Reference Work Entry

Encyclopedia of Genetics, Genomics, Proteomics and Informatics

pp 931-932


Hybrid Dysgenesis

A historical term for various genetic phenomena caused by transposable elements in Drosophila. It entails mutations and chromosomal rearrangements in hybrids of two genetic stocks. P-M system: The P-strains carry a transposable element and a suppressor of transposition. In the other strains (M), there is a genetic factor that derepresses the transposase and thus the hybrid becomes genetically unstable, while both the parental forms are stable. (The P and M originally indicate paternal and maternal conditions, respectively.). In Drosophila, over 30 different hybrid dysgenesis systems have been identified. The best known among them is the P-M system. The physical structure of a complete P element is shown in the diagram (see Fig. H80).
Figure H80.

The structure of a complete P element of Drosophila (2,907) flanked in the genomic DNA target site by 8 bp direct repeats. The numbers in parenthesis indicate the nucleotide positions (beginning with the 5′ end). The horizontal arrows indicate the direction of the repeats; the vertical arrows point toward the positions in the linear sequence of the P element. (After Engels WR 1989. Mobile DNA, Berg DE & Howe MM, eds., pp. 437–484. American Society of Microbiology, Washington, DC.)

Some P elements have internal deletions, duplications, and substitutions (such as π2, Pc[ry]). The P element can be autonomous (transpose by their own power) and nonautonomous (requires a more complete [helper] element to move it). The autonomy of an element has been successfully tested by introduction through transformation of an in vitro engineered P element (Pc[ry]) carried on a plasmid along with the rosy gene (rosy is the structural gene for xanthine dehydrogenase [map position 3–52] and mutants have rosy eyes). The wild type allele introduced into ry homozygotes have normal eyes and are capable of moving into the sn (singed, map location 1–21, responsible for bristle [microchaetae] deformations), and this fact can easily be monitored.

The movement of P is controlled by the transposase function that is encoded by the four exons that extend to almost the entire length of the element (see diagram). The transposase begins transcription at base 85 and terminates at 2696, thus the transcript includes about 2.5 kb. The transposase enzyme is an 86.8-kDa protein. The P element can excise almost completely and leave behind the original genomic sequence or it may delete internal sequences, sometimes including even flanking nucleotides, involving genes with a total length rarely exceeding 7 kb. The imprecise excisions generate the defective elements. The frequencies of these excisions vary, ranging from 0.4 to nearly 2% per generation of the dysgenic flies. The mutability at the sn locus varies from 20 to 60%, but may reach up to 90% when two reverse oriented P elements (double P) are present at the target site. The targets for insertion are not distributed at random and P is inserted by several orders of magnitude more frequently in the sn locus than into the alcohol dehydrogenase (Adh) gene. Also, there is a tendency for P elements to become clustered. For insertion, the non-translated upstream regions of genes are favored compared to the coding regions. Insertion into euchromatic sites is favored over heterochromatin. Interbands appear more likely targets over chromosomal bands. P elements integrate with preference for 5′-end of the genes. The integration does not seem to have base-specificity, rather some structural properties of DNA are the basis of choice for insertion. The 8-bp target site duplication created by the P insertion is situated within a 14-bp palindromic sequence. Also, transposition in the germline is much more frequent than in the somatic cells. The suppression of transposition of the P chromosomes may only partially be relieved in the strains designated as M'. These transposable elements induce a variety of genetic events, including recombination in the male Drosophila, mutation, and chromosomal rearrangements. P elements carrying visible markers or the molecularly defined insertions can be used for chromosome mapping (Zhai RG et al 2003 Proc Natl Acad Sci USA 100:10860). The P-M system slightly boosts the effect of other mutagens. The frequency of X-chromosomal rearrangements was estimated to be 10% per generation and the second breakpoint tends to stay within the same chromosome. The active transposable elements also cause segregation distortion because the transmission of the P chromosomes is reduced compared to the M chromosomes. The transposons are also associated with gonadal abnormality and (GD) sterility at temperatures particularly above 27° C. The cytological sites and cis-effects too may affect the activity of the P elements. Various mutations induced by P are subject to suppressors. This transposon, similar to others, can be used for gene tagging and isolation, particularly its special constructs carrying selectable markers such as neomycin resistance so they can be screened efficiently (smart ammunition). The element pogo (about 2.2-kb) is somewhat similar to P, and it has either 23-bp inverted terminal repeats and no target site duplication, or a 21-bp inverted terminal repeat flanked by duplication of TA. The transposable element hobo (variable up to 3-kb) with up to 50 copies and 8-bp targets site duplication. Some (H) hobo elements are located in euchromatic regions and others are empty (E) sites. Reciprocal crosses do not activate Hobo but its presence is associated with high degree of instabilities. “HB” is a small (1.6-kb) element with 20 copies and 8-bp target site duplication. HB contains one reading frame of 444 bp that shares 25% homology with the amino acids of the Tc element of Caenorhabditis elegans.

