Spermatogenesis and sperm structure of Neopanorpa lui and Neopanorpa lipingensis (Mecoptera: Panorpidae) with phylogenetic considerations

Abstract

Spermatogenesis and sperm ultrastructure were investigated in Neopanorpa lui Chou & Ran and Neopanorpa lipingensis Cai & Hua by light and transmission electron microscopy. Two lateral lamellae run along the lateral grooves of the spermatid nucleus during spermiogenesis, but disappear in the mature spermatozoa. Sperm cells are very similar in ultrastructure between the two species. The spermatozoa are elongated cells, each comprising an apical bilayered acrosome inserted on the anterior nuclear region, an elongate nucleus that occupies the anterior sperm region, a neck region, and a posterior long flagellum. The nucleus has two prominent lateral grooves and assumes a helical appearance. The flagellum is helical and consists of a 9 + 2 axoneme, two mitochondrial derivatives of unequal size, one accessory body, and two extra-axonemal accessory structures. The uniqueness of spermatozoa of Neopanorpa lies in the specifically spoon-shaped larger mitochondrial derivative along most of the flagellum length. A tentative phylogenetic analysis based on sperm structure supports a possible monophyletic clade comprising Panorpidae, Panorpodidae, Bittacidae, and Boreidae of Mecoptera with two non-homoplasious characters: glycocalyx organised in longitudinal ridges and two extra-axonemal accessory structures/rods in flagellum. Closer affinities of Siphonaptera and the mecopteran clade are suggested by the characters of two asymmetric mitochondrial derivatives and the presence of mitochondrial derivative with crystalline structure. The sister group relationship of Panorpidae with Panorpodidae is supported by the presence of nuclear fiber and the irregular shape of the larger mitochondrial derivative.