Mitochondrial Placement and Function in Insect Ion-transporting Cells

The ion-transporting epithelia of insects possess some unusualmorphological adaptations which promote close juxtapositionof mitochondria and the ion-transporting plasma membranes. Aparticularly striking example of this adaptation is providedby the movement of branches of mitochondria into and out ofthe apical microvilli in the Malpighian tubules. In the hemipteranRhodmus prohxus, the microvilli in the resorptive lower tubuleare small and contain no mitochondria during the non-transportingperiod. When ion transport is stimulated,either in vivo or invitro, there is a concomitant growth in microvillar volume andsurface area. In addition, branches of mitochondria enter thesemicrovilli. It has been shown that these mitochondrial movementsare driven by an actinassociated process involving the microvillarcore microfilaments. The stimulation for this movement in vivois the insect diuretic hormone. In the lepidopteran Calpodesethhus, the rates of fluid transport which the Malpighian tubulescan sustain vary during the insect's life stages. Larvae andadults show rapid transport, while pupal Malpighian tubulesshow none. In the larvae and adults, microvilli in the Malpighiantubules are large and contain mitochondria. In the pupae, reducedtransport is associated with mitochondrial retraction and microvillarshrinkage. These ultrastructural changes appear tobe regulatedby the insect's developmental hormones. In the Malpighian tubulesof adult female mosquitoes of the genus Aedes, intracellularinfection by microfiliarial nematodes has hen shown to causemitochondrial retraction and reduced rates of fluid transport.A model is presented which serves to summarize currently proposedmechanisms of membrane and mitochondrial function in the ion-transportingepithelia of insects.