Vairimorpha imperfecta n.sp., a microsporidian exhibiting an abortive octosporous sporogony in Plutella xylostella L. (Lepidoptera : Yponomeutidae)

The microsporidian genus Nosema is characterized by development in direct c ontrol with host cell cytoplasm, diplokaryotic nuclei throughout developmen t and disporous sporogony. The genus Vairimorpha exhibits the same features plus an octoporous sporogony producing uninucleate spores in a sporophorou s vesicle. A microsporidium from diamondback moth, Plutella xylostella, fal ls between Nosema and Vairimorpha in that it initiates but fails to complet e the octosporous sequence in this host. The name Vairimorpha imperfecta n. sp. is proposed. Merogony is mainly by formation of buds from multinucleate meronts, the buds remaining attached in chains. Diplokaryotic spores measu re 4.3 x 2.0 mu m (fresh) and have 15.5 coils of the polar tube in 1 rank. The octosporous sporogony is aborted owing to irregular formation of nuclea r spindles, incomplete cytoplasmic fission and bizarre deposition of electr on-dense episporontal secretions. Phylogenetic analyses of the sequences of the small subunit rRNA genes of V. imperfecta and of several Nosema and Va irimorpha spp. place V. imperfecta in a clade with Nosema spp. from Lepidop tera rather than in the clade containing the more typical species of Vairim orpha. It is suggested that the ancestors of the Vairimorpha/Nosema complex of species exhibited both disporous and octosporous sporogonies, as does t he type species of Vairimorpha, Vairimorpha necatrix. It would follow that true Nosema spp. have lost the ability to express an octosporous sequence a nd that V. imperfecta is in the process of losing it. It is proposed that t he genera Nosema and Vairimorpha be placed in the same family Nosematidae L abbe 1899, rather than in separate families and orders as at present.