Abstract

Disease is one of the primary threats to reptile populations, leaving them imperiled globally. However, modern immune research on these ectotherms has lagged behind endothermic birds and mammals. This imbalance leaves conservation biologists to act without a full understanding of reptile host-pathogen dynamics. Innate immunity involves cellular and acellular responses, including natural antibodies (NAbs) and the serum complement cascade, responsible for eliciting a nonspecific, fast-acting response against microbial infections. The goal of this experiment was to examine interclade variation in acellular innate immune function amongst three major extant reptile clades. Specifically, we characterized innate immune performance in two Squamates (Nerodia sipedon and Sistrurus miliarius), two Testudines (Chrysemys picta and Pseudemys rubriventris), and a Crocodilian (Alligator mississippiensis). We hypothesized that immune performance would vary among groups and reflect phylogenetic relationships. We employed immune performance assays challenging individuals’ plasma against a gram-negative bacterium, E. coli, and mammalian erythrocytes in vitro. Bactericidal assays (BKA) provide an assessment of complement mediated-lysis via the classical and lectin-mediated pathways. Erythrocyte hemolysis assesses complement-mediated cell lysis via the classical and alternative assays, and erythrocyte agglutination reflects the activity of NAbs. Complement performance of squamates was significantly better than turtles and crocodilians, which did not perform differently from each other. Our results support the hypothesis that the major reptile clades differ in complement performance and that differences may reflect broad phylogenic relationships among clades, therefore, highlighting the need for further exploring of interclade variation in immune function among reptiles.