Abstract
Bdelloura candida (Platyhelminthes, Tricladida, Maricola) is an ectocommensal symbiont on the American horseshoe crab Limulus polyphemus, living on the book gills and appendages, where it spends its entire life. Given its limited dispersal capabilities and its inability to live outside of the host, we hypothesized a genetic structure that parallels that of its host. We obtained 84 planarian individuals from 19 horseshoe crabs collected from 10 sites from Massachusetts to Florida. We amplified the mitochondrial 16S rRNA and the nuclear internal transcribed spacer 2 and conducted phylogeographic and population genetic analyses, which show a clear and strong genetic break between the populations in the Atlantic and the Gulf coasts. Among the Atlantic populations, two additional, weaker barriers located along Cape Hatteras and Cape Cod restrict gene flow. Even though previous studies have suggested that the populations of the host may be in decline, those of B. candida remain stable, and some even shows signatures of expansion. Our results indicate that the phylogeography of these marine ectocommensal triclads closely mirrors that of its Limulus host, and highlight the challenges to both host and symbiont to genetically connect populations across their distribution.
Highlights
The study of symbiosis is a growing field in biology, requiring integration of multiple disciplines (McFall-Ngai 2008)
Since genetic data are not available for B. candida to test whether its genetic structure mirrors that of L. polyphemus, we examined specimens from hosts at ten sites along the Atlantic and Gulf coasts of the USA, as we wanted to ascertain whether the commensal showed the signature of the host’s phylogeographic structure
For most populations, haplotype diversity was higher for the internal transcribed spacer-2 region (ITS2) than for 16S rRNA, given that less individuals were sequenced for ITS2 than for 16S rRNA
Summary
The study of symbiosis is a growing field in biology, requiring integration of multiple disciplines (McFall-Ngai 2008). Symbiotic relationships among different phyla are often “loose”, especially among ectocommensal animals, which are commonly non-specific Many such ectocommensal relationships are known to be strictly specific, such as those of cycliophorans with their nephropid (clawed) lobsters (Funch and Kristensen 1995; Obst et al 2006), where a faithful one-to-one species–host relationship exists, even in places where two host species coexist (Baker et al 2007; Baker and Giribet 2007).
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