Nuclear and mitochondrial ribosomal ratio as an index of animal growth rate

Quantification of growth is one of the important steps to understand the flow of elements and energy in food webs although in-situ estimation of the growth rate in general is very difficult. The present study proposes the nuclear and mitochondrial ribosomal ratio as a novel index of animal growth rate. The ribosome is a biomolecule composed of ribosomal-RNAs and -proteins, through which various proteins are synthesized using information encoded in mRNAs. Two types of ribosomes can be found in metazoan cells: mitochondrial ribosome and nuclear-encoded cytosolic ribosome. Functional roles of the mitochondrial ribosome (production of the proteins required to generate adenosine triphosphate, which provides energy to the cell) and cytosolic ribosome (production of diverse proteins required for various biological reactions, including cell growth and division) are different. Furthermore, ribosomes are macromolecules that require large amounts of environmentally limiting elements, such as phosphorus and nitrogen. Therefore, it is hypothesized that the allocation of those elements between the two ribosomes changes with nutrient availability. The present study conducted laboratory growth experiments of Daphnia magna using various food concentration and temperature treatments. As a result, positive correlation was found between nuclear-encoded cytosolic to mitochondrial ribosomal (Nuc/Mito ribosomal) ratio and somatic growth rate. A significant positive correlation was found between food concentration treatments and the ratio, but not between temperature treatments and the ratio. Our results demonstrate that the ratio between nuclear-encoded cytosolic and mitochondrial ribosome is an effective growth rate estimator.