The other best-studied transposable system of Drosophila causing hybrid dysgenesis is the I-R system. The complete I element is 5.4-kb (5–15 copies/genome scattered among the chromosomes) and has many features of a retrotransposon (its transposition is via an RNA intermediate), but it does not have long terminal repeats. Thus, it is structurally quite different from P, yet some of its functions warrant its description. The counterpart of the M cytotype of the P-M system is the R (responsive) cytotype. Hybrid dysgenesis is observed in the crosses of R females with I males. These sterile/semisterile females are called SF (stérilité femelle) whereas the reciprocal non-dysgenic ones are RSF. The female sterility of I-R does not involve gonadal anomalies (in contrast to GD in P-M) but hatching of the eggs is reduced. Eventually, the R strains may be converted to I by “chromosome contamination”, i.e., accumulation of chromosomes derived from an I strain by crossing and segregation. I factor activity involves mutations (recessive and dominant) that are frequently clustered, indicating their occurrence shortly before or at meiosis. The frequency of mutation varies at different loci and does not follow the same pattern as with P. The molecular structure of the complete I element of 5,371 bp is known. It does not have terminal repeats, however, four TAA reiterations are near the 3′-end of one strand and in the genomic DNA there are 12-bp duplications at the target site. One of the strands of I has open reading frames (ORF) I (1,278-bp) and II (3,258-bp), separated by 471 bases. There is probably another ORF of 228-bp. The base sequences in ORF II are similar to viral and virus-like transposases and reverse transcriptases. There are apparent coding sequences at the COOH-end for RNase H (ribonuclease digesting RNA in RNA-DNA hybrid molecules as required in reverse transcription). Elements similar to I have been detected in the mammalian L1, Drosophila non-viral retrotransposable elements (F family), R2 ribosomal insertions in silk worm, Cin4 element in maize, and in the ingi elements of Trypanosoma brucei. Near the 3′ end of ORF II and the longest ORFs of L1 and Cin4 code the amino acid sequence: Cys-Pro-Phe-Cys-Gln-Gly-Asp-Ile-Ser-Leu-Asn-His-Ile-Phe-Asn-Ser-Cys that resembles the metal-binding domain of general transcription factor TFIIIa. ORF I has a sequence with some homology to the DNA-binding viral gag polypeptides (group-specific antigen). The I elements are most common near the centromeric regions. Mutations induced by I are stable and do not revert (unlike to P). Many of the I elements are truncated and show internal deletions and rearrangement. The R factor is quite complex, and it is determined by both nuclear and cytoplasmic regulatory components. Its role is to release the I elements' expression. The FB family of transposable elements is complex ca. 6.5-kb, or smaller size transposons cause a variety of genetic effects. They seem widespread in Drosophila but are present in small copy number. They cause chromosomal rearrangement in 1/1,000 chromosomes. transposable elements, retroposons, copia, HEI; Engels WR 1996 Curr Top Microbiol Immunol 204:103; Simmons MJ et al 2002 Proc Natl Acad Sci USA 99:9306; Rio DC 2002 In Craig NL et al eds Mobile DNA II. Am. Soc. Microbiol. Press, Washington, DC, USA, p 484.

Copyright information

© Springer Science+Business Media 2008
Show all