Influence of environmental factors on the homoeotic effect ofloboid-ophthalmoptera inDrosophila melanogaster.

This paper deals with the influence of environmental factors, particularly those that may be expected to change the rate of growth in the larval period, on the homoeotic wing-like outgrowths in the eyes of the strainloboid-ophthalmoptera (see Ouweneel, 1969a, b).The penetrance of the homoeotic effect increases with maternal age (Table 1); Delcour (1968) furnished evidence that there is a relation between maternal age and growth rate of tissues in the larva. Penetrance is inversely related to temperature; at low temperatures the development of the whole larva is strongly retarded, but the growth of the eye disc is probably less retarded, so that it grows relatively more than at higher temperatures. It is notable that the temperature-sensitive period (24-60 h after hatching, at 25° C; Fig. 1) precedes, and coincides with, the period of abnormal hyperplasia in the disc (Ouweneel, 1969b).Acetamide, given in the food, enhances the number of facets so that even the wild type eye size may be reached; in proportion to its concentration it moreover strongly increases the penetrance of the wing-like outgrowths. At higher concentrations, however, when eye enlargement is maximal, the penetrance of the outgrowths decreases again (Figs. 2-5). The optimal concentration (at which the penetrance is maximal) is higher at higher temperatures (Figs. 2-5; cf. Fig. 7). The sensitive period for acetamide extends from about 0-55 h after hatching (at 25° C) (Fig. 6). Uracil increases the penetrance of the homoeotic effect (Table 2). Sodium tetraborate seems to increase the penetrance of the homoeotic outgrowths, but to reduce their expressivity slightly at higher concentrations. Starvation (Tabel 3) and farnesol medium prolong the larval period, but do not show clear-cut effects on the homoeotic phenomenon, probably because they only postpone metamorphosis, and do not retard the whole of larval development (compare temperature effect, above). UV irradiation influenced penetrance, but lower dosages had a greater effect than higher ones; the former suppressed the homoeotic effect at early larval ages, but stimulated it at later ages; this points to the occurrence of two separate sensitive periods for UV (Table 4). In all experiments clear-cut sexual differences in sensitivity were observed, which may be ascribed to the sex-linked location of theophthalmoptera modifier (Ouweneel, 1969a).Evidence can be found in the literature that enhanced proliferation is a prerequisite for many homoeotic phenomena in the broad sence (cf. Fig. 8). The present study and previous data show that also homoeotic mutations often interact with agents causing changes in growth rate; it is possible that the basic action of all homoeotic mutations has to do with such changes.