Meiosis is an event that occurs prerequisitely and specifically in gametogenesis. However, the mechanisms of conversion from mitosis to meiosis are poorly understood. I will review the results so far obtained by us using newt testis as a model system, and discuss about the extrinsic mechanism(s) controlling the conversion from mitosis to meiosis. In the newt spermatogonia enter meiosis in the 8^th generation after 7 mitotic divisions. We developed organ and reaggregate culture systems with a chemically defined medium in which porcine follicle-stimulating hormone (pFSH) promotes spermatogonial proliferation and differentiation into primary spermatocytes. Human recombinant stem cell factor (RhSCF) in vitro stimulates the spermatogonial proliferation and progression to the 7^th generation, but not the differentiation into primary spermatocytes; instead they die of apoptosis. The reason why rhSCF does not stimulate meiosis entrance seems to be due to the low level expression of c-kit protein at the 7^th generation of spermatogonia. Ovine PRL induces apoptosis in the 7^th generation of spermatogonia in vivo and in vitro. Incubation of newts at low temperature causes spermatogonial apoptosis by the elevation of plasma PRL titer. In the absence of FSH in organ culture spermatogonia can progress until the 7^th generation, but the 8^th generation never appear due to the apoptosis. Altogether there seems to be a regulatory checkpoint for entrance into meiosis in the 7^th generation. Spermatogonia could circumvent the checkpoint by the influence of some factor(s) produced by Sertoli cells upon activation by FSH. Trial to isolate factor(s) responsible for the meiosis-initiation is now underway.

The male and female genitalia are finely designed to match each other for copulation in the cricket Gryllus bimaculatus. Copulatory acts of the male, stereotyped and time-fixed, are elicited by stimulation of mechanoreceptors on particular regions of the abdomen, cerci and genitalia. Sequential execution of each motor act proceeds as a chain reaction in which one act stimulates some receptors which in turn elicits another act and so on, while the female remains immobile on the male's back. Each key stimulus for a motor act appears as a result of the male's own act, except for copulatory papilla protrusion by the female. The final sequence of spermatophore extrusion and transfer are irreversible fixed motor actions which are triggered when the female copulatory papilla stimulates the epiphallic hairs. They proceed without continual central drive from the brain, and apparently without sensory feedback. In addition, they are well coordinated with movement and posture in the entire body. Some neural mechanisms of controlling mating behavior and switching the reproductive cycle are discussed.

Contrary to most other Diptera, the inter-rhabdomeral spaces of the retina of the Jamaican cavefly Neoditomya farri are filled neither by extracellular matrix nor dense cytoplasmic material. Instead, a foamy organization of loose vacuoles, measuring approximately 0.7 μm in diameter, appears to keep the rhabdomeres of retinula cells 7 and 8 in place. The vacuoles are bounded by membranes and traces of actin, determined immunocytochemically, are present. The origin of the vacuoles is unclear, but evidence in support of a retinula cell rather than cone cell origin is advanced.

Effects of rearing conditions were examined in the development of agonistic behaviors in the male fighting fish. In group-I (highly social), fish were communally reared. In group-II (highly social and isolate), fish were individually housed and exposed to the group-I fish through transparent walls until the sexual maturity (from 6 to 12 weeks post-hatch). In group-III (social and isolate), individually housed fish were similarly exposed to other fish within the group. In group-IV (non-social and isolate), individually housed fish were further visually isolated. Agonisitc behaviors were compared among males of the groups-II, -III, and -IV in their fights against the group-I male. The group-IV males showed significantly higher rate of wins than the groups-II and -III males, without differences in the incidence of agonistic behaviors (butt-or-bite, chase, and gill-cover erect) before the termination of the mutual fights. Increased incidence of agonistic behaviors was found after the termination (particularly in the unilateral chase), suggesting that the group-IV males continued to fight even after the opponent male displayed a submission. The aggression was also enhanced in the group-II, when they were thereafter reared in a social isolation after the sexual maturation; a critical period was thus not found. The enhanced aggression was not reversed in the group-IV, when they were thereafter exposed to social stimuli; shift to the continued fights was irreversible. Possible fitness gain of the enhanced aggression was discussed in terms of the adjustability to altered biological resources.

