High Polymorphism of TPE Repeats Within Natural Populations of Drosophila melanogaster: A Gradient of the 5TPE hobo Element in Western Europe

Eric Bonnivard*,2, Claude Bazin{dagger} and Dominique Higuet*

*Laboratoire Dynamique du Génome et Evolution, Institut J. Monod, Paris Cedex, France;
{dagger}Laboratoire Population, Génétique et Evolution, Gif sur Yvette, France


    Abstract
 TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Acknowledgements
 References
 
To find out whether the polymorphism of TPE repeats of the hobo transposable element observed in some populations results from polymorphism within flies or from variability between flies, or both, we carried out isofemale line analyses of 25 populations. We found that polymorphic populations result from the presence of polymorphic flies combined with interfly variability within these populations. The fact that populations display different levels of polymorphism, i.e., different types of element and different frequencies of polymorphic flies, can be used to differentiate between qualitatively identical populations. This showed that the geographical structuring previously observed is reinforced and, in particular, that the western European populations, which have 3TPE and 5TPE elements, display a centrifugal decrease in the frequency of 5TPE hobo elements which start in western France. This gradient supports the hypothesis of a dynamic invasion by this type of elements: a total invasion by 3TPE elements, followed by further invasions involving other types of hobo elements. Moreover, the analysis of numerous sequences in current populations revealed the existence of seven types of never-previously described hobo elements with regard to TPE repeats. This diversity, which contrasts with the conservation of other parts of the element, highlights the high mutation rate of the S region.


    Introduction
 TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Acknowledgements
 References
 
Temporal surveys of natural populations of Drosophila melanogaster have revealed historical patterns, suggesting that worldwide invasions of this species by at least three transposable elements, I, hobo, and P, have occurred during the last century (Bregliano and Kidwell 1983Citation ; Kidwell, Frydryk, and Novy 1983Citation ; Anxolabéhère, Kidwell, and Périquet 1988Citation ; Périquet et al. 1989Citation ; Pascual and Périquet 1991Citation ). These transposable elements, which are currently found in all natural populations, are not found in long-established laboratory strains. The dynamics of the P element in natural populations is now clearly understood (Anxolabéhère et al. 1984Citation , 1985Citation , 1990Citation ; Boussy and Kidwell 1987Citation ; Boussy et al. 1998Citation ; Bonnivard and Higuet 1999Citation ; Itoh et al. 1999Citation ), but the dynamics of I and hobo remain unclear. In the case of the hobo element, previous studies, based on the correlation between collection date and the presence of the full-size element, suggested that a recent worldwide invasion started in America some time before about 1950 (Périquet et al. 1989Citation ; Boussy and Daniels 1991Citation ; Pascual and Périquet 1991Citation ). To clarify the dynamics of hobo elements, and thus to help understand the hobo invasion, current, natural populations were assessed with regard to the repetition polymorphism of a tandem "Threonine (T) Proline (P) Glutamic acid (E)" motif. These TPE repeats consist of repetitions of a 9-bp "actccagaa" sequence (Streck, Mac Gaffey, and Beckendorf 1986Citation ; Calvi et al. 1991Citation ; Bazin and Higuet 1996Citation ), corresponding to the polymorphic S region localized in the ORF1 of the element. In hobo108, Streck, Mac Gaffey, and Beckendorf (1986)Citation described 10 perfect copies, flanked by five degenerate ones, three in 5' and two in 3'. The autonomous hobo element of reference, Hfl1 (Calvi et al. 1991Citation ), has only three perfect copies, which are also flanked by degenerate ones.

Using TPE repeats as molecular markers revealed a characteristic geographical distribution, which seems to have been stable since the early 1960s (Bonnivard et al. 2000Citation ). Most populations are monomorphic and contain only hobo elements with three TPE repeats (3TPE elements). These populations belonging to the [3] class (with solely 3TPE elements) are found worldwide. In contrast, polymorphic populations (with several types of element) are restricted to three regions: western Europe, South America, and Equatorial Africa. These polymorphic populations display marked geographical structuring, with adjacent populations showing similar polymorphism. In the light of this stable distribution, we proposed a new scenario for the dynamics of hobo elements based on there having been two distinct invasion stages: a successful and total invasion by 3TPE elements followed by the start of new invasions involving other types of hobo elements, especially 5TPE elements (Bonnivard et al. 2000Citation ).

