Macrobdella decora: Old World Leech Gut Microbial Community Structure Conserved in a New World Leech

Abstract

Leeches are found in terrestrial, aquatic, and marine habitats on all continents. Sanguivorous leeches have been used in medicine for millennia. Modern scientific uses include studies of neurons, anticoagulants, and gut microbial symbioses. Hirudo verbana, the European medicinal leech, maintains a gut community dominated by two bacterial symbionts, Aeromonas veronii and Mucinivorans hirudinis, which sometimes account for as much as 97% of the total crop microbiota. The highly simplified gut anatomy and microbiome of H. verbana make it an excellent model organism for studying gut microbial dynamics. The North American medicinal leech, Macrobdella decora, is a hirudinid leech native to Canada and the northern U.S.A. In this study we show that M. decora symbiont communities are very similar to those in H. verbana. This similarity allowed for an extensive study in which wild caught animals were sampled to determine effects of geographic separation, time of collection, and feeding on the microbiome. Through 16S V4 rRNA deep sequencing we show that: i) the M. decora gut and bladder microbial communities are distinct, ii) the M. decora gut community is affected by feeding and long periods of starvation, and iii) geographic separation does not appear to affect the overall gut microbial community structure. We propose that M. decora is a replacement for H. verbana for studies of wild-caught animals and offer evidence for the conservation of annelid symbionts. Successful culturing and comparison of dominant symbionts from M. decora and H. verbena will provide the ability to assess host-symbiont co-evolution in future work.\n\nIMPORTANCEBuilding evidence implicates the gut microbiome in regulating animal digestion, nutritional acquisition, immune regulation, development, and even mood regulation. Because of the difficulty of assigning causative relationships in complex gut microbiomes a simplified model for testing hypotheses is necessary. Previous research in Hirudo verbana has suggested this animal as a highly simplified and tractable animal model of gut symbioses. Our data show that Macrobdella decora may work just as well as H. verbana without the drawback of being an endangered organism and with the added convenience of easy access to field-caught specimens. The similarity of the microbial community structure of species from two different continents reveals the highly-conserved nature of the microbial symbionts in sanguivorous leeches and confirms the medicinal leech as a highly simplified, natural animal model in which to study gut symbioses.