Highly divergent 18S rRNA gene paralogs in a Cryptosporidium genotype from eastern chipmunks (Tamias striatus).

Cryptosporidium is an apicomplexan parasite that causes the disease cryptosporidiosis in humans, livestock, and other vertebrates. Much of the knowledge on Cryptosporidium diversity is derived from 18S rRNA gene (18S rDNA) phylogenies. Eukaryote genomes generally have multiple 18S rDNA copies that evolve in concert, which is necessary for the accurate inference of phylogenetic relationships. However, 18S rDNA copies in some genomes evolve by a birth-and-death process that can result in sequence divergence among copies. Most notably, divergent 18S rDNA paralogs in the apicomplexan Plasmodium share only 89-95% sequence similarity, encode structurally distinct rRNA molecules, and are expressed at different life cycle stages. In the present study, Cryptosporidium 18S rDNA was amplified from 28/72 (38.9%) eastern chipmunks (Tamias striatus). Phylogenetic analyses showed the co-occurrence of two 18S rDNA types, Type A and Type B, in 26 chipmunks, and Type B clustered with a sequence previously identified as Cryptosporidium chipmunk genotype II. Types A and B had a sister group relationship but shared less than 93% sequence similarity. In contrast, actin and heat shock protein 70 gene sequences were homogeneous in samples with both Types A and B present. It was therefore concluded that Types A and B are divergent 18S rDNA paralogs in Cryptosporidium chipmunk genotype II. Substitution patterns in Types A and B were consistent with functionally constrained evolution; however, Type B evolved more rapidly than Type A and had a higher G+C content (46.3% versus 41.0%). Oocysts of Cryptosporidium chipmunk genotype II measured 4.17 µm (3.73-5.04 µm) × 3.94 µm (3.50-4.98 µm) with a length-to-width ratio of 1.06 ± 0.06 µm, and infection occurred naturally in the jejunum, cecum, and colon of eastern chipmunks. The findings of this study have implications for the use of 18S rDNA sequences to infer phylogenetic relationships.

Odontogenic myxoma in an 8-year-old Labrador Retriever dog.

Odontogenic myxoma (OM) was diagnosed in an 8-year-old Labrador Retriever dog with an ulcerohemorrhagic mass located on the caudal area of the right maxillary gingiva. The neoplasm was characterized by a low mitotic index and moderate numbers of spindle, stellate, and round cells that were sparsely distributed in an alcian blue reactive myxomatous matrix. Individual neoplastic cells were characterized by small amounts of faintly eosinophilic staining cytoplasm, prominent nucleoli, and stippled amphophilic staining chromatin that was immunoreactive for vimentin but negative for cytokeratin and actin. To the authors' knowledge, this is the first reported case of canine OM from North America, and it shares histomorphologic and histochemical features with 3 other cases reported in dogs elsewhere. Whereas, a literature review suggests untreated canine OM is insidious and locally aggressive, the prognosis in the present dog remains unknown. These findings support previous recommendations for inclusion of canine OM on the World Health Organization list of odontogenic tumors.