Supplementary online resource to Matzke-Karasz, Smith & Heß: Removal of extracellular coat from giant sperm in the female’s receptacle induces sperm motility in Mytilocypris mytiloides (Cyprididae, Ostracoda, Crustacea), DOI: 10.1007/s00441-016-2507-6
Chronology of the dispute on how to interpret findings in the field of sperm coats and sperm motility in ostracods
The first hypothesis on the gain of motility in ostracod giant sperm was phrased by Zenker (1854). He listed three arguments to support his hypothesis: (1) he erroneously interpreted the sperm pump as a gland, providing a ‘slime’covering the sperms during insemination; (2) he discovered sperm cell content partly separated from their sheaths, and many empty sperm sheaths in the females’seminal receptacles; (3) he found sperms were motile only inside the females’receptacles and not in the male animal. He concluded that the secretion of the ‘slime gland’cover the sperm and keep them immotile by hardening. Later, in the receptacle, these hardened sheaths swell, break apart, and finally, the sperms moult and develop motility. The empty sheaths are left behind, and visible through the microscope, in the female’s receptacle.
While Stuhlmann (1886) corrected Zenker’s wrong interpretation of the sperm pump as a gland (‘ejaculation apparatus’in Stuhlmann(1886), later named ‘Zenker organ’ by Vávra (1891)), he concurred with Zenker that the sperms become motile only after shedding their ‘hyaline sheath’in the female’s receptacle. Müller (1889) presented a slightly modified hypothesis on sperm motility. He opined that the material covering the sperm as a sheath in the vas deferens would subsequently shrink (in the male), putting the sperm under tension. The passage through the narrow inlet of the ductusejaculatorius would result in breaking loose the contact between the actual sperm cell and its sheath, allowing for a moult from the sheath when it arrived in the seminal receptacle of the female. According to Müller, the movements seen in sperm taken from the female’s receptacle are only those induced by the release of the described tension.
Lowndes (1935), who presented a comprehensive study on sperm motility in a dozen ostracod species, also recorded “empty husks”in his sperm preparations from female specimens. He further described the observation of sperm frequently “boring”into others, “tearing off a considerable portion of the chitinous membrane”(Lowndes 1935: 39). From the presence of an obviously unaltered reproductive system in animals of all-female ostracod species Lowndes erroneously concluded that in species with both genders present, “coition continued to take place long after syngamy ceased”(Lowndes 1935: 47). He thus considered the giant sperms of ostracods as functionless.
Electron microscopy studies of the 1960s potentially came closer to the problem’s solution with their first evidence of a multi-layered sperm coat. While Tétart (1967: 66) found three cortical layers (“trois couches corticales”), Gupta (1968) defined two coats, i.e. a chitinous sheath (however, without providing evidence for its chitinous nature) and an outer fibrous coat. He further remarked that sperm cells appear in “various stages of exhaustion”when taken from older females, assuming that a sperm disintegrates in the seminal receptacle, once the stored energy is consumed (Gupta 1968: 120). Gupta therefore rejected Müller’s hypothesis of sperm moulting in the female’s seminal receptacle. In a figure caption, Gupta further elaborates that the fibrous coat is not present in sperms from the seminal receptacles, which may be “one of the factors responsible for the lack of motility in the spermatozoa from the male specimens of ostracods”(Gupta 1968: 122).
Later, Reger and Florendo (1969b: 121) described a “spermatophore sheath”present in sperms taken from males, while Zissler (1970: 85) found that an outer coating substance (“eineäußereHüllsubstanz”) covers the inner coat; however, referring to Zenker (1854) Stuhlmann (1886), and Müller (1889), he repeatedthe idea of sperm cells moultingin the female (Zissler 1969b). Wingstrand (1988), in his monograph on ostracod sperm, described and illustrated the sperms’extracellular coats in great detail. He distinguished between an inner, “permanent coat” covering the entire (cypridoidean) sperm and forming e.g. the anterior drill, and an outer “deciduous coat”, which covers the surface structures of the permanent coat and is finally shed or disintegrates inside the female’s receptacle. However, while Gupta strictly excluded a sperm moult in the seminal receptacle, Wingstrandconsidered both, moulting and disintegration of the outer coat being possible. Therefore, neither the source of empty sperm coats in the seminal receptacles has so far been explained satisfactorily, nor have the possible rise of sperm motility in the female and the disappearance of the outer sperm coat been prescinded from pure speculation.