The existence of polymorphic populations raised questions about the origin of this polymorphism. Does it result from existence of polymorphic flies (i.e., different types of hobo sequences within a fly) or from variability between flies within the same population (i.e., flies belong to different classes with regard to TPE repeats), or from both? To describe the components of the polymorphism that was observed previously and to test our model, we investigated 25 current, natural populations through an analysis of the isofemale lines. In current populations, this analysis revealed the existence of seven new types of hobo elements with regard to the TPE repeats. Moreover, we observed interfly variability within the populations that made it possible to distinguish between populations that belong to the same polymorphic class. In particular, we describe in western Europe a centrifugal decrease in the frequency of 5TPE hobo elements that start from western France.


    Materials and Methods
 TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Acknowledgements
 References
 
Strains
Twenty-five natural populations, collected between 1992 and 1999, derived from various localities around the world, were studied using isofemale lines (table 1 ). The choice of these populations was based on the distribution described in Bonnivard et al. (2000)Citation and their maintenance as isofemale lines. Four of the chosen populations (k, l, r, and t) displayed unvarying regions, where only [3] monomorphic populations are found. We then chose three [3] populations (j, m, and v), located in or near the regions where polymorphic populations were found, as well as eight polymorphic populations, four populations each from South America (n, o, p, and q) and Equatorial Africa (u, w, x, y). Finally, nine polymorphic populations (a to i) were used to investigate the marked geographical structuring seen in western Europe. In all these populations, the TPE motif characteristics were investigated in 10 isofemale lines (eight lines for Liberia, Nimba [w] and Algeria, Skikda [s]).


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Table 1 Location and molecular characteristics in the H-E system of 25 natural populations collected between 1992 and 1999

 
Determining the TPE Status of Isofemale Lines
For each isofemale line, DNA was extracted from a single fly, using the method described by Di Franco et al. (1995)Citation . PCR amplification was performed using the h6 and h11 internal primers of the Hfl1 element, as described in Bazin and Higuet (1996)Citation . Preliminarily, PCR products were electrophoretically segregated by size in 2% agar gel on a Mupid-21 (Eurogentec), migration 3 h at 100 V. This made it possible to identify the different profiles present in a population by comparing the length of the PCR product with the sequenced control fragments. Then, for each of the populations, we identified lines that were representative of the different migration profiles observed.

For the 50 representative lines identified, PCR products were also separated on polyacrylamide gel to confirm the different numbers of TPE repeats observed. For this purpose, PCR amplification was performed using a 33P-labeled h6 primer. After amplification, samples were diluted with one volume of loading dye (95% formamide, 0.005% xylene cyanol FF, and 0.005% bromophenol blue), heat-denatured at 94°C for 5 min, and then directly cooled on ice. For each sample, a 5- to 7-µl aliquot was loaded onto a 6% denaturing polyacrylamide gel. After migration, gels were transferred to Whatman 3 MM paper and vacuum dried at 80°C for 1 h, and the dried gels were exposed to X-ray film.

More than 35 TPE repeat bands were chosen on the basis of their size and the population from which they had come, such that at least an example of each type of TPE repeat was included from each of the geographical regions. PCR amplifications were performed on these selected TPE repeat bands excised from the dried gels, according to Melayah et al. (2001)Citation , and sequenced using the specific amplifying primer h11.

Southern Blot
The Southern blot method was used to estimate the number of full-size hobo elements (2.6 kb XhoI fragments) using the method described by Bonnivard and Higuet (1999)Citation . For this purpose we used a hobo probe corresponding to the 1756–2168 internal sequence obtained by PCR amplification using the h6 and h11 primers. The full-size elements were quantified using the CyHBL1 strain as a reference (one Hfl copy per diploid genome, Calvi and Gelbart 1994Citation ). This enabled us to estimate the number of these elements present in the different strains.

