Buoyancy

Buoyancy, fat and metabolism

In a previous stage of my research career, I spent a considerable amount of time screening a collection of recessive lethal Drosophila strains for abnormal mitotic phenotypes. I looked for such phenotypes in squash preparations from third instar larval brains, and to get the larvae out of the fly food, we often floated them out in salt solution.  So, while scanning through an alert of new publications in PLoS Genetics, one particular title popped out at me:

A Buoyancy-Based Screen of Drosophila Larvae for Fat- Storage Mutants Reveals a Role for Sir2 in Coupling Fat Storage to Nutrient Availability

One of my mantras in the lab is that if one can think of a suitable selection, one can find any kind of mutant phenotype. In this paper, the authors have used a seemingly trivial selection system to identify mutants with a phenotype of altered fat metabolism. You can see the selection in this figure (Panel A in Figure 1): [caption id="attachment_1189" align="alignleft" width="160" caption="Selecting mutant larvae by buoyancy"][/caption] The plastic cuvette is filled with 10% sucrose, and the larvae added and left to float or sink to equilibrium.  You can see the wild type larvae in the left cuvette mostly sink, while all the adp mutant larvae float. Clearly there's a difference in buoyant density between the two genotypes.  adp is a spontaneous mutation identified many years ago.  Somewhat oddly, the authors refer to adp as "a conserved anti-obesity gene first identified as a naturally-occurring mutation in Drosophila".  This sounds kind of odd to me.  The FlyBase record for adp describes it thus:

There is experimental evidence that it is involved in the biological process: lipid metabolic process; response to desiccation; negative regulation of sequestering of triglyceride.

Still, it clearly affects lipid deposition such that adp mutants mostly float in 10% sucrose, thereby demonstrating the efficacy of the selection. The authors used this buoyancy test to screen nearly 900 strains, each bearing transposon insertions, that represent about 500 distinct loci.  66 genes were identified by this assay as affecting body fat composition - some were previously characterised as having a function related to fat physiology, and about two thirds of the genes had mammalian orthologues.