Behavior of hobo and P transposons in yellow2-717 unstable line of Drosophila melanogaster and its derivatives after crossing with a laboratory strain

Using fluorescent in situ hybridization technique (FISH), the frequency of hobo and P mobile elements transpositions on X chromosomes from the y2-717, isolated from the Uman' population of Drosophila melanogaster, as well as from its phenotypically normal and mutant derivatives, obtained as a result of crosses the males examined with the C(I)DX, ywf/Y females, was evaluated. It was demonstrated that the maximum frequency of hobo transpositions on X chromosomes of the males from derivative strains, subjected to repeated hobo-dysgenic crosses reached a value of 1.2 x 10(-2) per site per X chromosome per generation. The number of hobo copies in male X chromosomes from derivative strains was 3 times higher than in the original initial strain. Furthermore, the "old" hobo sites remained unchanged. In derivative strains, the frequency of hobo insertions was higher than that of excisions. One of the derivative strains, y1t-717alk3-2, was characterized by high intra-strain instability of hobo element localization. In the y2-717a1k3 and y1t-717alk3-2 strains a large inversion, In(1)1B; 13CD, was described. At the absence of the full-sized P element in the strains involved in crosses, maximum frequency of P element transpositions in the derivative strains reached a value of 1.2 x 10(-2) per site per X chromosome per generation.     

Genetic Instability in Drosophila Melanogaster Mediated by Hobo Transposable Elements

Eight independent recessive lethal mutations that occurred on derivatives of an unstable X chromosome (Uc) in Drosophila melanogaster were analyzed by a combination of genetic and molecular techniques. Seven of the mutations were localized to complementation groups in polytene chromosome bands 6E; 7A. In situ hybridization and genomic Southern analysis established that hobo transposable elements were associated with all seven of the mutations. Six mutations involved deletions of DNA, some of which were large enough to be seen cytologically, and in each case, a hobo element was inserted at the junction of the deletion's breakpoints. A seventh mutation was associated with a small inversion between 6F and 7A-B and a hobo element was inserted at one of its breakpoints. One of the mutant chromosomes had an active hobo-mediated instability, manifested by the recurrent production of mutations of the carmine (cm) locus in bands 6E5-6. This instability persisted for many generations in several sublines of an inbred stock. Two levels of instability, high and basal, were distinguished. Sublines with high instability had two hobo elements in the 6E-F region and produced cm mutations by deleting the segment between the two hobos; a single hobo element remained at the junction of the deletion breakpoints. Sublines with low instability had only one hobo element in the 6E-F region, but they also produced deletion mutations of cm. Both types of sublines also acquired hobo-mediated inversions on the X chromosome. Collectively, these results suggest that interactions between hobo elements are responsible for the instability of Uc. It is proposed that interactions between widely separated elements produce gross rearrangements that restructure the chromosome and that interactions between nearby elements cause regional instabilities manifested by the recurrence of specific mutations. These regional instabilities may arise when a copy of hobo transposes a short distance, creating a pair of hobos that can interact to produce small rearrangements.     

Interacting Hobo Transposons in an Inbred Strain and Interaction Regulation in Hybrids of Drosophila Melanogaster

A transposable hobo element in the Notch locus of the Uc-1 X chromosome, which does not interfere with the normal expression of the locus, interacts with other hobo elements in the same X chromosome to produce Notch mutations. Almost all of these mutations are associated with deficiencies, inversions or other rearrangements, and hobo elements are present at each of the breakpoints. The Uc-1 X chromosome produces the Notch mutations at a rate of 4-8% in both sexes of flies in a strain that has been inbred for 96 generations. At least two-thirds of the mutations are produced in clusters suggesting that they have originated in mitotic (premeiotic) germ cells of the Uc-1 inbred strain. The interaction of hobo elements in the Uc-1 X chromosome can be repressed by at least two different mechanisms. One found in three inbred strains not related to the Uc-1 strain involves a maternal effect that is not attributable to the actions or products of hobo elements. Repression by this mechanism is manifested by a clear reciprocal cross effect so that the production of Notch mutations is repressed in the daughters of Uc-1 males, but not in the daughters of Uc-1 females. The other mechanism apparently requires genetic factors and/or hobo elements in a particular strain of Oregon-R; complete repression is present in both types of hybrids between Uc-1 and this strain.     

