How do insect tubules achieve the highest transport rates in biology?

Insects are poised on the edge of osmoregulatory oblivion; scaling arguments dictate that terrestrial insects live on the brink of desiccation. It thus seems paradoxical that excretory Malpighian tubules secrete water faster on a per-cell basis than any other epithelium. How is this accomplished, and how is it controlled? A convergence of classical physiology and molecular genetics has allowed rapid progress in the small model, Drosophila melanogaster. We have developed a two cell-model, in which active, electrogenic cation transport takes place through the large, mitochondria-rich principal cell, whereas passive anion and water fluxes are confined to the smaller stellate cell. Multiple endocrine pathways converge on these cell types, providing independent control of cation and anion fluxes.

Can the insights obtained in Drosophila be applied more widely? Classically, the stellate cells – a prerequisite of the two-cell model – is thought to be Dipteran-specific; and chloride flux through stellate cells is activated by the kinin neuropeptides (1). To probe for the existence of non-Dipteran stellate cells,  we developed a new, fluorescent labelling strategy to systematically map kinin-binding cells in tubules of insect species chosen to provide representation of 90% of insect biodiversity (2). Remarkably, specialized kinin-binding cells could be found across the advanced Endopterygotes (except the beetles!), but kinin binding was a general property of all tubule cells in the Exopterygota. These data provide the first opportunity to map renal function across the Insect Class.

(1) Cabrero et al. (2014).  PNAS 111, 14301-6.

(2)  Halberg et al. (2015). Nature Communications 6. doi:10.1038/ncomms7800