Studies in Quantitative Inheritance. Xiii. Interrelations between Genetic Behavior and Development in the Cellular Constitution of the Drosophila Wing.

HE preceding paper of this series (ROBERTSON 1959) and the present one Tdeal with different parts of a single study of the relations between cell size and number in the wing of Drosophila melanogaster. The reasons for examining this problem together with details of procedure and statistical analysis are given in the earlier paper but a brief resunik will help the reader to appreciate what follows. Wing size is highly correlated with body size. Since it is possible to escmate cell size and number in the wing, we can follow changes in the cellular constitution of the wing which accompany the alteration of body size for either genetic or environmental reasons. The wing is made up of a double layer of cells, each of which carries a tiny bristle so that the bristle density in a given area on one surface of the wing provides an estimate of cell area. The evidence presented in the earlier paper showed that variation in the pattern of bristle density in different regions of the wing is relatively unimportant in the material studied in these experiments and that the bristle density in a central region of the wing provides an adequate measure of general cell area or cell size. The records of wing and cell area are expressed in natural logarithms of squared hundredths of a millimeter. Cell number can be estimated from the difference between log wing and log cell area. The effects of environmental variation were studied by growing larvae at different temperatures and also on chemically defined, aseptic media, deficient in some essential nutrient, which led to smaller wing and body size. Lower temperature during growth results in larger body size; the observed differences in wing size are expressed almost entirely by changes in cell size and the average estimated cell number is about the same at high and low temperatures, which are responsible for big differences in wing area. On the other hand, when the larval diet is suboptimal, provided it is not too deficient, there is a distinct tendency for cell size to remain constant and cell number to decline. But with more extreme conditions, cell size declines as well. Thus equivalent changes in total wing and body size, resulting from different kinds of environmental change are associated with different relations between cell size and number. The relative contribution of genetic effects to the total within-culture variance for various …