Polymorphism of <Emphasis Type="Italic">yellow</Emphasis> locus in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> mutants from natural populations

We have studied the molecular characteristics of the yellow locus (y; 1–0.0), which determines the body color of phenotypically wild-type and mutant alleles isolated in different years from geographically distant populations of Drosophila melanogaster. According to the Southern blot, data restriction maps of the yellow locus of all examined strains differ from one another, as well as from Oregon stock. FISH analysis shows that, in the neighborhood of the yellow locus in the X chromosome, neither P nor hobo elements are found in y^1–775 stock, while only hobo is found in these region in y^1–859 and y^1–866 stocks, only the P element is found in y⁺sn⁸⁴⁹ stock, and both elements are found in y^1–719 stock. Thus, all yellow mutants studied are of independent origin. Locus yellow located on the end of X chromosome (region 1A5–8 on the cytologic map) carries significantly more transposon than retrotransposon induced mutations compared to the white locus (region 3C2). It is possible that, at the ends of Drosophila melanogaster chromosomes, transposons are more active than retrotransposons.     

Association of the yellow leaf (y10) mutant to soybean chromosome 3

At least 19 single recessive gene yellow leaf mutants and one duplicate recessive gene mutant have been described in soybean. This study was conducted to associate a yellow leaf mutant, y10, with a specific soybean chromosome by using primary trisomics (2n = 41). Seven soybean primary trisomics were hybridized as female parent with genetic stock strain, T161, carrying y10. F(1) disomic and primary trisomic plants were identified cytologically. One disomic (control) and all primary trisomic plants were allowed to self-pollinate and F(2) populations were classified for green versus yellow leaf mutant. The F(2) population of Triplo 3 segregated in a 17:1 ratio, while a disomic (3:1) ratio was observed with Triplo 8-, 17-, 18-, and 20-derived F(2) populations, suggesting that the y10 locus is on chromosome 3. The y10 locus was examined with four simple sequence repeat (SSR) markers (Satt584, Sat_033, Satt387, and Satt022) from molecular linkage group (MLG) N and y10 was found linked with Satt022. Therefore we confirmed the association of MLG N with chromosome 3. The possible association of y10 with Triplo 16 and Triplo 19 are discussed.     

Persistent locus-specific instability of yellow(27-717) and Noth(Uc-1) in Drosophila melanogaster coincides with hobo multiplication

According to FISH data the presence of multiple hobo element copies in the unstable yellow and Notch loci in y(2-717) and Uc-1 Drosophila melanogaster stocks, respectively, was found. Locus-specific instability in these strains is caused by hobo multiplication in the respective loci and its subsequent recombination with neighboring hobo copies rather than its insertion-excision.     

Isolation and genetic analysis of double hyperunstable systems in Drosophila melanogaster]

Hyperunstable mutations were described previously at the yellow locus of Drosophila melanogaster. These mutations are related to the insertion of the complex sequence containing two deleted copies of the P element at the termini and central unique regions from different sites of the X chromosome. In this work, double hyperunstable mutations at loci yellow and scute were obtained. These events were shown to occur from the inversion induced by the P elements located at the loci yellow and scute.     

Transposition of the hobo element in Drosophila melanogaster somatic cells

Somatic mutation and recombination test on wing cells of Drosophila melanogaster showed that the recombination frequency in the somatic tissues of strains studied correlated with the presence of a full-length copy of the hobo transposable element in the genome. Transposition of hobo in somatic tissue cells at a frequency 3.5 x 10-2 per site per X chromosome was shown by fluorescence in situ hybridization with salivary gland polytene chromosomes of larvae of one of the D. melanogaster strains having a full-length hobo copy.     

Wanderings of Hobo: A Transposon in Drosophila Melanogaster and its Close Relatives

The transposon hobo is present in the genomes of Drosophila melanogaster and Drosophila simulans (and D. mauritiana and probably D. sechellia, based on Southern blots) as full-size elements and internally deleted copies. The full-size melanogaster, simulans and mauritiana hobo elements are 99.9% identical at the DNA sequence level, and internally deleted copies in these species essentially differ only in having deletions. In addition to these, hobo-related sequences are present and detectable with a hobo probe in all these species. Those in D. melanogaster are 86-94% identical to the canonical hobo, but with many indels. We have sequenced one that appears to be inserted in heterochromatin (GenBank Acc. No. AF520587). It is 87.6% identical to the canonical hobo, but quite fragmented by indels, with remnants of other transposons inserted in and near it, and clearly is defunct. Numerous similar elements are found in the sequenced D. melanogaster genome. It has recently been shown that some are fixed in the euchromatic genome, but it is probable that still more reside in heterochromatic regions not included in the D. melanogaster genome database. They are probably all relics of an earlier introduction of hobo into the ancestral species. There appear to have been a minimum of two introductions of hobo into the melanogaster subgroup, and more likely three, two ancient and one quite recent. The recent introduction of hobo was probably followed by transfers between the extant species (whether 'horizontally' or by infrequent interspecific hybridization).     