Sequence Polymorphism Analysis
To research polymorphism beside the S region, we first used the sequences of h6-h11 PCR amplification products. Second, we investigated part of the no-coding region in 3' of the ORF1 (Streck, Mac Gaffey, and Beckendorf 1986Citation ) using two primers, h21 and h4, which have sequences corresponding to bases 2267–2286 (5'-ACAAAAACCTAAACAACTCG-3') and 2879–2859 (5'-ACCCTACTTGCGGCAACACA-3'), respectively, in the Hfl1 element (Calvi et al. 1991Citation ). PCR amplifications were performed on DNA extracted from single fly with only one type of element with regard to the TPE repeats and were sequenced using the specific amplifying primer h21.


    Results
 TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Acknowledgements
 References
 
High Polymorphism of TPE Repeats
PCR amplifications of the S region were carried out on 246 isofemale lines obtained from 25 different natural populations. Nine different hobo elements were detected (fig. 1 ). They correspond to 3TPE or 5TPE elements, which have already been reported elsewhere, and to seven new types of hobo elements, which have never previously been described. Five of them correspond to variations in the number of TPE motifs, resulting in 2TPE, 4TPE, 6TPE, 8TPE, or 9TPE elements, respectively. Our results do not support the existence of a 7TPE hobo element suggested by Bonnivard et al. (2000)Citation . In two cases, the sequenced S regions did not match the expected number of TPE copies due to two particular types of hobo elements (fig. 1 ). The first element, the size of which corresponds to four TPE repeats, actually contains only three perfect TPE repeats linked to a duplication of the 3'-adjacent 9-bp sequence that encodes a "Serine (S) Leucine (L) Glutamic acid (E)" motif. This element is defined as 3TPE + 1SLE element (simplified notation: 3 + 1SLE). The second element, corresponding in size to one TPE repeat, in fact has two perfect TPE repeats that are linked to a deletion of the 5'-adjacent 9-bp sequence encoding a "Threonine (T) Proline (P) Arginine (R)" motif. This element is defined as -1TPR 2TPE element (simplified notation: -1TPR 2).



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Fig. 1.—Sequence variability of the S region of hobo elements reported in current, natural populations. Sequences have been aligned on the Hfl1 element sequence (Calvi et al. 1991Citation ) chosen as a reference (at the foot). The TPE-repeats motif is indicated in bold, and the adjacent motifs are indicated by gray boxes. Dashes indicate sequence deletion, and blanks have been introduced in the reference sequence to allow for nucleotide insertions

 
Characterization of Natural Populations with Regard to TPE Repeats
To determine how to characterize an isofemale line, 16 lines were analyzed, each using three to five flies separately. Whatever the conditions (geographical origin, monomorphic or polymorphic flies, flies with rare types of hobo elements, flies from the same or different generations), no difference in TPE profile could be detected between flies from the same isofemale line. Thus, a single fly corresponds to an isofemale line.

Existence of Polymorphism Within Flies
This study of isofemale lines enables us to define a new class of monomorphic flies, the [5] class, represented by two French lines (fig. 2 ). Hence, it appears that some flies do not contain the 3TPE hobo element. However, this element type is still found to be predominant, as it is present in all the other 244 lines tested (being sequenced in 14 flies representing 11 populations, table 1 ). The 137 other monomorphic flies all belonged to the [3] class. In addition, several different polymorphic classes of flies can be defined (fig. 2 ). Two of them seem to be preponderant: 5TPE elements were found in 75 flies from 14 different populations (sequenced in eight of them, table 1 ), and 4TPE elements were found in 18 flies from six different populations (sequenced in all of them, table 1 ). All the other types of elements seemed to occur at low frequencies, as they occurred in less than two lines (six lines for the 8TPE element). Moreover, the flies that harbor these types of elements present a higher signal for 3TPE, 4TPE, or 5TPE elements in agarose gel. Such differences in the intensity of the PCR product can be interpreted as differences in the ratio of the number of elements of each type, according to Brunet et al. (1996)Citation . To confirm this hypothesis, we used two transformed lines, which have a single 3TPE or 5TPE hobo element, respectively. DNA extractions were performed on different mixes of these flies (ratio 1:9, 2:8, 5:5, 8:2, and 9:1). PCR amplification results show a difference in the intensity of the PCR products that correlates with the ratio of the transformed flies. Hence, according to the intensity of the PCR product, these particular types of element also seem to be in the minority within the genome.



