Morphology and development ofloboid-ophthalmoptera, a homoeotic strain inDrosophila melanogaster.

In a previous paper (Ouweneel, 1969) the genetic basis has been described of a uew homoeotic strain, calledloboid-ophthalmoptera (ld-opht). In the present paper the morphological appearance and the development of the homoeotic effect are dealt with. In the adult fly the eye is often considerably reduced and kidney-shaped (Fig. 2). In the hypodermal part of the eye area frequently outgrowths occur; the smaller ones consist of head hypodermis, the larger ones of wing tissue, mostly wing spread (Figs. 3 and 4). The average localization of the outgrowths in the eye area was determined (Fig. 5). In the homoeotic outgrowths beside the wing spread, the costal region of the wing is often represented. Also the border bristles ("triple row" and "double row") and the wing veins, as well as parts of the wing base and possibly the humeral plate, were found (Figs. 6-10).In vivo it appeared that protuberances later giving rise to homoeotic outgrowths arise at the time of first folding of the disc; this folding is much more extensive than in the wild type disc, so that the size of the flat, unfolded part of the disc (which will form the ommatidia) is considerably reduced (Fig. 11). The appearance of the protuberances seems to be the result of a strong, local stimulus to increased growth rate. Histological examination ofld-opht eye discs has confirmed that the area occupied by ommatidial precursors is often strongly reduced by the thickened folds of the disc. The protuberances arise as evaginations from the folded part of the disc, and consequently must evert during the pupal stage, along with the eversion of the cephalic complex (Figs. 12-15). The average localization of the disc protuberances turned out to correspond to the average localization of the homoeotic outgrowths in the eye area (Fig. 16).A developmental history is proposed for theld eye, based upon two actions of the gene, first a reduction of eye disc size (first instar ?), and secondly a local hyperplasia (extensive folding) of the disc reducing the ommatidial field (third instar). The occurrence of wing tissue inld-opht is ascribed to local strong proliferation (due to certain modifiers present in the strain) leading to transdetermination, its chance being proportional to the rate of proliferation.