Well-fed larvae switch ion transport in rectal complex

The key to a good larval life is to eat as much as possible, regardless of the havoc wreaked on crops. So, gorging larvae have to balance their fluid intake and losses depending on how moist or dry their diet; which is why some species have evolved a specialised organ for reclaiming fluid from the gut, known as the rectal complex. Michael O'Donnell and Esau Ruiz-Sanchez from McMaster University, Canada, explain that the rectal complex also regulates the levels of Na+ and K+ ions in the insect's haemolymph, which would otherwise fluctuate wildly when the larvae are gorging and growing rapidly. However, little was known about the fine details of fluid transport between the rectal complex and the larva's other excretory organ, the Malpighian tubule. So, O'Donnell and Ruiz-Sanchez teamed up to build a more detailed understanding of the delicate interplay of fluid and ion transport between the two structures in well-fed and hungry larvae.Collecting fourth instar larvae from a lab colony of cabbage looper moths (Trichoplusia ni), the duo first investigated the relative architecture of the Malpighian tubule and rectal complex. They identified the distributions of two different cell types – known as principal (type I) cells and secondary (type II) cells – in the two structures. Then the duo began the incredibly intricate task of measuring ion transport at different locations across the structures using thin ion-selective microelectrodes positioned with computer-controlled micron precision at different locations on both structures.Having discovered that the rectal complex unexpectedly reabsorbs Na+ and K+ ions from the gut and returns them to the haemolymph, O'Donnell and Ruiz-Sanchez also realised that the larvae's ability to transport Na+ and K+ ions varies dramatically – depending on whether or not they are well fed and the location in the rectal complex and Malpighian tubules – despite the apparently identical appearance of many sections of the tubule. In one example, the duo describes how ion transport differs along the section of the gut known as the rectal lead when the larvae are hungry and well fed. They explain that larvae with full guts reabsorb K+ in the distal rectal lead, but when the larvae's guts are empty K+ is secreted in the proximal rectal lead. In contrast, well-fed larvae secrete Na+ ions across the distal rectal lead while reabsorbing the ions across the proximal rectal lead. And when the duo analysed which cell types were involved in ion transport, they found that the type I cells in the section of tubule linking the Malpighian tubule to the rectal complex – known as the ileac plexus – secreted K+ ions. However, they were surprised that the type II cells reabsorbed K+ and Na+ ions, with the type II cells transporting Na+ at twice the rate of the type I cells.O'Donnell and Ruiz-Sanchez admit that they are surprised by the complex pattern of secretion and ion reabsorption and they are also intrigued about the change in direction of ion secretion in different regions of the Malpighian tubule when the larvae are fed. However, they are optimistic that they will be able to learn more about these puzzles using the techniques that they have developed in this study and O'Donnell says, ‘I see 5–10 years of exciting work ahead’.

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