翼手類

Breeding behaviors of Microchiroptera, particularly the mechanism of fertilization and development are different from those of the other mammals. Namely, most of the vespertilionid bats of the temperate zones (e.g., Pipistrellus abramus etc. from Japan) copulate during the autumn, and the spermatozoa are stored in the uterus until the next spring, when ovulation and fertilization occur. On the other hand, in Miniopterus schreibersi fuliginosus from Japan, ovulation and fertilization take place soon after copulation. Therefore, the spermatozoa of this species are not stored for a long time in the uterus, but a single fertilized egg in free blastocyst stage is recognized in the uterus during the winter. The aim of our study attempts to examine the mechanism of fertilization and development, particularly, of capacitation of the spermatozoa during their passage through the genital tracts in the two species, P. abramus and M. s. fuliginosus, the breeding behaviors of which are different. Mossman (1953) has shown that the characters of the genitalia and foetal membranes are far more conservative, and hence, can be better utilized for determining the phylogenetic relationships of the mammals than other characters. Sperm morphology as a criterion of phylotaxonomic relationships among animals belongs to a relatively recent innovation. The present work is to make use of the fine structure of sperm as a criterion of phylotaxonomic relationships among bats. In order to provide a logical solution for these problems, it is necessary to pursue the process of spermiogenesis at electron microscopic level. Testes and epididymides of the following six species of bats belonging to two families and four genera, i.e., Rhinolophus cornutus cornutus, R. ferrumequinum nippon (Rhinolophidae), Myotis nattereri bombinus, My. macrodactylus, Pipistrellus abramus (Vespertilioninae of Vespertilionidae) and Miniopterus schreibersi fuliginosus (Miniopterinae of the same family) were examined in the present study. The spermatids and spermatozoa of each species were observed and compared morphologically with Hitachi HU-11A electron microscope, but the descriptions were based chiefly on M. s. fuliginosus. /I. The late maturation phase in spermiogenesis/ The spermatids continue to undergo alterations from the Golgi phase to the maturation phase of the spermatid and consequently take on the shape characteristic of the species (Fig. 1). In the Golgi phase of the spermatid of M. s. fuliginosus, a large Golgi complex is visible, and a acrosomal granule is fixed to the limiting membrane of the vacuole where the latter is adherent to the nuclear envelope (Figs. 2 and 3). In the acrosomal phase, the acrosomal vacuole has begun to spread laterally over the anterior pole of the nucleus, and the space in the acrosomal vacuole is filled with a homogeneous material that is continuous with the acrosomal granule. In the space between the inner acrosomal membrane and outer leaf of the nuclear envelope, a little cytoplasm is visible, and in this space of the spermatid in the maturation phase an electron dense material of the apical body is observed (Fig. 4 a and b). The nucleus of the spermatid is in progress at a pole of the spermatid. The outer acrosomal membrane becomes adherent to the plasma membrane of the spermatid (Fig. 5). In this stage, the caudal sheath extends backwards from a nuclear ring at the posterior margin of the acrosomal cap, and continuously the spermatid nucleus becomes elongated and flattend during this period, and the chromatin becomes coarser and larger (Fig. 6 a and b, Fig. 7 a and b). In the late maturation phase of spermatid, the nucleus takes on the shape characteristic of the species, and the nucleus is transformed into a homogeneous dense mass. The post-nuclear cap is gradually formed while the nuclear ring migrates backwards between the plasma membrane and nuclear envelope of the spermatid. It seems to us that the functional morphology of the topographical relationship between the nuclear ring and two membranes is similar to the structure and function of the slide fastener. Thus, posterior to the nuclear ring, the posterior portion of the sperm head remains incomplete. The nucleoplasm is often left between the nuclear envelope and the chromatin (Fig. 8). In the next stage, the nuclear ring and caudal sheath become extinct. Each of them is generally regarded as a transient structure. By this time, the sperm head is completed and a redundant portion of the nuclear envelope is reflected from the surface of the nucleus, and forms a membranous scroll around the neck. When the connecting piece reached to its completion, the middle piece is not yet completed in this stage. Just behind the connecting piece the annulus is still present, and posterior to the annulus, the base of the flagellum devoid of the mitochondria is observed (Fig. 9). The fibrous sheath is already formed before the annulus has migrated. When the annulus migrates from the posterior part of the connecting piece to the boundary part between the middle piece and principal piece, the mitochondria arrange in a helix around the base of the flagellum to form the mitochondrial sheath (Figs. 10 and 11). In the tail of the mature sperm, nine outer dense fibers are seen in the cross section of the middle piece, and four of them are unusually larger than the others (Fig. 12). Fig. 13 shows semischematically a series of the changes in the late spermiogenesis stated above. /II. Phylogenetic considerations of the bats based upon the fine structure of the spermatozoa/ The fine structure of the spermatozoa is distinctive at the familial, subfamilial, generic and specific levels. In the spermatozoa of bats belonging to Rhinolophidae, the shape of the nucleus in frontal view is broader at the anterior portion than at the posterior portion, and the acrosome is larger than in Vespertilioninae of Vespertilionidae; the total number of mitochondria in the spermatozoon is large in number (i.e., in R. cornutus 130, R. ferrumequinum 160), but the size of each mitochondrion is small. On the other hand, in the sperm of bats belonging to Vespertilioninae of Vespertilionidae, the shape of the nucleus is tapering anteriorly, and the acrosome is smaller than in Rhinolophidae, but the large mitochondria are generally large in number (i.e., in My. nattereri 135, My. macrodactylus 117, P. abramus 138). In the spermatozoa of M. s. fuliginosus belonging to Miniopterinae of the same family, the acrosome is larger than in both of Rhinolophidae and Vespertilioninae, and its anterior portion swells out; the total number of mitochondria in the spermatozoon is a comparatively small numerical value (78), and the size of each mitochondrion is small. Between R. cornutus and R. ferrumequinum, or between My. nattereri and My. macrodactylus, belonging to the same genus, the fine structure of sperm is distinctive at specific level (Table 1). When considered from the viewpoint of fertilization physiology, the spermatozoa of bats belonging to Vespertilioninae of Vespertilionidae are stored in the uterus from the autumn to the following spring when ovulation and fertilization occur. In these species, large mitochondria are generally large in number. However, in M. s. fuliginosus ovulation and fertilization occur soon after the autumn copulation. Therefore, the spermatozoa of this species are not stored for a long time in the uterus. In this connection, it is worthy to note that in this species the total number of mitochondria of the spermatozoon is small and each mitochondrion is small in size. Based upon these data, we believe that the fine structure of the spermatozoon may be reasonably utilized as a criterion for determining the phylogenetic relationships of bats.