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Fig. 2.—Distribution of flies within natural populations with regard to their TPE status. For each population (bars), 10 flies (single boxes) from different isofemale lines are analyzed (only eight flies for Nimba [w] and Algeria [s]). Flies belong to six different classes, according to both the number and the type of elements detected. For clarity, types of elements present in less than two flies or populations are not distinguished but are grouped in the [3 & others] class. (A) Populations from America and Africa. (B) Focus on populations from western Europe. Letters refer to table 1

 
Interfly Variability Within Populations Explains the Variability Between Populations
The level of polymorphism of the 25 populations studied is shown in table 1 and figure 2 . Five populations were monomorphic and consisted solely of flies with 3TPE elements. Four of these populations (Stavenger [k, not shown in fig. 2 ], Boston [l], Marrakech [r], and Dubai [t]) are located in regions where only monomorphic populations had previously been observed. This confirms that such populations present only 3TPE hobo elements. The fifth population (Bissau [v]) came from western Africa, where polymorphic populations also were found.

Polymorphic populations show different levels of interfly variability, depending on the frequency of polymorphic flies that they display. Some populations have only a few fractions of polymorphic flies, about 1 or 2 out of 10. They are mainly located around the edge of the region where TPE-repeat polymorphism is observed (e.g., Poznan [j] or Costa [m]). For the sake of clarity, the uncommon classes of flies are not shown in figure 2 . They harbor: the 2TPE element in one fly in Nimba (v); the 9TPE element in one fly in Madagascar (y); the 3 + 1SLE element in two independent flies in Poznan (j) and Nimba (v); and -1TPR, 2TPE, or 6TPE elements in few flies from Kourou (p), some of which harbored up to four different types of hobo elements, belonging to the [-1TPR 2, 3, 5, 6] class. In contrast, several populations contain a majority of polymorphic flies. This polymorphism mainly results from 4TPE or 5TPE elements in addition to 3TPE elements. For example, elements with four and five TPE repeats are accountable for the high polymorphism level observed in eastern Africa and western Europe, respectively.

A Gradient of 5TPE hobo Elements in Western European Populations
In western Europe, populations contain a majority of flies of the [3,5] class (fig. 2B ), especially in France, where only two monomorphic [5] flies were observed. The other 48 French flies were polymorphic, and all but four belonged to the [3,5] class. The remaining four flies also contained 8TPE elements. In the three German populations, monomorphic flies of the [3] class were observed but were still a minority. They make up a majority only in the Italian population Firenze (i), and, to the east, in Poland, Poznan (j). This implies that the western European populations display marked geographically determined structuring, as geographically adjacent populations show similar kinds of interfly variability.

To study the marked geographical structuring, 3TPE or 5TPE hobo elements were considered separately, and their frequency in each of the European populations was determined (fig. 3 ). 3TPE hobo elements were present in all flies, except one in Brest (a) and one in Naussac (d). On the other hand, there was a marked decrease in the number of isofemale lines harboring 5TPE elements that correlated with the distance from Brest. 5TPE hobo elements were present in all French flies but were present in progressively fewer and fewer flies in the German and Italian populations and were completely absent from the Polish flies. The data therefore revealed a relationship between the number of flies presenting 5TPE elements and the location of each population. This conclusion is reinforced when we consider the intensity of the different PCR products. Indeed, the eight [3,5] flies in which the 3TPE repeats product could not be detected easily were all French. This fits in with the fact that two French flies belonged to the [5] class. In contrast, the 11 [3,5] flies in which the 5TPE repeat product could not be detected easily were all German. In conclusion, the frequency of 5TPE hobo elements seems to decline steadily and centrifugally with distance, irrespective of the direction from Brest.



