Presence of the deleted hobo element Th in Eurasian populations of Drosophila melanogaster

Presence d'elements Th defectifs pour la sequence hobo dans des populations naturelles eurasiennes de D.m., mise en evidence a l'aide de la methode Southern


Introduction
In Drosophila melanogaster, the progeny of certain out-crosses is characterized by a number of germline abnormalities, including chromosome breakage, high mutation rates, sterility, and male recombination. This syndrome has been termed hybrid dysgenesis (Kidwell et al., 1977). Three independent systems of transposable elements (i, P, hobo) can produce these anomalies through the interaction of chromosomal, cytoplasmic, and environmental factors (see reviews by Blackman and Gelbart, 1988;Engels, 1988;Louis and Yannopoulos, 1988). In the hobo system, molecular analysis has determined 2 classes of strains defined by their hobo elements. H strains contain 3.0-kb full-sized elements and numerous smaller derivatives, whereas E strains lack all such elements.
In most strains examined, the number of 3.0-kb elements is low, about 2-10 copies per genome, while smaller elements appear to be more numerous, from 30 to 75. These elements usually form only a few size classes, with each member of a class having the same internal deletion (Streck et aL, 1986;Blackman et al., 1987). However, different H strains, tested from different laboratory stocks, harbor different classes of defective elements. The homogeneity of defective elements within a given strain contrasts with the heterogeneity of the size classes among different strains. The presence of identical defective elements throughout these strains has suggested the predominance of preference for the amplification of defective elements rather than complete elements (Blackman and Gelbart, 1988).
In this paper, we report the analysis of current strains collected from natural populations over the Eurasian continent and show the presence of 2 major classes of hobo elements, a 3.0-kb element class and one particular deletion-derivative class of elements which have accumulated in all naturally occurring strains throughout the continent.

Materials and Methods
Southern blot analyses were performed on DNA extracted from 31 strains of D. melanogaster, establised by mass culture, from natural populations collected from France to China in 1986-87 (11 strains), and in 1981-84 (20 strains). Standard techniques were used for DNA extraction, gel electrophoresis, blotting, hybridization, and ligation (Maniatis et al., 1982). All Southern blots were hybridized and washed at I x SSC; 0.1 % SDS at 65°C. Genomic DNA was digested by Xhol and probed by the pRG 2.6 X plasmid containing the internal 2620 bp Xhol fragment from a complete hobo element inserted into pUC8 (Fig. 1 ).

Results
The results of Southern blot analyses are shown in Fig. 2a for natural strains collected in 1986-87, and in Fig. 2b for the 1981-84 sampling. About 4 gg of each drosophila genomic DNA was digested with Xhol and probed with pRG 2.6 X. The presence of full-sized 3.0-kb hobo elements gives rise to a 2.6-kb Xhol fragment with this probe. Any other bands are due to the presence of hobo deletion-derivatives which either have a deletion between the Xhol sites or have lost a Xhol site.
All strains contain sequences homologous to the probe. However, in the Paris strain the 2.6-kb band was not detected, and in the other strains strong differences in the intensity of this band were observed, reflecting variations in the number of full-sized elements present in each strain. More strikingly, all 31 of the Eurasian strains tested show a marked band of hybridization at 1.1 kb. This band appears to be derived from several copies of a particular class of deleted hobo element, in turn derived from a 3.0-kb element by an internal deletion of about 1.5 kb located between the 2 Xhol sites (Fig. 1 ). We refer to this element as the Th element as it was first detected from the French Tours (82) strain. The presence of this element was also detected in current populations of the United States and Mexico, but not in the early collected strains: Oregon-R S (USA, 1920-30) Paris (1945), and Marseillan (France, 1965).

Discussion
Our survey of natural populations shows that the 3.0-kb hobo element and its deletionderivative Th element are present in all Eurasian populations examined. No other derivative hobo elements have been accumulated to such a great extent, in terms of either geographic distribution or copy-number.
Hobo elements have been implicated as determinants of genetic instability, but their contribution to hybrid dysgenesis remains to be determined (Blackman and Gelbart, 1988;Louis and Yannopoulos, 1988). The hobo system has genetic analogies with the P-M system, although the molecular sequences of the elements are different. In the P-M system, genetic instability is clearly promoted by complete 2.9-kb P elements which encode for a transposase (Rio et aL, 1986;Engels, 1988). Other smaller and defective P elements are also present in the Drosophila genome, either associated with complete P element or alone.
The distributions of the P and hobo elements in the Eurasian population show striking similarities. In the P-Msystem, molecular and genetic analysis has revealed a specific P deletion-derivative, the KR element, present in all naturally occurring strains in Europe-USSR (Black et al., 1987) and China (Anxolabehere et al., unpublished data). The KP element appears to be implicated in the regulatory mechanisms of P-induced hybrid dysgenesis (Black et al., 1987). These authors suggested that the accumulation of KP elements in natural populations is due to the selection of individuals with the highest numbers of KP elements, in which P hybrid dysgenesis is suppressed. In the hobo system, all 3.0-kb elements found in nature are not necessarily functional, as other analyses have revealed microheterogeneity in this class of elements for labora-tory strains (Blackman and Gelbart, 1988). However, the induction of genetic instabilities by strains isolated from a natural Greek population suggests that intact hobo elements are present in the wild and may express their dysgenic properties (Yannopoulos et al., 1987). According to this hypothesis, the presence of the Th element may be interpreted as a contribution to the regulatory mechanisms of the hobo system. Moreover, their absence in some old laboratory strains raises the question of their putative recent origin and expansion.