Significance of peristaltic squeezing of sperm bundles in the silkworm, Bombyx mori: elimination of irregular eupyrene sperm nuclei of the triploid

Abstract

Silkworm (Lepidoptera) males produce dimorphic sperm: nucleate eupyrene sperm and anucleate apyrene sperm. The eupyrene sperm are ordinary sperm to fertilise the eggs, while the function of apyrene sperm remains uncertain. After meiosis, 256 sperm cells are enclosed by a layer of cyst cells, forming a sperm bundle. We have previously documented that the nucleus of eupyrene sperm anchors to the head cyst cell, which locates at the anterior apex of the bundle, by an acrosome tubule-basal body assembly. Neither the basal body attachment to the nucleus nor the acrosome is seen in apyrene sperm, and the nuclei remain in the middle region of the bundle. Peristaltic squeezing starts from the anterior of the bundles in both types of sperm, and cytoplasmic debris of the eupyrene sperm, and both the nuclei and debris of apyrene sperm, are eliminated at the final stage of spermatogenesis. Since the irregularity of meiotic division in apyrene sperm is known, we used triploid silkworm males that show irregular meiotic 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

Roles of actin networks in peristaltic squeezing of sperm bundles in Bombyx mori

“…The slenderizing process coincides with that of Bombyx mori spermatogenesis (Kawamura et al, 2000). Elimination of irregular eupyrene nuclei in the silkworm is an important function of peristaltic squeezing (Kawamura et al, 2001). Insects other than Bombyx probably use the same mechanism to avoid irregular fertilization.…”

Application of artificial insemination technique to eupyrene and/or apyrene sperm in Bombyx mori

“…Apyrene sperm bundles in DH males and eupyrene sperm bundles in triploid males had been confirmed to be morphologically abnormal (Kawamura et al, 2001, Sahara and. Eupyrene sperm bundles in DH males and apyrene sperm bundles in triploid males, however, accomplish maturity through peristaltic squeezing (Kawamura et al, 2000(Kawamura et al, , 2001Sahara and Kawamura, 2002).…”

“…Eupyrene sperm bundles in DH males and apyrene sperm bundles in triploid males, however, accomplish maturity through peristaltic squeezing (Kawamura et al, 2000(Kawamura et al, , 2001Sahara and Kawamura, 2002). It was previously ascertained that these males produce few offspring (0 to 0.10%), but fertility is greatly improved by copulation of both types of males with a single female (74.64 to 78.74%) .…”

Double copulation of a female with sterile diploid and polyploid males recovers fertility in Bombyx mori

“…Some morphological differences between eupyrene and apyrene sperm in the process of spermatogenesis are documented in (Osanai et al, 1989) and dissociation of eupyrene sperm bundles by acids and endopeptidase (Osanai & Isono, 1997). Kawamura et al (2001) showed that the peristaltic squeezing that occurs at the very late stage of spermatogenesis eliminates the nuclei of polyploid eupyrene sperm bundles. Sugai & Kiguchi (1967) reported that the treatment of male silkworm pupae with a high temperature produces highly sterile individuals.…”