Origin and decay of the P element-associated latitudinal cline in Australian Drosophila melanogaster

The latitudinal dine in P transposable element-associated characteristics i n eastern Australian populations of Drosophila melanogaster has changed bet ween 1986 and 1991-1994. New collections were made in 1991-1994 from locali ties along the eastern coast of Australia. P element-associated properties of 256 isofemale lines from 43 localities were evaluated using gonadal dysg enesis and/or singed-weak hypermutability assays. The overall results indic ate that both P activity and P susceptibility have declined, with all popul ations showing a tendency towards a state with little P activity potential but with P repressor function (neutral or 'Q'). P repressor function is str ong in all populations except some of the most southerly. P activity potent ial peaks at about 27 degrees SLat, and drops off to the south las in 1983- 1986 collections) and to the north tin contrast to 1983-1986 collections); thus the dine is no longer a simple P-to-Q-to-M pattern from north to south , but is now Q-P-Q-M. A mtDNA RFLP that putatively distinguishes North Amer ican and European populations varies in frequency among the populations but the frequency does not vary clinally with latitude, ruling out massive int roductions from North America and Europe as causing the dine.