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A P element is a transposon that is present specifically in the fruit fly ''Drosophila melanogaster'' and is used widely for mutagenesis and the creation of genetically modified flies used for genetic research. The P element gives rise to a phenotype known as hybrid dysgenesis. P elements were discovered in the 1970s when laboratory strains used since 1905 were compared to wild type flies (i.e. found in nature). It seemed that these P elements had swept through all wild type populations of ''D. melanogaster'' subsequent to the isolation of the laboratory strains, which did not contain P elements. The P element encodes for the protein P transposase and is flanked by terminal inverted repeats which are important for its mobility. Unlike laboratory strain females, wild type females are thought to express an inhibitor to P transposase function. This inhibitor reduces the disruption to the genome caused by the P elements, allowing fertile progeny. Evidence for this comes from crosses of laboratory females (which lack P transposase inhibitor) with wild type males (which have P elements). In the absence of the inhibitor, the P elements can proliferate throughout the genome, disrupting many genes and killing progeny. ==Characteristics== The P element is a class II transposon, and moves by a DNA-based "cut and paste" mechanism. The sequence comprises 4 exons with 3 introns. Complete splicing of the introns produces the transposase enzyme, while alternative partial splicing of intron 1 and 2 leaving in only intron 3 encodes the P element repressor. The complete, autonomous P element encodes a transposase enzyme, which recognizes the 31 bp terminal inverted repeats of the P element and catalyzes P element excision and re-insertion. The complete element is 2907 bp; non-autonomous P elements contain an internal deletion of varying length which abolishes transposase production, but such elements can still be mobilized if transposase is encoded elsewhere in the genome. P element insertion and subsequent excision results in the production of 8 bp direct repeats, and the presence of such repeats is indicative of previous P element activity. ==Hybrid dysgenesis== Hybrid dysgenesis refers to the high rate of mutation in germ line cells of ''Drosophila'' strains resulting from a cross of males with autonomous P elements (P Strain/P cytotype) and females that lack P elements (M Strain/M cytotype). The hybrid dysgenesis syndrome is marked by temperature-dependent sterility, elevated mutation rates, and increased chromosome rearrangement and recombination. The hybrid dysgenesis phenotype is affected by the transposition of P elements within the germ-line cells of offspring of ''P strain'' males with ''M strain'' females. Transposition only occurs in germ-line cells, because a splicing event needed to make transposase mRNA does not occur in somatic cells. Hybrid dysgenesis manifests when crossing P strain males with M strain females and not when crossing P strain females (females with autonomous P elements) with M strain males. The eggs of P strain females contain high amounts of a repressor protein that prevents transcription of the transposase gene. The eggs of M strain mothers, which do not contain the repressor protein, allow for transposition of P elements from the sperm of fathers. In P strain females, the repressors are found in the cytoplasm. Hence, when P strain males fertilize M strain females (whose cytoplasm contain no repressor), the male contributes its genome with the P element but not the male cytoplasm leading to P strain progeny.〔 This effect contributes to piRNAs being inherited only in the maternal line, which provides a defence mechanism against P elements. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「P element」の詳細全文を読む スポンサード リンク
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