Silkworm males produce dimorphic sperm, nucleate eupyrene sperm and anucleate apyrene sperm. Apyrene sperm have been speculated to have an assisting role in fertilisation. However, the coexistence of eupyrene and apyrene sperm in the testis and female reproductive organs has made it difficult to define the role of apyrene sperm. Polyploid males are highly sterile. Microscopic observation revealed that the elimination of eupyrene nuclei by peristaltic squeezing caused the sterility of polyploids. Heat-shock applied to pupae of Daizo males (DH) also induced high sterility due to the lack of normal apyrene sperm. When eupyrene sperm of sterile DH males and apyrene sperm of sterile polyploid males were mixed by double copulation, a remarkable increase in fertility of the double-mated females was observed. This finding strongly suggests that the apyrene sperm are indispensable in fertilisation of the silkworm and that polyploid apyrene sperm function as a substitute for diploid sperm. We established an experimental system in which we can separate the two types of sperm for further studies on their functions without chemical and/or mechanical treatments.

Silkworm (Lepidoptera) males produce dimorphic sperm: nucleate eupyrenesperm and anucleate apyrene sperm. The eupyrene sperm are ordinary sperm tofertilise the eggs, while the function of apyrene sperm remainsuncertain. After meiosis, 256 sperm cells are enclosed by a layer of cystcells, forming a sperm bundle. We have previously documented that thenucleus of eupyrene sperm anchors to the head cyst cell, which locates atthe anterior apex of the bundle, by an acrosome tubule-basal bodyassembly. Neither the basal body attachment to the nucleus nor the acrosomeis seen in apyrene sperm, and the nuclei remain in the middle region of thebundle. Peristaltic squeezing starts from the anterior of the bundles inboth types of sperm, and cytoplasmic debris of the eupyrene sperm, and boththe nuclei and debris of apyrene sperm, are eliminated at the final stage ofspermatogenesis. Since the irregularity of meiotic division in apyrenesperm is known, we used triploid silkworm males that show irregularmeiotic division even in eupyrene spermatocytes and are highly sterile. The irregular nuclei of the triploid are discarded by the peristaltic squeezing just as those of the apyrene sperm. Transmission electron microscopic observations disclose the abnormality in the acrosome tubule and in the connection to the basal body. The peristaltic squeezing of sperm bundles in the silkworm appears to be the final control mechanism to eliminate irregular nuclei before they enter female reproductive organs.