Sperm ultrastructure in the water beetles (Insecta, Coleoptera)

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Abstract The ultrastrucrure of conjugated spermatozoa of some dytiscid beetles was examined in ultrathin sections and freeze‐fracture replicas. Whereas spermatozoa of Melanodytes pustulatus are conjugated pairwise, those of Colymbetes fuscus are conjugated in larger groups. Other members of the same subfamily (Colymbetinae) may have single or multiple spermatozoa. Features seen in the ultrathin section that were found to characterize spermatozoa from this subfamily include the hooded structure of the sperm nucleus, the absence of a ‘paraflagellar body’, a prominent mass of homogeneous appearance that seems to act as a cementing material when conjugation occurs, and the feature that the sperm nucleus turns its convex side toward the cementing material. In these respects the Colimbetinae spermatozoa seem to differ from those of the Dytiscinae subfamily. The spermatozoa of Graphoderus cinereus (subfamily Dytiscinae) were seen to be conjugated pairwise and to have mitochondrial derivatives which consist of conspicuous globular subunits. Upon transfer to the female spermatheca the paired spermatozoa of Dytiscus marginalis dissociate to single spermatozoa and were seen to loose the granular cementing material. The glycocalyx that covers the plasma membrane along the ‘accessory bodies’ of the sperm tail similarly is lost, although the corresponding intramembrane particles made visible by freeze‐fracture replication remain. A tubular material makes its appearance in the fluid surrounding the dissociated spermatozoa.