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

We here compare morphological and molecular characters of some putative endosymbiotic elements of the digestive gland of three ampullariid species (Pomacea canaliculata, Pomacea scalaris and Asolene platae) which coexist in Lake Regatas (Palermo, Buenos Aires). The putative endosymbionts were reported in these species and were identified as C and K corpuscles. The three species show tubuloacinar glands, each adenomere was constituted mainly by two distinct cell types (columnar and pyramidal). C and K corpuscles together occupied from one-fourth to one-fifth of the tissue area in the three host species, where C corpuscles were round and greenish-brown, were delimited by a distinct wall, stained positively with Alcian Blue and were associated with columnar cells. K corpuscles were oval, dark-brown multilamellar bodies and were associated with pyramidal cells. Under TEM, C corpuscles occurred within vacuoles of columnar cells and contained many electron-dense clumps and irregular membrane stacks and vesicles spread in an electron-lucent matrix. Sometimes a membrane appeared detached from the inner surface of the wall, suggesting the existence of a plasma membrane. In turn, K corpuscles were contained within vacuoles of pyramidal cells and were made of concentric lamellae, which were in turn made of an electron-dense fibrogranular material. No membranes were seen in them. Interspecifically, C corpuscles vary significantly in width and inner contents. K corpuscles were also variable in length and width. However, both C and K corpuscles in the three studied species hybridised with generalised cyanobacterial/chloroplast probes for 16S rRNA. Also, both corpuscle types (isolated from gland homogenates) were sensitive to lysozyme digestion, which indicates that bacterial peptidoglycans are an integral part of their covers. The reported data confirm and extend previous studies on P. canaliculata in which the endosymbiotic nature of C and K corpuscles were first proposed. We further propose that the endosymbiotic corpuscles are related to the Cyanobacteria/chloroplasts clade. Based on the known distribution of these corpuscles in the major clades of Ampullariidae, we hypothesise they may be universally distributed in this family, and that may constitute an interesting model for studying the co-evolution of endosymbionts and their gastropod hosts.