Apyrene Sperm as a Key Factor for Sperm Competition

Abstract

Both types of sperm, eupyrene and apyrene, are individually structured in the testis as bundles. Until accumulation in the seminal vesicles, apyrene sperm bundles disappear, and free apyrene spermatozoa are ready to transfer although the eupyrene sperm is still bundled. During copulation, apyrene spermatozoa and eupyrene sperm bundles are transferred, and consequently, immediately after copulation, the spermatophore contains free apyrene spermatozoa and eupyrene sperm bundles. The eupyrene sperm bundles disappear in the spermatophore after copulation, and free apyrene spermatozoa start to migrate towards the spermatheca before the free eupyrene spermatozoa move. Although free apyrene spermatozoa arrive at the spermatheca earlier than free eupyrene spermatozoa, the spermatheca contains both apyrene and eupyrene spermatozoa. The spermathecal gland, which is a cul-de-sac tube connected to the spermatheca, is also filled gradually by apyrene and then eupyrene spermatozoa after sperm migration. Eupyrene spermatozoa in the spermatheca are used in fertilisation during oviposition, and decrease with the number of eggs laid. Changes in the number of apyrene spermatozoa both in the spermatophore ejaculated and in the spermatheca immediately after sperm migration suggest that females could absorb apyrene spermatozoa, probably for intake of additional nutrients. Many hypotheses on the role of apyrene sperm have been proposed from the aspects of developmental biology, physiology and behavioural ecology. Although the physiological aim of apyrene spermatozoa in male reproductive organs is considered to be sperm transfer, each hypothesis is focussed on sperm competition because a considerable number of apyrene spermatozoa arrive at the final destination of the female, the spermatheca, and because females receive much more apyrene spermatozoa with a single copulation, and furthermore because of counter-adaptation on the multiple mating of females. The apyrene sperm must have an important role for butterflies to consider sperm competition.