Depression,heterosis and carry-over effects after many generations of inbreeding withDrosophila melanogaster

Abeele and Gabarros₄, two inbred lines originating from natural populations, and among which the size was stable at least up to generation 12 and generation 22 respectively, while the viability was decreasing, have been studied in generation 63 and 73 respectively. Different environments including temperature and egg-density conditions were used. The viability and size were diminished to a different extent in the two lines. Hybrids obtained by crossing the 87th and 82nd generation, respectively, of these two lines show a definite heterosis for size and a viability percentage smaller than the one of the parental inbred lines. No reciprocal differences, except in high density conditions, were observed. A second generation, obtained from the F₁ reciprocals raised in different density environments show a breakdown of heterosis for size and a large increase in viability. Carry-over effects due to the different environments in which the reciprocal hybrids were grown were also observed in the second generation. The interference of cytoplasm for transmission of acquired characters seems almost certain. Questions are raised concerning the selection opportunity in inbred lines, the influence of parental inbreeding on the character of the hybrids, the opportuneness of the generalization of the homeostasis idea, and the transmission of acquired characters.