Prey, populations, and the pleistocene: evidence for low COI variation in a widespread North American leech

Abstract

Placobdella rugosa has long presented challenges to leech biologists. Its extreme morphological variability and similarity to some congeneric species has confounded classification for over a century. Recent molecular analyses revealed a surprising lack of genetic variation among morphologically disparate, geographically widespread specimens of P. rugosa. Given the lack of any obvious mechanism by which this species could disperse between distant habitats, it was expected that widespread populations would be genetically isolated from each other. In the present study, we investigate the relationship between geographic distance and genetic diversity in P. rugosa using COI sequences from specimens collected across Canada and the United States. Although we find preliminary evidence for a barrier to gene flow between eastern and western collecting localities, our vastly expanded dataset largely corroborates prior studies, showing minimal phylogeographic signal among the sequences and negligible levels of genetic isolation by distance. A recent range expansion following the last ice age and/or host-mediated dispersal are discussed as potential explanations for this unexpected phylogeographic pattern.

Keywords:

• Hirudinea
• Placobdella
• leech
• COI variation
• phylogeography
• North America

Acknowledgements

We are grateful to Kristen Choffe for assistance in DNA sequencing. Rafael Iwama, Claire Manglicmot, Madeleine Foote, Kevin Anderson, and Don Stacey were all instrumental in collecting the Royal Ontario Museum specimens used in this study. We thank Michael Tessler for the script used to run TNT, and the Willi Hennig Society for making TNT freely available. One anonymous reviewer and the Editor provided feedback and suggestions that greatly improved a previous version of this manuscript. SK was funded by a NSERC Discovery Grant and an Olle Engkvist Byggmästare Foundation grant, JM was funded by an NSERC Undergraduate Student Research Award, and Dd was funded by an NSERC Postgraduate Doctoral Scholarship (PGSD2-518435-2018).

Disclosure statement

No potential conflict of interest was reported by the authors.