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Fig. 3.—Decrease in the frequency of 5TPE elements in western European populations. The number of flies that present 3TPE elements (rhombus) or 5TPE elements (square) was plotted against the geographic distance from Brest. (Letters refer to populations in table 1 and fig. 2B. )

 
Number of Full-Size Elements
The presence of 5TPE elements, or of other types of elements, in addition to 3TPE elements raises the question of how many hobo elements are found within polymorphic populations. Indeed, the fact that populations present different levels of polymorphism, i.e., different types of elements and different frequencies of polymorphic flies, could influence their number of hobo elements. For example, populations consisting of a majority of polymorphic flies (or populations that include many different types of elements) could have a greater number of elements. This implies that the gradient detected in western Europe could be reflected by the number of elements. To examine this possibility, we looked at the estimated number of full-size hobo elements (Hfl) because they are the main hobo elements carrying TPE repeats (Bonnivard et al. 2000Citation ). The data (table 1 ) show that there were between 2 and 20 copies of Hfl, with an average of eight, which is in agreement with our previous results (Bonnivard, Higuet, and Bazin 1997Citation ). The wide range of numbers of Hfl makes it impossible to detect any particular distribution of either the geographical position of the populations or their invasion by new types of elements with more than three TPE repeats. First, in western Europe, there is no correlation between the number of Hfl and the location of the populations (Spearman rank correlation: rs = 0.2, n = 10, P > 0.05). Hence, there is no particular gradient (fig. 4 ) to match the one found for the frequency of flies with 5TPE elements (fig. 3 ). Second, the number of Hfl does not reflect the level of interfly variability. For example, Perpignan, where all the flies seemed to be polymorphic, has only four copies (i.e., only about one-third the number of copies as other French populations), whereas Kircherhenbach, where only half of the flies are polymorphic, has about 12 copies. This may be because there is a considerable variability in the number of copies, which ranges from 4 to 20 in populations of the [3] class.



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Fig. 4.—Number of Hfl elements in western European populations versus their geographic distance from Brest. (Letters refer to populations in table 1 and fig. 2B. )

 
Sequence Polymorphism
To find out whether each type of element observed on different continents has a single origin or not, we were interested in finding a linkage disequilibrium between the number of TPE repeats and other polymorphic loci in the hobo sequence. For this purpose, the sequences of h6-h11 PCR products were compared with the Hfl1 sequence. These PCR products come from 18 different populations and correspond to all the different types of elements described. Our data do not reveal polymorphism for any type of hobo element anywhere other than in the S region. Then, we investigated about 600 bp of the no-coding region in 3' of the ORF1 of the hobo element. Four monomorphic isofemale lines were studied, three belonging to the [3] class (from Freiburg, Costa, and Dubai) and the other to the [5] class (from Brest). Sequences performed directly on h21-h4 PCR products (see Materials and Methods) did not reveal any polymorphism, even though these flies came from widely distant regions. These findings show that even though some hobo elements could display sequence polymorphism other than the TPE repeats, most of them have the same sequence that Hfl1 has. The same consensus sequence therefore is always found beside the S region, and no polymorphism of hobo elements in natural populations can be used to infer the origin of 4TPE or 5TPE hobo elements.


    Discussion
 TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Acknowledgements
 References
 
Results reveal that polymorphic populations result from the existence of polymorphic flies combined with interfly variability. Actually, a given population can contain monomorphic flies as well as polymorphic flies belonging to different classes. It is the ratio between these different flies that accounts for the polymorphism observed at the population level. Moreover, the polymorphism also results from an unexpected diversity of the hobo elements with regard to TPE repeats, which raises the question of their origin, both at the molecular and geographical levels.

Characterization of Populations with Regard to TPE Repeats
The polymorphism observed in some populations using mass culture can be redefined on the basis of the interfly variability. Of the seven populations of the [3] class, chosen to confirm that they really are monomorphic, only populations from Costa Rica and Poznan contained one polymorphic fly. Hence, if we assume that mass cultures had been established at least two generations before the molecular analysis, it is quite conceivable that they had escaped detection in the previous study. It also appears that these two populations were chosen for their particular location, on the borders of regions where polymorphic populations were found (see Materials and Methods). On the other hand, all the populations located in regions where only monomorphic populations were found displayed only 3TPE hobo elements, as expected.

