Department of Ecology and Evolutionary Biology, The University of Arizona, Tucson, Arizona
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Abstract |
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Key Words: Drosophila willistoni transposons MITEs hAT elements
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Introduction |
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MITEs were first discovered about 10 years ago in maize (Bureau and Wessler 1992) and have subsequently been identified in many animals and plants, including Homo sapiens, Caenorhabditis elegans, fish and Xenopus species, Arabidopsis thaliana, and a variety of other plants (Feschotte, Zhang, and Wessler 2002). Within Diptera, MITEs appear to be abundant in the mosquito species Anopheles gambiae, Aedes aegypti, and Culex pipiens. Ten MITE families have been reported to date in Anopheles gambiae (Besansky et al. 1996; Tu 2001) and one family reported in Anopheles stephensi (Luckhart and Rosenberg 1999). Twelve families have been described in Aedes aegypti (Tu 1997, 2000; Tu and Orphanidis 2001) and three families described in Culex pipiens (Feschotte and Mouches 2000; Feschotte, Zhang, and Wessler 2002).
MITEs are very short nonautonomous elements that are often present in high copy number (>1,000) per family per genome. These elements can occupy significant portions of their host genomes, and can insert close to structural genes, potentially influencing gene action. MITEs have terminal inverted repeats (TIRs) or subterminal inverted repeats (IRs) and are typically smaller than 1,000 bp. They have no coding potential, are AT-rich, and may have a stable secondary structure.
Different families of MITEs have common structural features but little if any sequence similarity. Until recently, it was not obvious to which class of transposons (I or II) they should be assigned nor how they attained such high copy numbers. It is now apparent that MITEs are nonautonomous DNA (class II) elements that originated from a subset of autonomous DNA transposons (Feschotte, Zhang, and Wessler 2002). MITEs are distinct from internally deleted class II elements in that they have no internal homology to their derivative transposons. Instead, they seem to have a propensity for including nonhomologous sequences in their internal regions. Investigations within plants and other organisms have demonstrated that MITEs are active and may be transposed by their parent DNA transposons (Feschotte, Zhang, and Wessler 2002). MITEs derived from the same superfamily of class II elements are generally similar in size.
In the first systematic attempt at classification (Feschotte, Zhang, and Wessler 2002), MITEs have been grouped into superfamilies based on their association with superfamilies of transposases. Possible superfamilies of transposase (and MITEs) include Tc1/mariner, PIF/Harbinger, piggyBac/TTAA, and hAT.
The hAT superfamily of autonomous transposons is so named because members all share homology and transposition characteristics with hobo from Drosophila, Ac from Maize, and Tam3 from the snapdragon (Calvi et al. 1991; Atkinson, Warren, and O'Brochta 1993). They are class II DNA transposable elements, which transpose with a cut and paste mechanism, and are found in a wide variety of eukaryotic taxa, with many described from plants, nematodes, and fungi. They typically have short TIRs and duplicate 8 bp of host target site DNA (TSD) upon transposition.
Within insects, a number of hAT superfamily members have been identified. Members of this superfamily include Homer, and the nonautonomous element HLE, from the tephritid fly Batrocera tryoni (Pinkerton et al. 1999), Hermes from the housefly Musca domestica (Warren, Atkinson, and O'Brochta 1994), Hermit from the blowfly Lucilia cuprina (Coates et al. 1996), and the ancestral element hopper from the tephritid Batrocera dorsalis (Handler and Gomez 1997). Partial hAT-like sequences have also been derived from the Australian bushfly Musca vetustissima (Warren, Atkinson, and O'Brochta 1995) and the Lepidopterans Heliothis virescens and Helicoverpa zea (DeVault and Narang 1994).
Within the genus Drosophila, the hobo element (McGinnis, Shermoen, and Beckendorf 1983) is the only hAT element thus far identified. Daniels et al. (1990) performed a Southern hybridization screen of hobo elements from 134 taxa within the genus Drosophila and found hobo elements restricted to the melanogaster and montium subgroups of the melanogaster species group. Only D. melanogaster and its sibling species, D. simulans and D. mauritiana, contained potentially complete hobo elements. This observation provides additional evidence for a recent origin of hobo within the melanogaster species group, perhaps by horizontal transfer, akin to the recent acquisition of P elements by D. melanogaster (Anxolabéhère, Kidwell, and Périquet 1988). This scenario is also supported by the presence of both H (hobo+) and E (hobo-) strains in D. melanogaster (Streck, MacGaffey, and Beckendorf 1986; Periquet et al. 1989; Boussy and Daniels 1991; Pascual and Periquet 1991) and D. simulans (Boussy and Daniels 1991). Furthermore, the hobo element appears to have transferred horizontally within the species group (Simmons 1992). Using squash and dot blotting, Loreto et al. (1997), detected hobo signal from isolated lines of D. willistoni. Although our repeated attempts to isolate hobo from this species have been unsuccessful (unpublished data), recent studies in another laboratory suggest the presence of multiple copies of this TE family (V. Valente, V. Valiati, and C. C. Klein, personal communication). Evidence that hobo-like sequences are an ancient component of Drosophila genomes is provided by the presence of divergent hobo-homologous sequences in the genomes of D. melanogaster E strains (Galindo et al. 2001) and of coding genes with hobo homology in the D. melanogaster genome (Adams et al. 2000; Robertson 2002).