The photosensitivity of the central nervous system (CNS) of the horseshoe crab, Limulus polyphemus, was investigated by analyzing changes in motor nerve activity in the segmental nerves of prosomal and opisthosomal ganglia. Spontaneous efferent impulses were recorded in the dark from all the investigated segmental nerves. Impulse trains from the 7th dorsal nerve in the prosomal CNS were inhibited in response to illumination of the whole CNS. Impulse trains from each of the 9–13th dorsal nerves in the isolated opisthosomal CNS were inhibited, and the impulse train from each the 14–16th dorsal nerve was elicited or inhibited upon illuminating the whole CNS. Spontaneous rhythmic bursts at 20–80 s intervals were recorded in the dark from the ventral nerves of the isolated opisthosomal CNS. In the presence of light, the rhythmicity of spontaneous bursts disappeared and other species of impulse trains were elicited. In single ganglion preparations, isolated from the rest of the CNS by surgically severing the connectives, similar photoresponses were recorded before and after isolation. These results demonstrate that the CNS of Limulus is a photosensitive organ.

To obtain basic information on the properties of the intraovarian cavity leucocytes (IOCLs) of the viviparous teleost, Neoditrema ransonneti, morphological characteristics and numerical changes of IOCLs during the reproductive cycle were investigated. In the ovaries of newborn females, leucocytes exuded into the lumen were observed first in November, prior to insemination of semen. These cells were primarily macrophages, neutrophils and lymphocytes. Among them, macrophages were invariably the largest population throughout the reproductive cycle. They began to phagocytize spermatozoa in December, when spermatozoa were first detected in the ovary. The number of IOCLs gradually increased from November in the newborn female. However, this increase is not ascribed to the effect of copulation or the presence of semen, because the number of leucocytes also increased in non-mating fish. While developing embryos were discharged into the ovarian lumen at the latest in January, a number of spermatozoa and spermatozoa-phagocytizing macrophages were seen until March. Even after the extinction of sperm cells, numerous IOCLs remained in the lumen and coexisted with fetuses until their parturition. These results suggest that IOCLs play roles in successful pregnancy, besides elimination of remaining spermatozoa.

Early developmental staging from the zygote stage to the gastrula is a basic step for studying embryonic development and biotechnology. We described the early embryonic development of the loach, Misgurnus anguillicaudatus, based on morphological features and gene expression. Synchronous cleavage was repeated for 9 cycles about every 27 min at 20°C after the first cleavage. After the 10th synchronous cleavage, asynchronous cleavage was observed 5.5 h post-fertilization (hpf), indicating the midblastula transition. The yolk syncytial layer (YSL) was formed at this time. Expressions of goosecoid and no tail were detected by whole-mount in situ hybridization from 6 hpf. This time corresponded to the late-blastula period. Thereafter, epiboly started and a blastoderm covered over the yolk cell at 8 hpf. At 10 hpf, the germ ring and the embryonic shield were formed, indicating the stage of early gastrula. Afterward, the epiboly advanced at the rate of 10% of the yolk cell each hour. The blastoderm covered the yolk cell completely at 15 hpf. The embryonic development of the loach resembled that of the zebrafish in terms of morphological change and gene expression. Therefore, it is possible that knowledge of the developmental stages of the zebrafish might be applicable to the loach.

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor (GHS-R), was isolated from the rat stomach and determined to be n-octanoylated 28-amino-acid peptide. In this study, we studied the distribution of ghrelin-producing cells (ghrelin cells) in the gastrointestinal tract of male and female rainbow trout (Oncorhynchus mykiss) by immunohistochemistry using N-terminal region-recognizing antibody and also by in situ hybridization using a trout ghrelin-specific cRNA probe. Ghrelin cells were found in the mucosal layer of the stomach but not in the myenteric plexus, and no ghrelin cells were observed in other regions of the gastrointestinal tract. Ghrelin cells could be classified into two types: closed- and opened-type cells. The density of ghrelin cells increased gradually in the direction from the cardiac to pyloric portions of the stomach in both sexes. The number of ghrelin cells per unit area seemed to be higher in females than in males. In conclusion, trout ghrelin cells exist in the stomach and are classified into two types of cells, closed- and opened-type cells.