Considering polymorphic populations, isofemale lines analyses not only make it possible to achieve better distinction between populations in different classes but also are able to discriminate between populations belonging to the same polymorphic class. Some differences appear between mass culture and isofemale lines data (table 1 ). Such differences have two causes. The first cause is that the isofemale line approach is able to detect types of elements that are only present at a low frequency within a population, such as 2TPE, 8TPE, or 9TPE elements, and may not be detected using mass culture analysis. The second cause is that polyacrylamide gel analysis can be used for directly sequencing elements of interest. This can account for the detection of 4TPE elements that were probably mistaken for 5TPE elements in earlier studies using only agarose gel.

Molecular Origin of the Different Types of Elements
According to the length of the repeats unit, TPE repeats can be consider as microsatellites or minisatellites, depending on the authors (Tautz 1993Citation ; Debrauwere, Gendrel, and Dutreix 1997Citation ), even though their size corresponds more closely to microsatellites. Two mechanisms have been proposed to account for the instability of such repetitive sequences: DNA polymerase slippage, which could account for the low modification of the number of repeats, and unequal recombination, which reshuffles repeat variants (for review see Debrauwere, Gendrel, and Dutreix 1997Citation ). The existence of 3TPE, 4TPE, and 5TPE elements in the same region (e.g., Bolivia or Kenya), as well as the deletion of the degenerate TPR motif in 5' or the duplication of the degenerate SLE motif in 3', can be used to argue for a mechanism linked to polymerase slippage. On the other hand, the absence of elements with an intermediate number of TPE repeats, between five and eight in Europe or between four and nine in Madagascar, conflicts with such an evolutionary model and could argue for recombination events. The instability of microsatellites depends on numerous parameters such as the size and sequence of the repeated motif, the number of repeats, the monotony of the series, the locus, and positive mutational bias (Estoup and Cornuet 1999Citation ). All these parameters can also interact with the mutation rate; hence, the evolution processes of microsatellites depend on the locus considered. In this special case of a microsatellite within the coding region of a transposable element, more information is needed to infer the evolution of the number of TPE repeats. However, some particularities of this locus can already be pointed out. First, TPE repeats present unexpectedly great diversity, given that their worldwide invasion of D. melanogaster is supposed to be a recent event, occurring within the past 70 years. This diversity reflects a high mutation rate that contrasts with the conservation of other parts of the element and with the average mutation rates of microsatellite loci in D. melanogaster (6.5 x 10-6 per locus per generation; Schug, Mackay, and Aquadro 1997Citation ). This high mutation rate could be related to the number of copies and to replication resulting from the transposition of the elements. Second, variations always involve sequences of nine nucleotides, usually TPE motifs but also motifs that are different at the protein level. Third, it is necessary to consider that the S region may not be neutral because it is located in the coding sequence. Thus, it is also possible than some types of elements were undetected because they were eliminated by selection pressure.

The Two-Step Invasion Model
The results obtained in this study are still consistent with the scenario of a two-stage invasion, consisting of a total invasion by 3TPE elements followed by new invasions involving other types of hobo elements (Bonnivard et al. 2000Citation ). Each of the less common types of hobo element is found only in restricted geographical regions; as a result, at present, no data can argue for their invasion capacity. Indeed, elements may be considered as invasive only if they are found in several populations on a large geographical area, for example, 5TPE elements in Europe.

