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Molecular Divergence Defines Two Distinct Lineages of Crithidia bombi (Trypanosomatidae), Parasites of Bumblebees


Corresponding Author: R. Schmid-Hempel, Institute of Integrative Biology (IBZ), ETH Zürich, Universitätsstrasse 16, CH-8092 Zürich, Switzerland—Telephone number: +41 44 633 6038; e-mail:


ABSTRACT. This study provides, for the first time, sequence data for the protozoan flagellates Crithidia bombi and Crithidia mellificae (Kinetoplastea: Trypanosomatidae). We amplified the partial sequences of the small subunit ribosomal RNA (SSU rRNA), glycosomal glyceraldehyde phosphate dehydrogenase (gGAPDH), cytochrome b (Cyt b), and the complete internal transcribed spacer region 1 (ITS1) of the ribosomal RNA gene region for 66 clones of C. bombi from Switzerland and Alaska. Furthermore, we sequenced the same stretch of SSU rRNA and gGAPDH for one isolate of C. mellificae from Switzerland. All four molecular markers classified the C. bombi samples into two distinct lineages A and B. Both lineages were found in the two sampling locations. Variation within lineages was small or non-existing. Sequence differences between lineages were 1.64% for SSU rRNA, 4.36% for gGAPDH, and 12.02% for Cyt b. The ITS1-sequences of lineages A and B have diverged so much that no alignment was possible. With regard to ITS1, we additionally found fragment length polymorphism (variation in microsatellite repeat numbers) as well as nucleotide diversity within each lineage. Furthermore, the sequences of SSU rRNA and gGAPDH of C. mellificae were different from both lineages of C. bombi. The separation of lineages A and B, based on sequence differences and phylogenetic reconstruction, is so pronounced as to characterize two species of “C. bombi.” We propose to retain C. bombi for the more common lineage A and suggest the name Crithidia expoeki n. sp. for lineage B.