GnRH (gonadotropin-releasing hormone) is well-known as the central regulator of the reproductive system through its stimulation of gonadotropin release from the pituitary. Progress in studies on GnRH demonstrated that GnRH has both inhibitory and stimulatory effects on cell proliferation depending on the cell type, and the mechanisms of these effects have been intensively studied. However, even human GnRH receptors which mediate GnRH stimulation have not been completely identified. In the present study, we showed that the inhibitory and stimulatory effects of GnRH on colony-formation using four cell lines and have demonstrated that the inhibitory and stimulatory effects of GnRH exhibit distinctly different patterns of ligand sensitivity. This result strongly suggests that the two opposite effects of GnRH occur via different types of GnRH receptors, however expressional analyses of human GnRH receptors did not exhibit the significantly different pattern between negatively and positively responding cell lines. Then, in order to identify the GnRH receptors involved in the two opposite effects, effects of GnRH were analysed under the conditions that human GnRH receptors were knocked down by the technique of RNA interference. Consequently, it was found that human type II GnRH receptor mediates GnRH stimulation and its splice variant determines the direction of the response to GnRH. These results are the first clear evidence for the functionality of human type II GnRH receptor. Therefore our novel findings are quite noticeable and will greatly contribute to the studies on the mechanisms of the effects of GnRH on cell proliferation in the future.

Many albatross remains have been found in the Japanese Islands and the surrounding areas, such as Sakhalin and South Korea. These remains are interesting for two reasons: numerous sites from which albatross remains have been found are located in coastal regions of the Far East where no albatrosses have been distributed recently, and there are some sites in which albatross remains represent a large portion of avian remains, although albatrosses are not easily preyed upon by human beings. We collected data on albatross remains from archaeological sites in the Far East regions during the Holocene and arranged the remains geographically, temporally and in terms of quantity. Based on these results, we showed that coastal areas along the Seas of Okhotsk and Japan have rarely been used by albatrosses in Modern times, though formerly there were many albatrosses. We proposed two explanations for the shrinkage of their distributional range: excessive hunting in the breeding areas, and distributional changes of prey for albatrosses.

Phylogenetic relationships among asteroids remain to be extremely controversial in spite of many morphological and molecular studies have been applied to this issue. In the present study, especially focusing on resolving the relationship of Asterina and Solasteridae, we reconstructed the molecular phylogenetic tree of asteroids using nuclear 18S rDNA. A close relationship between Asterina and Solasteridae, which has been suggested from analyses of mitochondrial 12S rDNA and 16S rDNA, is supported here by the nuclear 18S rDNA dataset. The support is even stronger when the sequences of mitochondrial rDNAs and nuclear 18S rDNA are combined as a total dataset. The independent support from both nuclear 18S rDNA and mitochondrial rDNAs strongly argues for a close relationship between the Asterina and Solasteridae.

This report deals with an unusual mode of mitochondrial gene introgression between Cobitis hankugensis (C. sinensis) and C. longicorpus which is mediated by a unisexual hybridogenetic system of diploid-triploid C. hankugensis-longicorpus complex. Mitochondrial DNA sequences of 3329-3330bp encompassing from upstream ND6 to 12S rDNA indicated that mitochondrial genomes from the diploid hybrids, triploid hybrids, and their parental species are almost identical. Because triploid hybrids produce haploid ova with C. hankugensis chromosome set, normal diploid C. hankugensis regenerates upon insemination with C. hankugensis sperm. If the hybrid carries C. longicorpus mitochondrial genome, the regenerated C. hankugensis is a nucleo-cytoplasmic hybrid, thus accomplishing the unusual mode of mitochondrial gene introgression.