Molecular and morphological characterisation of a novel microsporidian species, Tubulinosema suzukii, infecting Drosophila suzukii (Diptera: Drosophilidae)
A microsporidium showing morphological characteristics typical of a Tubulinosema species was discovered in Drosophila suzukii. All developmental stages were diplokaryotic and grew in direct contact with the host cell cytoplasm. Spores from fresh preparations were ovoid to slightly pyriform and measured 4.29 x 2.47 μm in wet mount preparations. The spore wall consisted of a 125 nm thick endospore covered by a double layered exospore of 39 nm and 18 nm. The polar filament measured 67 μm in length, was slightly anisofilar and was arranged in ten coils in one or rarely two rows. The two posterior coils were 95 nm in diameter while the anterior coils were 115 nm in diameter. Early developmental stages were surrounded by electron-dense, 35.3 nm diameter, surface ornaments scattered over the membrane. Tubular elements with diameters of approximately 75 nm were seen attaching to the periphery of meronts and sporonts. Tissues infected included fat body, midgut and muscle. A 1915 bp rDNA fragment, covering the small subunit (SSU), the internal transcribed spacer (ITS) and the 5' end of the large subunit ribosomal DNA, was amplified by PCR and sequenced. Phylogenetic analyses of the SSU rDNA fragment revealed closest relationship to Tubulin osema pampeana (Host: Bombus atratus, South America) and Tubulinosema loxostegi (Host: Loxostege sticticalis, ubiquitous), but using the complete dataset of SSU-ITS-LSU rDNA genes revealed T. hippodamiae (Host: Hippodamiae convergeru) as the most closely related species. Based on the morphological and genetic features a new species, Tubulinosema suzukii sp. nov., is proposed for this microsporidium isolated from D. suzukii.