Ultrastructure of the spermatozoon of the tuatara ( Sphenodon punctatus ) and its relevance to the relationships of the Sphenodontida

Abstract

Spermatozoa of the New Zealand tuatara , Sphenodon punctatus punctatus (Gray), are described from light and electron microscopic observations and compared with spermatozoa of other living ‘reptiles’ (Chelonia, Crocodilia, Squamata), birds and mammals. Mature Sphenodon spermatozoa consist of an acrosomal complex (length 4 µm), elongate, helical nucleus (54-56 µm), a relatively short midpiece (7-8 µm), elongate principal piece (74-78 µm) and short end piece (2-4 µm). The acrosomal vesicle and underlying subacrosomal material form a double, curved, conical sheath around the nucleus anteriorly. Two parallel, loosely helical, endonuclear canals each containing perforatorial material, extend posteriorly from the apex of the nucleus to at least 2.5 µm below the base of the acrosomal complex. Rings of several spherical mitochondria are stacked around the elongate distal centriole to form the midpiece. Each mitochondrion has concentric cristae surrounding a dense central body. Proximal and distal centrioles, although differing markedly in length, are similar in having triplets with an open C tubule. Nine peripheral fibres are intimately associated with the triplets of the distal centriole. A well developed annulus defines the posterior extremity of the midpiece. The principal piece consists of a 9 + 2 axoneme (accompanied anteriorly by nine peripheral fibres) surrounded by a highly electron-dense fibrous sheath and the plasma membrane. Absence of a penis in Sphenodon has not resulted in recognizable modifications of the spermatozoon. Sphenodon shares many spermatozoal features here interpreted as plesiomorphies with crocodiles and turtles, particularly the latter group, but exhibits none of the advanced character states (apomorphies) diagnostic of the Squamata. These data not only underscore the primitive status of living tuatara (recently questioned in the literature) but also militate against a close, sister-group relationship between the Sphenodontida and Squamata.