Kinetic Analysis on the Motility of Liverwort Sperms Using a Microscopic Computer-Assisted Sperm Analyzing System

Abstract

Embryophytes (bryophytes and ferns) perform both parthenogenesis and sexual reproduction in their life cycle, and sperms are used for their sexual reproduction, in place of pollens in higher plants (Renzaglia et al., 2000; Renzaglia and Garbary, 2001). Because sperm motility is one of the most important features for the fertility in human and animals, percentage of motility and the kinetic parameters for swimming and the guidance to eggs (known as taxis) are well focused in the field of reproductive biology to evaluate and improve the male infertility. However, the nature of plant sperms is rarely known even more than centuries has passed since the first description on the discharge of samen from embryophytes by Schmidel in 1747 (as note by Mirbel, 1835a; 1835b; Thuret, 1851; Cavers, 1903; 1904; see also recent review by Araki, 2012). The discovery of a freely swimming sperm in Ginkgo biloba at the end of the nineteenth century by Sakugoro Hirase was one of the most significant moments in botany (Ridge et al., 1997). In the end of nineteenth century, the freely swimming sperms in embryophytes, thus motile under microscopes, were first identified in Ginkgo biloba (Hirase, 1896a; 1896b) and Cycas revoluta (Ikeno, 1896; 1897; Ikeno and Hirase, 1897), nowadays known as members of sperm-generating plant species. As the first step in fertilization mechanism in bryophytes, semen enriched with the biflagellated sperms are discharged from the antheridia of males as the top of antheridial receptacle are wet by the rain drops (Carothers and Kreitner, 1967; 1968, Shimomura, 2012). Discharge of semen is promoted by a droplet of water but not by touch, indicating that swelling rather than mechanical stimuli, and some following signal transduction might be playing key role(s) in the process. For the opening of covering jacket of an antheridium and distending of the antheridium wall cells which give a pressure to discharge sperms, swelling of the cells and mucus are supposed to be important (Renzaglia et al., 2000; Shimomura, 2012), which is different from the motive contraction of alveoli of human breast for lactation, which initiated by baby’s suck, a mechanical stress. A recent study on the “airborne” sperms of C. conicum indicated that liverwort sperms are discharged as pre-motile form, then shift to the motile form when they swell or they are subjected to hypo-osmotic stress (Shimomura et al., 2008). Thus, swelling and/or hypoosmotic stress, but not mechanical stress may distinctly play key roles in the phases of sperm discharge and transition to motile form in liverwort sperms. In general, swimming ability is one of the most important features of sperms because they must reach and fertil-