Localisation polymorphism of mdg-1, copia, I and P mobile elements in genomes of Drosophila melanogaster, from data of inbred lines

Abstract

Using data from in situ hybridisation of the giant chromosomes from salivary glands, mdg-1, I, copia, and P mobile element polymorphism was studied in 17 highly-inbred lines of Drosophila melanogaster and in the expected hybrids obtained from the theoretically-crossed inbred lines. The mean copy numbers of each element on the inbred lines were close and equal to 16-8 for mdg-1, 17•5 for I, 178 for copia and 186 for P (these values included the insertions in the centromeric regions). The P element differed from the other three for variance in copy number and distribution of the number of insertion site occurrences. A low frequency of X-linked copia element insertions (as compared with frequency of insertions in the other chromosomes) was reported, suggesting that natural selection acts against insertional deleterious mutations of this element. Such a low frequency of X-linked insertions was not observed for the other three elements. The mdg-1, I and copia element copy numbers were positively correlated, thus leading to lines with high or low copy number. No correlation was observed between the P element copy number and the numbers of mdg-1, I and copia elements. Moreover, a genomic control of copy number was detected for mdg-1, I and copia, but not for P. The number of labelling sites, the number of copies, the number of heterozygous insertion sites, and various indices of genetic variability were estimated from the expected hybrids obtained from the theoretically-crossed inbred lines; the calculated numbers of labelling sites for mdg-1 and I did not differ from those obtained directly from the initial wild-derived population (we had no estimate of the number of labelling sites available in this population for copia and P). An average proportion of heterozygous sites of 086, 092, 0•90 and 094, and an average number of copies of 33, 35, 38 and 37 were found in the theoretical hybrids for the mdg-1, I, copia and P elements, respectively.

Selection against transposable elements inD. simulansandD. melanogaster

“…The proportion of heterozygous sites (NH/NS) is thus an estimate of the degree of heterogeneity in insertion profiles between the populations. We found a value of degree of heterogeneity equal to 0-95, which is very close to values already reported in D. melanogaster for various elements (Biemont & Gautier, 1988;Biemont et al 1994), clearly indicating that our D. simulans populations bore different insertions. Low transposition rate is thus not the main factor determining the low copy number of the 412 element in D. simulans, in agreement with the lower proportion of insertions on the X, which was not expected under the low transposition rate hypothesis.…”

“…With a computer we analysed the expected insertion profiles of the individuals obtained from theoretically crossing the isofemale lines from the various populations. We then calculated the number of labelling sites (NS) and the number of heterozygous sites (NH) in these flies (Biemont & Gautier, 1988;Biemont, 1995). The proportion of heterozygous sites (NH/NS) is thus an estimate of the degree of heterogeneity in insertion profiles between the populations.…”

Mechanisms regulating the copy numbers of six LTR retrotransposons in the genome of Drosophila melanogaster

“…Interestingly, 412 and mdg-1 have been shown to be closely related to each other (Whalen and Grigliatti 1998); it may be that they share some ancestral feature that promotes recessive, or partly recessive, insertion mutations. It is possible that a potential creator of fitness loss in the insertion mutations may be a sequence involved in the expression of these two 516 Table 5 Pooled X chromosomal proportions using data from the studies cited in Strobel et al (1979); c Belyaeva et al (1984); d Biémont (1986); e Montgomery et al (1987);f Yamaguchi et al (1987); g Biémont and Gautier (1988); h Biémont (1992); i Charlesworth et al (1992); j Biémont et al (1994); k Aulard et al (1995); l Vieira and Biémont (1996) …”

“…a Carr et al this study); bStrobel et al (1979); cBelyaeva et al (1984); dBiémont (1986); eMontgomery et al (1987); fYamaguchi et al (1987); gBiémont and Gautier (1988); hBiémont (1992); iCharlesworth et al (1992); jBiémont et al (1994); kAulard et al (1995); lVieira and Biémont (1996) …”

Dynamic equilibrium between insertion and excision of P elements in highly inbred lines from an M? strain of Drosophila melanogaster

“…Lines 4, 5, and 9 had an M cytotype In Situ Hybridization. Polytene chromosome spreads from salivary glands of third-instar larvae taken directly from the inbred lines were prepared and treated with nick-translated, biotinylated DNA probes (Bi6mont and Gautier 1988). Note that it is often stated that in situ hybridization of polytene chromosome squashes in Drosophila leads to a deterioration of the chromosomes when protocols using biotin are used.…”