Within plants, and indeed within many other organisms, class II TEs are seldom complete, the vast majority being internally deleted forms created by imprecise transposition (Robertson 2002). These still have the ability to transpose, as long as an alternative source of transposase, provided by autonomous elements in trans, is available. Many of these, such as naturally deleted forms of hobo (Streck, MacGaffey, and Beckendorf 1986) and the P element (O'Hare and Rubin 1983), do not qualify as MITEs, because they do possess internal homology to their derivative transposons.
We describe here two new MITE families, Vege and Mar, from Drosophila willistoni, that are associated with the hAT super family of TEs. These may be the first true MITEs identified from Drosophila.
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Materials and Methods |
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Southern Hybridizations
Two micrograms of pooled genomic DNA from approximately 30 flies was digested to completion using PvuII (Gibco-BRL) a null cutter within the two sequenced MITEs. Restricted DNA was elecrophoresed and transferred to nylon membrane (Amersham) using a modified capillary transfer protocol (Sambrook, Fritsch, and Maniatis 1989). Radioactive probes were generated from a PCR-derived template amplified from the reference sequences using Radprime DNA labeling (Gibco-BRL). Membranes were hybridized overnight before being washed at high stringency (final wash 65°C, 0.1XSSC, 20 min). Membranes were exposed to Xomat-AR film (Kodak), and scored manually.
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Results and Discussion |
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Mar is similar to Vege in that it has 8-bp TSDs and 11-bp perfect TIRs. Mar has 38% GC over 610 bp. The Mar sequence is deposited in GenBank under accession number AF518731. Mar is also inserted within a nonautonomous P element and has been detected in a single population of D. willistoni. The Mar insertion is between nu 58 and 59 of the canonical P element, with the TSD of nu 50 to 58. Interestingly, the Mar insertion included a loss of 7 bp of P element sequence immediately adjacent to the 3' insertion site (but after the TSD). TBLASTn and BLASTx analysis indicate that like Vege, Mar has neither coding capacity nor significant sequence homology to known published sequences.
We hypothesize that both Vege and Mar are MITE members of the hAT superfamily of TEs. Although autonomous dipteran hAT members (and the nonautonomous HLE element) have different length TIRs and sequence-specific TSDs, there are distinct areas of homology between them (tables 1 and 2). These homologous nucleotides frequently extend to other, more diverged, members of the hAT superfamily from vertebrates and plants (Rubin, Lithwick, and Levy 2001).
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The apparent rarity of Drosophila MITEs may well be related to the relative paucity of class II transposon sources. In contrast to some other taxa, relatively few class II TEs are present within the genus Drosophila (Robertson 2002). In the euchromatic portion of the D. melanogaster genome that has been sequenced, only 10 out of 54 families are identified as class II transposons (Rizzon et al. 2002). Those present in chromosomes 2R, X, and 4 are limited to 1360, BariI, Foldback, HB1, hobo, hopper, pogo and the S element (Bartolome, Maside, and Charlesworth 2002). A shortage of transposase sources may have limited the formation and expansion of MITE-like elements within this and related species.
Feschotte, Zhang, and Wessler (2002) list Dm-mPogo and SGM-IS as the only known Drosophila MITEs. Dm-mPogo, with dinucleotide (TA) TSDs and perfect 21 bp (or imperfect 25 bp) long TIRs are essentially identical over their length to the sequence of the autonomous pogo element and may actually be considered severely truncated non autonomous versions of these elements (Kapitonov and Jurka 2002b). Likewise, SGM-IS sequences have recently been identified as members of the mini-me element family, common among species of the genus Drosophila (Wilder and Hollocher 2001). Mini-me TEs have one terminal and one subterminal repeat, have no TSDs, and may have hairpin formation at the 5' terminus when single stranded. These and other factors lead the authors to conclude that these elements may be a new group of non-LTR retroposon (Wilder and Hollocher 2001). It may be, therefore, that there are no previously described true MITEs in the genus Drosophila.
Within the willistoni species group of Sophophora, no hAT TE members are known, although recent studies have indicated the presence of hobo-related elements in this group (V. Valente, V. Valiate, and C. C. Klein personal communication). The presence of hAT-related MITEs and the polytypic insertion pattern of the Mar element suggest that D. willistoni does have hAT elements and that they may have provided a transposase source for the movement of hAT-related MITEs in recent times. Given that hAT elements have been shown to cross-mobilize other hAT sequences in vivo (Sundararajan, Atkinson, and O'Brochta 1999) it is plausible that any hAT element present in this species could mobilize Vege and Mar.
Presently, it is not possible to say if Vege and Mar are closely related to any one of the sequenced hAT elements from other species. Vege and the Anopheles gambiae MITE Pegasus share 100% similarity over their overlapping TIRs, However, Pegasus TSDs don't conform well to the Dipteran consensus (Besansky et al. 1996).
To conclude, we have described here what may be the first true MITEs from a Drosophila genome. We hypothesize, based on TIR and TSD similarities, that both Vege and Mar belong to a hAT superfamily of MITEs found across a broad taxonomic range of organisms. Although the current absence of hAT autonomous TEs within the willistoni species group of Drosophila is not consistent with this finding, we predict that hAT elements will eventually be found in these species.
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Acknowledgements |
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Footnotes |
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