Abstract
How multicellular organisms assess and control their size is a fundamental question in biology, yet the molecular and genetic mechanisms that control organ or organism size remain largely unsolved. The freshwater polyp Hydra demonstrates a high capacity to adapt its body size to different temperatures. Here we identify the molecular mechanisms controlling this phenotypic plasticity and show that temperature-induced cell number changes are controlled by Wnt- and TGF-β signaling. Further we show that insulin-like peptide receptor (INSR) and forkhead box protein O (FoxO) are important genetic drivers of size determination controlling the same developmental regulators. Thus, environmental and genetic factors directly affect developmental mechanisms in which cell number is the strongest determinant of body size. These findings identify the basic mechanisms as to how size is regulated on an organismic level and how phenotypic plasticity is integrated into conserved developmental pathways in an evolutionary informative model organism.
Highlights
How multicellular organisms assess and control their size is a fundamental question in biology, yet the molecular and genetic mechanisms that control organ or organism size remain largely unsolved
With decreasing temperature the number of epithelial cells increases by 83% (12 °C; 23,698 ± 3537; n = 18) with no further gain in cell number at 8 °C (21,441 ± 8565; n = 16) (Fig. 1b)
As in the insulin-like peptide receptor (INSR)-KD polyps, we found no evidence for cell size changes due to forkhead box protein O (FoxO)-KD (Fig. 3c)
Summary
How multicellular organisms assess and control their size is a fundamental question in biology, yet the molecular and genetic mechanisms that control organ or organism size remain largely unsolved. Environmental and genetic factors directly affect developmental mechanisms in which cell number is the strongest determinant of body size These findings identify the basic mechanisms as to how size is regulated on an organismic level and how phenotypic plasticity is integrated into conserved developmental pathways in an evolutionary informative model organism. Even though alterations of environmental factors or conserved pathways seem to affect body size, it is not well understood, (i) how environmental cues affect genetic programs, (ii) how size is measured by cells or tissue during growth, and (iii) by what processes termination of growth is initiated Understanding of these processes promises to contribute to the comprehension of ecological and developmental interactions on a molecular level, but may have major implication for evolutionary concepts
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