"Mode for mutation" in the natural population of Drosophila melanogaster from Umani is caused by distribution of a hobo-induced inversion in the regulatory region of the yellow gene

A mutation outburst of the yellow gene occurred in a Drosophila melanogaster population from the town of Uman' from 1982 to 1991 and was associated with the instability of several alleles. Molecular genetic analysis revealed a deletion variant of the hobo transposable element in the same site of the regulatory region of yellow in the mutant alleles and their derivatives. The outburst of the yellow-2 mutations was attributed to the spreading of the X chromosome, which contained an inversion of the yellow regulatory region, through the population. Reinversion resulted in the wild-type phenotype. Crossing lines carrying the inversion with laboratory line C(1)DX, ywf induced instability of the yellow alleles, which was associated with duplication or multiplication of a fragment of the yellow gene. Most derivative lines eventually became stable. The loss of instability was not associated with phenotypic changes; molecular genetic changes included a loss of the duplicated sequences or a deletion of the inverted regulatory region of the yellow gene.     

Cause for maintaining high instability at the yellow gene in Drosophila melanogaster lines, isolated during the "mode for mutation" period in Uman populations

Mobile genetic elements are responsible for most spontaneous mutations in Drosophila melenogaster. The discovered in the 1980s phenomenon of frequent change of the wild-type yellow phenotype for a mutant one, and vice-versa, in strains of Drosophila melanogaster isolated from the Uman' natural population can be, according to our data, explained by repeated inversions and reinversions of the gene regulatory region located between the two copies of the hobo transport. However, most molecular genetic events accompanying the process can occur without the phenotype change. After several generations, the strains, remaining phenotypically unchanged, can possess different molecular genetic properties with respect to yellow. Using genetically homogenous or isogenic strains for the genetic analysis or for production of the new plant cultivars or animal breeds, geneticists and breeders often face the problem of stability of the strains. In the present study, the mechanism underlying the generation of instability at the yellow locus of D. melanogaster determined by the hobo-induced genome instability is described.     

The effect of gamma-radiation on induction of the hobo element transposition in Drosophila melanogaster

The transposition frequency of the hobo mobile element in four successive generations of Drosophila melanogaster strain y2-717 after an acute gamma-irradiation with a dose of 30 Gr amounted to 7.5 x 10(-4) per site per genome per generation. Under the same conditions, PCR analysis of the genomic DNA of y2-717 flies detected new variants of defective hobo sequence. No changes in the hobo localization and PCR products compared with the control were detected in the case of single irradiation with doses of 3 and 30 Gr. The localizations of hobo element on polytene chromosomes of y2-717 strain did not change during 11 generations after five exposures of flies to 30 Gr. Irradiation of a highly unstable D. melanogaster strain y+743 did not increase the number of families with mutant progeny, yet increased the total number of mutant descendants almost twofold, from 5 to 9%.     

The effect of genetic environment on locus-specific instability of the yellow gene in Uman' population of Drosophila melanogaster

The effects of genotype of the laboratory strains, C(1)DX, ywf/Y, 23.5 MRF/CyL4, and C(1)DX,yf; pi2, on locus-specific instability in the yellow gene of the strains y(2-717, y(2-715), and y(2-700 ) from Uman' population of Drosophila melanogaster was studied. Crosses of the males from Uman'-derived lines with the C(1)DX, ywf/Y females yielded a cascade of derivatives, mostly consisting of y+ and y2 alleles, while their crosses with the 23.5 MRF/CyL4 and C(1)DX,yf; pi2 females mostly resulted in the appearance of y+ and y(1) derivatives. The genomes of laboratory strains used in the study contained the full-sized hobo elements, which could differ from one another relative to the structure of variable region and affinity to different DNA sequences.     

Patterns of hobo elements and their effects in natural populations of Drosophila melanogaster in Japan

We studied the dynamics of hobo elements of Drosophila melanogaster in Japan with the goal of better understanding the invasion and evolution of transposons in natural populations. One hundred and twenty-six isofemale lines and 11 older stocks were tested for the presence and genetic phenotype of hobo elements. The oldest H strain, containing complete and deleted hobo elements, is Hikone-H (1957), but Hikone-R (1952) has no hobo-homologous sequences. The findings suggest that the hobo element invaded Japanese populations in the mid-1950s, at about the same time as the P element invasion in Japan. This chronology is consistent with the hypothesis of a recent worldwide hobo element invasion into D. melanogaster in the mid-1950s. In recently collected populations, H degrees strains (low hobo activity and high repression potency) are predominant, whereas H+ strains (high hobo activity and high repression potency) are predominant in the Sakishima Islands, the most southwestern islands of the Japanese archipelago. H' strains (high hobo activity and low repression potency) were first found in limited island populations. Japanese populations have not only full-size hobo elements and 1.5 kb Th elements but also characteristic deletion derivatives (1.6 and 1.8 kb XhoI fragments) that we have named Jh elements. These results are consistent with transgenic experiments with complete hobo elements, in which populations evolved to H+ or H degrees via H', and in which 1.8 kb fragments appeared. We conclude that hobo elements invaded the central region of Japan, spread to the far islands, and that the invasion is currently at an intermediate, nonequilibrium stage.     

