A multiple comparative study of putative endosymbionts in three coexisting apple snail species

Abstract

We here compare morphological and molecular characters of some putative endosymbioticelements of the digestive gland of three ampullariid species (Pomacea canaliculata,Pomacea scalaris and Asolene platae) which coexist in Lake Regatas (Palermo, BuenosAires). The putative endosymbionts were reported in these species and were identifiedas C and K corpuscles. The three species show tubuloacinar glands, each adenomerewas constituted mainly by two distinct cell types (columnar and pyramidal). C and Kcorpuscles together occupied from one-fourth to one-fifth of the tissue area in the threehost species, where C corpuscles were round and greenish-brown, were delimited bya distinct wall, stained positively with Alcian Blue and were associated with columnarcells. K corpuscles were oval, dark-brown multilamellar bodies and were associatedwith pyramidal cells. Under TEM, C corpuscles occurred within vacuoles of columnarcells and contained many electron-dense clumps and irregular membrane stacks andvesicles spread in an electron-lucent matrix. Sometimes a membrane appeared detachedfrom the inner surface of the wall, suggesting the existence of a plasma membrane. Inturn, K corpuscles were contained within vacuoles of pyramidal cells and were madeof concentric lamellae, which were in turn made of an electron-dense fibrogranularmaterial. No membranes were seen in them. Interspecifically, C corpuscles varysignificantly in width and inner contents. K corpuscles were also variable in lengthand width. However, both C and K corpuscles in the three studied species hybridisedwith generalised cyanobacterial/chloroplast probes for 16S rRNA. Also, both corpuscletypes (isolated from gland homogenates) were sensitive to lysozyme digestion, whichindicates that bacterial peptidoglycans are an integral part of their covers. The reporteddata confirm and extend previous studies on P. canaliculata in which the endosymbioticnature of C and K corpuscles were first proposed. We further propose that theendosymbiotic corpuscles are related to the Cyanobacteria/chloroplasts clade. Basedon the known distribution of these corpuscles in the major clades of Ampullariidae,we hypothesise they may be universally distributed in this family, and that mayconstitute an interesting model for studying the co-evolution of endosymbionts andtheir gastropod hosts.