Ecological correlates of body size in relation to cell size and cell number: patterns in flies, fish, fruits and foliage

Body size is important to most aspects of biology and is also one of the most labile traits. Despite its importance we know remarkably little about the proximate (developmental) factors that determine body size under different circumstances. Here, I review what is known about how cell size and number contribute to phenetic and genetic variation in body size in Drosophila melanogaster, several fish, and fruits and leaves of some angiosperms. Variation in resources influences size primarily through changes in cell number while temperature acts through cell size. The difference in cellular mechanism may also explain the differences in growth trajectories resulting from food and temperature manipulations. There is, however, a poorly recognized interaction between food and temperature effects that needs further study. In addition, flies show a sexual dimorphism in temperature effects with the larger sex responding by changes in cell size and the smaller sex showing changes in both cell size and number. Leaf size is more variable than other organs, but there appears to be a consistent difference between how shade-tolerant and shade-intolerant species respond to light level. The former have larger leaves via cell size under shade, the latter via cell number in light conditions. Genetic differences, primarily from comparisons of D. melanogaster, show similar variation. Direct selection on body size alters cell number only, while temperature selection results in increased cell size and decreased cell number. Population comparisons along latitudinal clines show that larger flies have both larger cells and more cells. Use of these proximate patterns can give clues as to how selection acts in the wild. For example, the latitudinal pattern in D. melanogaster is usually assumed to be due to temperature, but the cellular pattern does not match that seen in laboratory selection at different temperatures.