The centrifugal gradient of 5TPE hobo elements described in western Europe can reflect a new invasion of the European populations by these elements, corresponding to the second invasion stage. An alternative hypothesis is that 5TPE elements occurred earlier in Europe, but if this were so, there is no way of explaining the origin of these elements or how they have faded out in many European flies. Fly migrations are preponderant factors in the transposable-element invasion of D. melanogaster (Quesneville 1996Citation ; Bonnivard 1999Citation ). Step-by-step recurrent migrations play a major role in establishing centrifugal gradients such as the one described in this study or the one described in the investigation of natural populations with regard to the number of full-size P elements (Bonnivard and Higuet 1999Citation ). Hence, if we compare the two systems, the stable distributions in western Europe present a remarkable spatial likeness with the centrifugal gradient centered in the west of France that decreases sharply in central Europe. This similarity is difficult to interpret because it cannot result from fly migration alone. Indeed, in the P-M system, the distribution also results directly from the activity and repression properties of the elements (Bonnivard and Higuet 1999Citation ). Indeed, mobility of the element within the genome is a second factor that may be implicated in their invasion capacity because elements that present activity can be amplified within the genome. Considering 5TPE elements, this mobility could reflect the activity of the elements themselves or their mobilization in trans because we are still not sure that a hobo element with more than three TPE repeats corresponds to complete elements. However, some data argued for the likely activity properties of 5TPE elements. First, most of the hobo elements carrying TPE repeats are full-size elements, as deleted elements that display the S region remain a minority, especially in the European populations (Bonnivard et al. 2000Citation ). Second, according to the preponderance of these elements in France, it seems that these elements not only invade French populations but also replace 3TPE elements.

Considering 4TPE elements, their geographical distribution, especially their high frequencies in eastern Africa, suggests that these elements are also potentially invasive and could participated in the second invasion stage. However, at present, the analysis of natural populations suffers from the lack of samples from South America and South Africa. Studying natural populations will allow us to determine if there is a continuum between these two regions and a gradient such as the one described for 5TPE elements.


    Acknowledgements
 TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Acknowledgements
 References
 
We would like to thank Cécile Balanant for her helpful technical assistance, Monika Ghosh for language revision, and anonymous referees for their helpful comments. This work was supported by GDR 2157-CNRS— "Evolution des éléments transposables: du génome aux populations" and Paris 6 University.


    Footnotes
 
Pierre Capy, Reviewing Editor

Keywords: hobo element TPE repeats microsatellites Drosophila melanogaster isofemale lines structuring populations Back

Address for correspondence and reprints: Eric Bonnivard, Laboratoire Dynamique du Génome et Evolution, Institut J. Monod, 2 place Jussieu, 75251 Paris Cedex 05, France. E-mail: bonni{at}ccr.jussieu.fr . Back


    References
 TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Acknowledgements
 References
 

    Anxolabéhère D., K. Hu, D. Nouaud, G. Périquet, 1990 The distribution of the P-M system in Drosophila melanogaster strains from the People's Republic of China Genet. Sel. Evol 22:175-188[ISI]

    Anxolabéhère D., H. Kai, D. Nouaud, G. Périquet, S. Ronsseray, 1984 The geographical distribution of P-M hybrid dysgenesis in Drosophila melanogaster Genet. Sel. Evol 16:15-26[ISI]

    Anxolabéhère D., M. G. Kidwell, G. Périquet, 1988 Molecular characteristics of diverse populations are consistent with a recent invasion of Drosophila melanogaster by mobile P element Mol. Biol. Evol 5:252-269[Abstract]

    Anxolabéhère D., D. Nouaud, G. Périquet, P. Tchen, 1985 P-element distribution in Eurasian populations of Drosophila melanogaster: a genetic and molecular analysis Proc. Natl. Acad. Sci. USA 82:5418-5422[Abstract]

    Bazin C., D. Higuet, 1996 Lack of correlation between dysgenic traits in the hobo system of hybrid dysgenesis in Drosophila melanogaster Genet. Res 67:219-226[ISI][Medline]

    Bonnivard E., 1999 Dynamiques des éléments transposables P et hobo dans les populations naturelles de Drosophila melanogaster Thèse de l'Université Paris 6

    Bonnivard E., C. Bazin, B. Denis, D. Higuet, 2000 A scenario for the hobo transposable element invasion, deduced from the structure of natural populations of Drosophila melanogaster using tandem TPE repeats Genet. Res 75:13-23[ISI][Medline]

    Bonnivard E., D. Higuet, 1999 Stability of European natural populations of Drosophila melanogaster with regard to the P-M system: a buffer zone made up of Q populations J. Evol. Biol 12:633-647[ISI]

    Bonnivard E., D. Higuet, C. Bazin, 1997 Characterization of natural populations of Drosophila melanogaster with regard to the hobo system: a new hypothesis on the invasion Genet. Res 69:197-208[ISI][Medline]

