A conserved domain of alkaline phosphatase expression in the Malpighian tubules of dipteran insects

Malpighian (renal) tubules are key components of the insect osmoregulatory system and show correspondingly great diversity in both number and length. Recently, the organisation of the Drosophila melanogaster tubule has been elucidated by enhancer trapping, and an array for functional properties has been shown to align with the functional domains. In Drosophila, there is a lower tubule domain, which coincides with expression of alkaline phosphatase and delineates the absorptive region of the tubule. Here, these observations are extended to three dipteran vectors of disease (Aedes aegypti, Anopheles stephensii and Glossina morsitans) and a non-dipteran outgroup, Schistocerca gregaria (Orthoptera). Despite a huge range in cell number and size, alkaline phosphatase was found on the apical surface of the lower 10% of each of the dipteran tubules but nowhere within the orthopteran tubule. An alkaline phosphatase lower tubule domain is thus conserved among Diptera. Cell counts are also provided for each species. As in Drosophila, stellate cells are not found in the lower tubule domain of Anopheles or Aedes tubules, confirming the unique genetic identity of this domain. As previously reported, we failed to find stellate cells in Schistocerca but, remarkably, also failed to find them in Glossina, the dipteran most closely related to Drosophila. The orthodoxy that stellate cells are unique to, and general among, Diptera may thus require revision.