Growth rate and life span in Drosophila, IV. Role of cell size and cell number in the biphasic relationship between life span and growth rate

Abstract

The patterns of variation of wing cell size and number were studied under developmental conditions leading to a biphasic relationship between life span and growth rate while duration of development remained constant (development on an agar-only medium with a varying added yeast amount, constant temperature (25°C) and constant larval density). Across the yeast range, a 125% increase of body weight was accompanied by a roughly 30% increase in the wing linear dimensions, wing cell size and wing cell number while estimated duration of cell division and its reciprocal mitotic division rate remained constant. Furthermore, cell size (but not cell number) varied with growth rate in a similar biphasic pattern to that observed for life span. Finally, from a simultaneous examination of the covariation patterns of life span, growth rate, cell size and cell number with decreasing yeast amount, it became apparent that there was a “critical“ yeast amount, approximately 125 mg/120 eggs, below which: (a) cell number abruptly started to decrease linearly from a roughly constant value; (b) the rate of the slow decrease of cell size now tripled and that of growth rate increased even more; and (c) life span which, in the upper yeast range, increased slowly with decreasing yeast, apparently reached a maximum at the critical yeast level and decreased three times faster below that level. These data taken together suggest that: (i) the decrease of all parameters (including life span) below the critical yeast results from a presumably suboptimal or disturbed development because of and in proportion to the lack of nutrients and (ii) the increase of life span with decreasing yeast amount above the critical yeast level has not been definitely explained but some possibilities are suggested such as changes in subcellular organelle numbers, size and/or functional properties, or other changes due to a phenomenon equivalent to food restriction in rats, probably without changes in overall metabolic rate of the flies.