A one-generation, white-peach assay for mariner DNA element activity in Drosophila simulans. 

Woodruff, R.C.,¹ and James N. Thompson, jr.^{2  1}Department of Biological Sciences, Bowling Green State University, Bowling Green, OH 43403;  ²Department of Zoology, University of Oklahoma, Norman, OK 73019.

            Transposable DNA elements are common in all organisms from prokaryotes to eukaryotes, including humans.  When these elements move they cause genetic damage such as insertions (which can disrupt genes and their control regions) and excisions (which can cause loss of nucleotides and chromosome breakage) (Lambert et al., 1988;  Berg and Howe, 1989;  McDonald, 1993).  Recombination between DNA elements can also lead to chromosome rearrangements, such as inversions (Caceres et al., 1999).  In humans, about 40% of the DNA is comprised of these elements and about 40-60 active elements are present in each genome (Prak and Kazazian, 2000).  The movement of DNA elements has been identified to cause mutations in a number of human genes, including Duchenne muscular dystrophy, type 2 retinitis pigmentosa, and hemophilia A.  The movement of DNA elements in somatic cells can also cause cancer (Miki et al., 1992).  Hence, these DNA elements are a major source of spontaneous genetic damage and disease in humans. 

            However, it is difficult to identify the presence or activity of DNA elements in higher organisms in a teaching environment, unless one has the equipment and supplies to perform molecular techniques, such as Southern blot analysis or polymerase chain reactions.  To overcome this limitation, we describe here a one-generation assay that can be used in a classroom laboratory to identify somatic movement of the mariner transposable DNA element in D. simulans obtained from natural populations. The movement is observed as mosaic eye-color spots in adults. 

            The mariner element is present in most organisms that have been tested and is found in the genome of humans (Oosumi et al., 1995;  Robertson, 1995).  This DNA element is especially common in Drosophila species, including the cosmopolitan species D. simulans (Hartl, 1989).  The mariner element has also been used as a vector to transfer DNA from one species to another (Warren and Crampton, 1994;  Gujeiros-Filho and Beverley, 1997;  Plasterk et al., 1999).

            To identify mariner elements that produce an active transposase protein, single D. simulans males captured on banana baits in nature are mated individually to females that are homozygous for the white-peach (w^pch/w^pch) mutation.  The only Drosophila species that resembles D. simulans is D. melanogaster, but these species can be differentiated by the amber-colored clam-shaped claspers of the male genitalia in D. simulans that are not seen in D. melanogaster when the male abdomens are viewed from the side.  If D. melanogaster males are placed together by mistake with white-peach D. simulans females, this will not be a problem, because the two sibling species almost never mate.  If they were to mate, however, only sterile female progeny survive (Ashburner, 1989).  Furthermore, mariner is not found in D. melanogaster (Brunet et al., 1994).

            The white-peach mutation is caused by a 1,300 base pair insertion of an inactive mariner element into the white gene on the X chromosome.  This mutation was originally isolated in D. mauritiana and was then placed into D. simulans by repeated backcrosses (Haymer and Marsh, 1986;  Capy et al., 1990).  This insertion can be activated to excise out of the white gene in germ and somatic cells by mariner transposase in D. simulans males from natural populations (Giraud and Capy, 1996).  This leads to red mosaic spots on a white-peach color background (see Haymer and Marsh, 1986, their Figure 1, p. 287;  Hartl, 1989, his Figure 2, p. 534). 

            In summary, all natural populations of D. simulans tested contained somatically active mariner elements, showing that the w^pch assay is a one-generation assay that can be easily used to identify active DNA elements in nature. 

            As a follow-up discussion, the class might be asked if they believe the somatically active mariner elements could cause genetic damage that would reduce the fitness of the F1 males.  Hint:  they do seem to reduce lifespan (Nikitin and Woodruff, 1995), and similar somatic movement of P DNA elements in D. melanogaster has been reported to reduce lifespan, mating activity, locomotion and fitness (Woodruff, 1992;  Woodruff and Nikitin, 1995;  Woodruff et al., 1999).  This could naturally lead to discussions of broader topics like the effects of mutagenic agents on somatic events like cancer and the possible role of increased mutation rates on life history in general. 

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