMOSOME SUBTELOMERIC HETEROCHROMATIN INDROSOPHILA-MELANOGASTER

In Drosophila melanogaster, crossing males carrying autonomous P eleme nts with females devoid of P copies results in hybrid dysgenesis in th e germline of progeny. The reciprocal cross produces non-dysgenic prog eny due to a maternally inherited state non-permissive for P transposi tion. The capacity of a P copy to repress transposition depends on bot h its structure and its chromosomal location. Naturally occuring regul atory P elements inserted at the telomere of the X chromosome have bee n genetically isolated in a genomic context devoid of other P elements . One or two copies of autonomous P elements at this site (1A) are suf ficient to elicit a strong P repression in the germline. These element s are flanked by Telomeric Associated Sequences, previously identified and described by Karpen and Spradling (1992) as having heterochromati c properties. The regulatory properties of P elements at 1A are strong ly impaired by mutations affecting Su(var)205, which encodes Heterochr omatin Protein 1, a non-histone heterochromatin protein. The regulator y properties of classical P strains are not sensitive to Su(var)205. M odels based on chromatin structure or on nuclear localisation of the t elomeres are discussed in order to explain both the strong regulatory properties of P elements at the X chromosome telomere and their sensit ivity to Su(var)205.