Ultrastructural and molecular characterization of Vairimorpha austropotamobii sp. nov. (Microsporidia: Burenellidae) and Thelohania contejeani (Microsporidia: Thelohaniidae), two parasites of the white-clawed crayfish, Austropotamobius pallipes complex (Decapoda: Astacidae)

Abstract

The microsporidiosis of the endangered white-clawed crayfish Austropotamobius pallipes complex has generally been attributed to only one species, Thelohania contejeani, the agent of porcelain disease. Species identification was mostly assessed by macroscopic examination or microscopic evaluation of muscle samples rather than by molecular or ultrastructural analyses. A survey conducted on A. pallipes complex populations in Northern Italy highlighted the presence of two different microsporidia causing similar muscular lesions, T. contejeani and an undescribed octosporoblastic species Vairimorpha austropotamobii sp. nov. Mature spores and earlier developmental stages of V. austropotamobii sp. nov. were found within striated muscle cells of the thorax, abdomen, and appendages of the crayfish. Only octosporoblastic sporogony within sporophorous vesicles (SPVs) was observed. Diplokaryotic sporonts separated into two uninucleate daughter cells, which gave rise to a rosette-shaped plasmodium, and eight uninucleate spores were produced within the persistent SPV. Ultrastructural features of stages in the octosporoblastic sequence were similar to those described for Vairimorpha necatrix, the type species. Mature spores were pyriform in shape and an average of 3.9 × 2.2 µm in size. The polar filament was coiled 11-14 times, lateral to the posterior vacuole. The small subunit ribosomal RNA gene (SSU rRNA) and the large subunit RNA polymerase II gene (RPB1) of V. austropotamobii sp. nov. were sequenced and compared with other microsporidia. The highest sequence identity of SSU rRNA (99%) and RPB1 (74%) genes was with the amphipod parasite Nosema granulosis and subsequently with V. cheracis, which infects the Australian yabby Cherax destructor. In our work we discuss about the reasons for placing this new species in the genus Vairimorpha. In addition, we provide for T. contejeani a RPB1 gene sequence, supplemental sequences of SSU rRNA gene and ultrastructural details of its sporogony in the host A. pallipes complex.

Keywords: Austropotamobius pallipes complex; Microsporidia; RPB1; SSU rRNA; Thelohania contejeani; Vairimorpha austropotamobii sp. nov..