Natural history of model organisms: The secret (group) life of Drosophila melanogaster larvae and why it matters to developmental ecology

Model organisms such as Drosophila melanogaster have been key tools for advancing our fundamental and applied knowledge in biological and biomedical sciences. However, model organisms have become intertwined with the idea of controlled and stable laboratory environments, and their natural history has been overlooked. In holometabolous insects, lack of natural history information on larval ecology has precluded major advances in the field of developmental ecology, especially in terms of manipulations of population density early in life (i.e., larval density). This is because of relativistic and to some extent, arbitrary methodologies employed to manipulate larval densities in laboratory studies. As a result, these methodologies render comparisons between species impossible, precluding our understanding of macroevolutionary responses to population densities during development that can be derived from comparative studies. We recently proposed a new conceptual framework to address this issue, and here, we provide the first natural history investigation of Drosophila melanogaster larval density under such framework. First, we characterized the distribution of larval densities in a wild population of D. melanogaster using rotting apples as breeding substrate in a suburban area in Sweden. Next, we compiled the commonly used methodologies for manipulating larval densities in laboratory studies from the literature and found that the majority of laboratory studies identified did not manipulate larval densities below or above the densities observed in nature, suggesting that we have yet to study true life history and physiological responses to low and high population densities during D. melanogaster development. This is, to our knowledge, the first direct natural history account of larval density in nature for this model organism. Our study paves the way for a more integrated view of organismal biology which re-incorporates natural history of model organisms into hypothesis-driven research in developmental ecology.