Studies on motile, nontubule-containing, filiform spermatozoa of the ostracod Cypridopsis. I. Spermiogenesis

Removal of extracellular coat from giant sperm in female receptacle induces sperm motility in Mytilocypris mytiloides (Cyprididae, Ostracoda, Crustacea)

Previous studies of cypridoidean ostracods have noted that (1) their giant spermatozoa are immotile inside the male, (2) these spermatozoa are motile in the female seminal receptacle and (3) these receptacles are often filled with empty sperm coats. Such findings have led previous authors to hypothesize that sperm must shed their coats in the female receptacle to become motile. We present light and electron microscopy results and video recordings of mating experiments with virgin specimens of Mytilocypris mytiloides. We show that the empty sperm coats frequently found in the female receptacles are not the result of sperm molting but are the resistant inner coats of exhausted sperm not used for egg fertilization. In contrast, we show that an outer granular coating material is successively removed from the sperm while resident inside the female receptacles before first oviposition occurs. During this period, previously immotile sperm gain motility, showing strong movement shortly before first oviposition takes place. By correlation of these phenomena, we suggest that dissolution of the outer coat material is required for motility to develop.

Giant spermatozoon coiled in small egg: fertilization mechanisms and their implications for evolutionary studies on Ostracoda (Crustacea)

Ostracods of the superfamily Cypridoidea have giant spermatozoa. However, little data exist on the sperm-egg interaction in this group: only two publications have so far given the most ambiguous indication that the entire sperm enters the egg on fertilization. These assumptions have not yet been tested with modern techniques, nor has their putative value for developmental and evolutionary investigations been realized. The present paper gives the first, clear, light- and scanning electron microscopical evidence of the entire giant ostracod spermatozoon being incorporated into the egg. Coiling of the sperm underneath the egg shell is shown in the early zygotes of the species Mytilocypris praenuncia and Pseudocandona marchica. Additionally, data on the morphology of female and male reproductive tracts are given for M. praenuncia. Hypotheses on the evolution of giant filiform sperm in the Animal Kingdom are reviewed, and their applicability to ostracods is discussed. The demonstrated ingression of the entire sperm implies the entry of the two giant paternal mitochondrial derivates into the zygote in Cypridoidea and potentially casts doubt upon the dogma of strict maternal inheritance of mitochondrial DNA. Evidence of paternal inheritance of mtDNA in several organisms has recently given rise to a controversial debate on this issue; the possible significance of this phenomenon for molecular studies on ostracod phylogeny and evolution is discussed.

Ultrastructure of mature spermatozoa of a minute featherwing beetle from Sri Lanka (Coleoptera, Ptiliidae: Bambara)

The fine structure of the mature spermatozoa of the feather-wing beetle (Coleoptera: Bambara new species no. 2) is described. Mature spermatozoa are extremely large in proportion to the size of the beetle. Under a light microscope a distinct head, midbody, and tail region can be seen with external coils in both the head and tail regions. Although the beetles were collected and preserved for skeletal anatomical study and not for histological preservation, the substructure comprising the spermatozoa was sufficiently well-preserved for their uniqueness to be appreciated. Observations with an electron microscope show that the axoneme is not contained in the tail region but is threaded through the head coils and then tightly wrapped around the midbody, terminating at the end of the midbody region. Sections through the midbody reveal three distinct layers: an inner electron dense core, a middle very elaborate crystalline Jattice-like matrix, and an outer axonemal wrapping. The structures observed in the head and tail regions appear to be modifications of those present in the midbody region. The possible functional significance of these unusual structures is discussed.

Studies on the fine structure of the mitochondrial derivative in spermatozoa of a gastropod

Spermatozoa of Limax sp. were studied by electron microscopy following thin section and freeze-fracture techniques. Mature spermatozoa were seen to be helically shaped, 150 microns long cells. A single mitochondrion extends the entire length of the spermatozoon. Its helical turn is the same as that of the spermatozoon. Freeze-fracture images of the spermatozoon reveal that the EF and PF, plasmalemmal faces contain scattered, 7-9 nm size particles, and that the PF, outer mitochondrial membrane face contains 8-10 nm size particles. The corresponding EF, outer mitochondrial face contains matching pits. A paracrystalline complex is situated between the inner and outer mitochondrial membranes. The complex is constructed of a series of 8-9 nm thick, 35 nm wide, helically orientated, tripartate elements which extend the full length of the spermatozoon. The helical tilt angle is approximately 55 degrees. Each element is composed of tightly approximated (interspace distance 10 nm), strands of particles 8-9 nm in diameter. Speculations as to the significance of this complex, and its location between inner and outer mitochondrial membranes are made. It is concluded that the paracrystalline order of the complex either reflects the molecular packing of enzyme systems present in the mitochondrion, or some other unknown function.

Locomotion of the filiform sperm of littorina (Gastropoda, Prosobranchia)

The filiform sperm of Littorina sitkana swims effectively in sea water and more viscous fluids, overcoming the problems of a non-uniform flagellar beat with an unusual mechanism, which involves three main events: (1) the sperm rotates anti-clockwise (looking from tail to head); then (2) stops rotating and stiffens itself to form a screw-shape, with the tail being held perpendicular to the middle piece, and finally; (3) reverses its rotation and propels itself forward in a clockwise spiral. The average velocity of sperm is approximately 184 micrometer/s with a rotational frequency of 24 revs/s. The mechanism of propulsion may involve two kinetic centers at opposite ends of the sperm, which coordinate their movements to produce anti-clockwise rotation, stationary twisting, or clockwise rotation, in a manner reminiscent of spirochaetes. Littorina sperm also exhibit slower methods of propulsion including swimming backwards (tail first) at 18 micrometer/s, and "gliding" at about 30 micrometer/s. The adaptive significance of the rapid propulsion is not obvious, because Littorina copulate and fertilize internally and at each stage in the transfer there are external aids to sperm transport, such as ciliary action (oviduct) and muscular expulsion (bursa and seminal receptacle). The filiform shape, however, is well-adapted for long-term storage in the female body. These points are discussed.