Possible influence of hobo mobile element on the frequency of somatic mutations in the wing cells of Drosophila melanogaster. 

Kovalenko, L.V., L.P. Zakharenko, and I.K. Zakharov.  Institute of Cytology and Genetics, Siberian Branch, Russian Academy of Sciences,  630090, Novosibirsk, Russia;  zakharlp@bionet.nsc.ru

The classic experiments of B. McClintock have demonstrated the induction of somatic mutagenesis by Ac-type mobile elements.  Molecular analysis of P-element transcripts (belonging to Ac-type transposons) in somatic and generative tissues demonstrates that P element transpositions are tissue-specific and occurring only in generative tissues due to a tissue-specific mRNA splicing (Engels et al., 1987).  However, a body of facts is inconsistent with this hypothesis: for example, P element transpositions occur premeiotically in the cells dividing by mitosis. Gonadal sterility depends on the temperature of raising of larvae and the period of sensibility begins very early at an embryonic stage when the generative tissue is still being formed.

The hobo element is closely related to the P element in its characteristics. In this work we study the influence of a full-size hobo element in the genome of flies on the frequency of mwh somatic mosaics in the wing cells of Drosophila melanogaster estimated by somatic mutation and recombination test. We revealed a two-fold increase in frequency of mwh mutant spots in fly stocks containing an unstable hobo element in the genome.

Materials and Methods

Drosophila melanogaster stocks:  mwh (provided by U. Graf );  flr³/TM3,Ser (provided by U. Graf);  Oregon R (contains only deleted hobo copies (Streck et al., 1986);  Canton S (lacking hobo copies) (Periquet et al., 1994);  C(1)DX, y f; π₂, (P-cytotype stock);  Harwich (contains P elements and is a strong inductor stock);  Uc-1, contains a full-size hobo element in an unstable Notch gene, stock provided by J. Lim (Eggleston et al., 1996);  y⁺N⁺ - recombinant between y^2-743+pn cv v f c B (strain containing full-size hobo copy and unstable for yellow gene) and Uc-1 strain;  The presence of hobo element in mwh and flr³/TM3,Ser stocks has not been studied. Presumably, these stocks do not contain hobo (at least, its full-size copy) as most of laboratory stocks.

Crosses:  We crossed virgin mwh females to the males of studied lines and analyzed the wings of F1 progeny under light microscope at 400´, as described previously (Graf  et al., 1992). Only typical mwh-type mutations were scored while the single cells with two wing hairs were not counted to rule out morphoses. The results were analyzed in two ways. In Table 1, the mutation frequency is calculated by dividing the total number of mutant spots by the number of wings analyzed. In Table 2, the mutation frequency is calculated by dividing the number of wings carrying the mutant spots by the total number of wings analyzed. Statistical validity of data was calculated by the χ ² method (Graf et al., 1992).

Results and Discussion

In mwh/y⁺N⁺ and mwh/Uc-1 heterozygotes containing full-size and deleted copies of hobo, the frequency of mutant spots in the wing cells is increased two-fold (Table 1). When calculating the number of cells with mutant spots (Table 2), a statistically valid increase in the number of mutations is detected only in mwh/y⁺N⁺.  When scoring only the spots comprised by more than a single cell (to avoid the possibility of scoring a morphosis cell as a mutation), the mwh/y⁺N⁺ and mwh/Uc-1 heterozygotes demonstrate a multifold increase in the number of such spots, as compared to control stocks lacking hobo element.

Thus, the somatic mosaicism frequency in the progeny of crosses between the stocks containing a full-size hobo copy and mwh stock is increased as compared to the crosses between the stocks lacking hobo elements (or its full-size copy) and mwh. These results can, possibly, be explained not only by the presence of hobo in the genome but also by a genetic background inducing its transpositions. A genetic instability in Notch locus detected in Uc-1 line and in Notch and yellow loci in y⁺N⁺ line is associated with the presence of the hobo element in these lines and induced by dysgenic crosses (Eggleston et al., 1996;  Zakharenko et al., 2000 a, b). An increase in the frequency of singed somatic mosaics in males has been shown by Zakharov and colleagues (Zakharov et al., 1983). Instability in the singed-49 allele was later reported to be associated with hobo (O’Hare et al., 1998). In the presence of the P element, a two-fold increase of somatic mutations in the wing of Drosophila melanogaster females has been shown when shifting the larvae from 25 to 29^oC degrees (Getz and Schaik, 1991). Golubovsky and colleagues have shown that in several fly lines the increase in the proportion of chromosomes carrying P elements reduces the mutation frequency in the singed gene both in generative and somatic tissues. Whatever the heterogeneity of the effects observed, the mutation events occurring both in generative and somatic tissue should be noted. We believe that Ac-type mobile elements can influence the mutation frequency in unstable alleles not only in the germ cell line but also in somatic tissues of Drosophila melanogaster.

Acknowledgments:  The work was supported by Russian Foundation for Basic Research grant N.02-04-49251.

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