Table 1 Hybrid dysgenesis

In Drosophila, some intraspecific crosses were observed to produce sterile females [50, 51, 80,81,82,83,84,85]. This phenomenon is called hybrid dysgenesis (Ovaries pictures from ref 81). It happens when males possessing a particular TE, hereafter referred as the inducer TE, are crossed with females whose genome is devoid of this TE. On the contrary, the reciprocal cross leads to viable and fertile individuals. The explanation is related to piwi-interacting RNAs (piRNAs), small RNAs repressing TEs with sequence complementarity (see the piRNA section). Because piRNAs are maternally transmitted, in dysgenic crosses the inducer TE insertion is transmitted to the progeny without the piRNAs directed against it [86]. In the reciprocal cross, both the inducer TE insertion and its piRNAs are transmitted, allowing the control of the TE family in the progeny, and the hybrid to be fertile.

Hybrid dysgenesis was documented in three systems in D. melanogaster: associated with P-element, I-element or Hobo [50, 51, 80]. P-element also appears to induce hybrid dysgenesis in D. simulans [81]. In addition, in D. virilis, a hybrid dysgenic cross potentially implying several TEs was reported [82,83,84,85]. From a historical perspective, the hybrid dysgenesis phenomenon played an important role not only in the discovery of horizontal transfers of TEs, but also in the study of host defenses against TEs [86,87,88].