The sperm aggregation in a whirligig beetle (Coleoptera, Gyrinidae): structure, functions, and comparison with related taxa

Spermatozoa may occur as physical aggregates of two or more cells, which is an array that frequently occurs in Adephaga. We investigated the spermatozoa of Gyretes sp. using light, scanning, and transmission electron microscopy techniques. The spermatozoa of this whirligig beetle were approximately 850 mu m in length and mainly exhibit a three-layered acrosome, flagellum with tiny amorphous accessory bodies, a paraflagellar body, membranous cisterns close the mitochondrial derivatives and axoneme. The spermatozoa occurred in two conformations depending on the organ along the male reproductive tract where they are found. Sperm bundles featuring up to 512 cells bond for the sperm heads formed at the end of the spermatogenesis and leave the testes in this conformation; however, before the vasa deferentia, the spermatozoa were in disarray but became arranged upon attachment to the spermatostyle in the vasa deferentia. The spermatostyle has only been identified in two Adephaga groups, the aquatic Gyrinidae and the terrestrial Carabidae, whereas the other aquatic Hydradephaga species have other mechanisms of sperm aggregation. The formation of a secondary sperm aggregate may be important for increasing the number of spermatozoa that are grouped and transferred to the female, increasing the gene pool of the ejaculate, or improving the selection of viable spermatozoa (ejaculate quality). The spermatozoa of Gyretes sp. share characters with other Gyrinidae species, and present a set of sperm features that distinguish them from those in other Adephaga families, such as apical three-layered acrosome, trapezoid nucleus in cross-section, the presence of a paraflagellar body, tiny and amorphous accessory bodies near membranous cisterns, symmetrical mitochondrial derivatives, among others.

Automated summary

This study investigated the sperm characteristics of Gyretes sp., a whirligig beetle, and found that its sperm has unique structures that distinguish it from other Adephaga species. The formation of secondary sperm aggregates may play an important role in reproduction and gene transfer.