    Boussy I. A., S. B. Daniels, 1991 hobo transposable elements in Drosophila melanogaster and D. simulans Genet. Res 58:27-34[ISI][Medline]

    Boussy I. A., M. Itoh, D. Rand, R. C. Woodruff, 1998 Origin and decay of the P element-associated latitudinal cline in Australian Drosophila melanogaster Genetica 104:45-57[ISI][Medline]

    Boussy I. A., M. G. Kidwell, 1987 The P-M hybrid dysgenesis cline in eastern Australia Drosophila melanogaster. Discrete P, Q and M region are nearly contiguous Genetics 115:737-745[Abstract/Free Full Text]

    Bregliano J. C., M. G. Kidwell, 1983 Hybrid dysgenesis determinants Pp. 363–410 in J. Shapiro, ed. Mobile genetic elements. Academic Press, N.Y

    Brunet F., F. Godin, C. Bazin, J. R. David, P. Capy, 1996 The mariner transposable element in natural populations of Drosophila teissieri J. Mol. Evol 42:669-675[ISI][Medline]

    Calvi B. R., W. M. Gelbart, 1994 The basis for germline specificity of the hobo transposable element in Drosophila melanogaster EMBO J 13:1636-1644[Abstract]

    Calvi B. R., T. J. Hong, S. D. Findley, W. M. Gelbart, 1991 Evidence for a common evolutionary origin of inverted repeat transposon in Drosophila and plants: hobo, ativator, and Tam3 Cell 66:465-471[ISI][Medline]

    Debrauwere H., C. G. Gendrel, M. Dutreix, 1997 Differences and similarities between various tandem repeat sequences: minisatellites and microsatellites Biochimie 79:577-586[ISI][Medline]

    Di Franco C., A. Terrinoni, D. Galuppi, N. Junakovic, 1995 DNA extraction from Drosophila individual flies Drosophila Inform. Serv 76:172-174

    Estoup A., J. M. Cornuet, 1999 Microsatellite evolution: inferences from populations data Pp. 49–65 in D. B. Goldstein and C. Schlötterer, eds. Microsatellites: evolution and applications. Oxford University Press, Oxford, London

    Itoh M., R. C. Woodruff, M. A. Leone, I. A. Boussy, 1999 Genomic P elements and P-M characteristics of eastern Australian populations of Drosophila melanogaster Genetica 106:231-245[ISI][Medline]

    Kidwell M. G., T. Frydryk, B. Novy, 1983 The hybrid dysgenesis potential of Drosophila melanogaster strains of diverse temporal and geographical natural origins Drosophila Inform. Serv 59:63-69

    Melayah D., E. Bonnivard, B. Chalhoub, C. Audeon, M. A. Grandbastien, 2001 The mobility of the tobacco Tnt1 retrotransposon correlates with its transcriptional activation by fungal factors Plant J 28:159-168[ISI][Medline]

    Pascual L., G. Periquet, 1991 Distribution of hobo transposable elements in natural populations of Drosophila melanogaster Mol. Biol. Evol 8:282-296[Abstract]

    Périquet G., M. H. Hamelin, Y. Bigot, A. Lepissier, 1989 Geographical and historical patterns of distribution of hobo elements in Drosophila melanogaster populations J. Evol. Biol 2:223-229[ISI]

    Quesneville H., 1996 Dynamique des populations d'éléments transposables: modélisation et validation Thèse de l'université Paris VI

    Schug M. D., T. F. C. Mackay, C. F. Aquadro, 1997 Low mutation rates of microsatellite loci in Drosophila melanogaster Nat. Genet 15:99-102[ISI][Medline]

    Streck R. D., J. E. Mac Gaffey, S. K. Beckendorf, 1986 The structure of hobo element and their insertion site EMBO J 5:3615-3623[ISI]

    Tautz D., 1993 Notes on the definition and nomenclature of tandemly repetitive DNA sequences Pp. 21–28 in S. D. J. Pena, R. Chakraborty, J. T. Epplen, and A. J. Jeffreys, eds. DNA fingerprinting: state of the science. Birkhäuser verlag, Basel

Accepted for publication August 22, 2002.