text: 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 chromosomal 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 defense mechanism against P elements.