STUDIES IN QUANTITATIVE INHERITANCE. XII. CELL SIZE AND NUMBER IN RELATION TO GENETIC AND ENVIRONMENTAL VARIATION OF BODY SIZE IN DROSOPHILA

Abstract

NDIVIDUALS may differ in body size for many distinct reasons, both environ-I mental and genetic. and it is of some interest to know how far different causes of variation and also different kinds of genetic behavior are associated with characteristic differences in development. Mere numerical record of genetic varia-tion of body size can identify different types of genetic behavior but lumps together different effects which must be recognized and studied if a fuller under-standing is to be achieved. Previous publications in this series have been con-cerned with various aspects of the genetic variation of body size in Drosophila melanoguster as revealed by selection, inbreeding, chromosome interchange be-tween lines and other tests. It is now necessary to examine such situations from a rather different viewpoint and see whether the combination of genetic and physiological evidence can take the analysis to a deeper level. Ideally the size of an animal or an organ can be described in terms of the size and number of its cells and it would be valuable to know how the cell size and number relations change in different genetic and environmental situations. Such information should, at the same time, throw light on the regulation of body size and of genetic differences in this capacity. It so happens that an insect like Drosophila is quite well suited for such a study. DOBZHANSKY (1929) first pointed out that changes in cell size and number might be estimated from the cell density in the wing membrane which consists of a double layer of cells, each bearing a tiny bristle. The cell outlines can be seen shortly after eclosion but later become obliterated. The number of bristles which occur in a given area of upper or lower surface of the wing provides a measure of the surface area of a cell or cell size. Although gradients in bristle density occur, the cells are regularly arranged and counts in different regions of the wing are quite highly correlated. It is well known that wing and body size vary together when nutrition is altered and many observations (ROBERTSON and REEVE 1952; REEVE and ROBERTSON 1953) have demonstrated a high genetic correlation as well and so differences in wing area generally afford a reliable indication of com-parable differences in body size. Hence records of wing area and cell density provide the basic data for this study in which the variation of size and cellular constitution of a single organ, which is highly correlated with body size, is used as an indication of genetic or environmental differences which influence body size generally.