Molecular Characterization of Hobo-Mediated Inversions in Drosophila Melanogaster

The structure of chromosomal inversions mediated by hobo transposable elements in the Uc-1 X chromosome was investigated using cytogenetic and molecular methods. Uc-1 contains a phenotypically silent hobo element inserted in an intron of the Notch locus. Cytological screening identified six independent Notch mutations resulting from chromosomal inversions with one breakpoint at cytological position 3C7, the location of Notch. In situ hybridization to salivary gland polytene chromosomes determined that both ends of each inversion contained hobo and Notch sequences. Southern blot analyses showed that both breakpoints in each inversion had hobo-Notch junction fragments indistinguishable in structure from those present in the Uc-1 X chromosome prior to the rearrangements. Polymerase chain reaction amplification of the 12 hobo-Notch junction fragments in the six inversions, followed by DNA sequence analysis, determined that each was identical to one of the two hobo-Notch junctions present in Uc-1. These results are consistent with a model in which hobo-mediated inversions result from homologous pairing and recombination between a pair of hobo elements in reverse orientation.     

Sequences homologous to the hobo transposable element in E strains of Drosophila melanogaster

Hobo is one of the three Drosophila melanogaster transposable elements, together with the P and I elements, that seem to have recently invaded the genome of this species. Surveys of the presence of hobo in strains from different geographical and temporal origins have shown that recently collected strains contain complete and deleted elements with high sequence similarity (H strains), but old strains lack hobo elements (E strains). Besides the canonical hobo sequences, both H and E strains show other poorly known hobo-related sequences. In the present work, we analyze the presence, cytogenetic location, and structure of some of these sequences in E strains of D. melanogaster. By in situ hybridization, we found that euchromatic hobo-related sequences were in fixed positions in all six E strains analyzed: 38C in the 2L arm; 42B and 55A in the 2R arm; 79E and 80B in the 3L arm; and 82C, 84C, and 84D in the 3R arm. Sequence comparison shows that some of the hobo-related sequences from Oregon-R and iso-1 strains are similar to the canonical hobo element, but their analysis reveals that they are substantially diverged and rearranged and cannot code for a functional transposase. Our results suggest that these ubiquitous hobo-homologous sequences are immobile and are distantly related to the modern hobo elements from D. melanogaster.     

The instability of the TE-like mutation Dp(2:2)GYL of Drosophila melanogaster is intimately associated with the hobo element

We have characterized molecularly several derivatives of the TE-like element Dp(2:2)GYL of Drosophila melanogaster. This highly unstable mutation occurred in a dysgenic cross involving the 23.5 MRF chromosome, and represents an inverted insertional duplication of approximately 130 polytene bands of the paternal 2L, at 50AB of the right arm of the maternal 2R. The instability of this mutation is characterized by deletion of some of duplicated material, by the induction of rearrangements in its vicinity and by the transposition of parts of the original element. We have found that the mobile element hobo is present at , or very near, the breakpoints of all GYL derivatives analysed, demonstrating that hobo is not only active in dysgenic crosses, but also that it can promote genetic instability reminiscent of transposable elements (TE).     

Horizontal transfer of hobo transposable elements within the Drosophila melanogaster species complex: evidence from DNA sequencing.

The hobo family of transposable elements, one of three transposable-element families that cause hybrid dysgenesis in Drosophila melanogaster, appears to be present in all members of the D. melanogaster species complex: D. melanogaster, D. simulans, D. mauritiana, and D. sechellia. Some hobo-hybridizing sequences are also found in the other members of the melanogaster subgroup and in many members of the related montium subgroup. Surveys of older isofemale lines of D. melanogaster suggest that complete hobo elements were absent prior to 50 years ago and that hobo has recently been introduced into the species by horizontal transfer. To test the horizontal transfer hypothesis, the 2.6-kb XhoI fragments of hobo elements from D. melanogaster, D. simulans, and D. mauritiana were cloned and sequenced. The DNA sequences reveal an extremely low level of divergence and support the conclusion that the active hobo element has been horizontally transferred into or among these species in the recent past.