A study of corpuscular DNA and midgut gland occupancy by putative symbiotic elements in Pomacea canaliculata (Caenogastropoda, Ampullariidae)

Abstract

Morphology of pigmented corpuscles (C and K types) in the midgut gland of Pomacea canaliculata has suggested that they may be morphotypes of a prokaryotic symbiont. Both corpuscles were isolated from midgut gland tissue to (1) quantify gland occupancy in different experimental conditions and (2) determine and preliminarily characterize DNA present in them. Pigmented corpuscles occupied 8.2–11% of the gland, and this percentage did not vary significantly according to sex or diet (either a mixed or a paper-only diet, during eight weeks). DNA was detected in both corpuscular types. DNA content in C corpuscles was estimated as 57 fg/corpuscle. Determination of the DNA/protein ratio in lysates of C corpuscles yielded values similar to those of bacterial cells used as controls (Escherichia coli, Synechococcus sp.). The DNA/protein ratio in K corpuscles, however, was much lower than in C corpuscles, probably owing to the high protein content of K corpuscles' envelop. PCR amplification of a ~1,500 bp fragment corresponding to the 16S rRNA gene was obtained when using template DNA extracted from both C and K corpuscles, as well as from an Escherichia coli culture used as control. The current results are consistent with the idea of a bacterial nature of C and K corpuscles.