Population cycle is commonly observed as diverse as unicellulates and mammals. Organisms in natural are affected by various factors and thus the underlying mechanism of the population fluctuation is enormously complicated. Accordingly, a laboratory-based approach is essential to clarify the mechanism underlying the population cycle, and we have been carrying out molecular biological experiments with a culture of the rotifer Brachionus plicatilis.
Insulin-like growth factor (IGF) signaling pathway regulates both reproduction and lifespan in some model organisms, and thus is one candidate at the molecular level affecting the population dynamics. We so far confirmed that the pathway regulates the rotifer’s lifespan, and succesfully cloned some genes involved in the pathway. It is promising to further investigate the IGF pathway for better understanding of drastic changes in the population cycle in various taxa of organisms.

　Yuzuru Suzuki (Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The Univ. of Tokyo)

Since water is perfect medium for bacteria and parasitic microbes, the fish skin has some defense mechanisms against pathogen invasion. We analyzed defense factors on the skin surface as well as internal defense systems in fish.
Lectins, carbohydrate-binding proteins, in the mucus might be one of the external barrier against pathogens. We analyzed the primary structures of the lectin in Japanese eel Anguilla japonica, fugu Takifugu rubripes, and ponyfish Leiognathus muchalis, and found four different types of lectin, demonstrating that skin mucus lectins in fish have wide molecular diversity.
In addition to the surface barrier, we analyzed factors of internal immunity in fugu, e.g., immunoglobulins, T cell markers, cytokines, and complement factors. We also found poly-Ig receptor, which is responsible for the transport of polymeric immunoglobulin across epithelial tissues into mucus secretions. This might be one of the key factors which link external barrier with internal one.

Biomineralization is thought to be a kind of adaptation to environment such as protection from enemies. In order to clarify the molecular mechanism of biomineralizatin, organic matrices contained in biominerals from some representative marine organisms were extracted and purified, and their structures were determined. We isolated and characterized two novel glycoproteins from otoliths of salmonid species, an acidic polysaccharide from coccoliths of the coccolithophorid alga, and a novel protein from the prismatic layer of the pearl oyster. It was estimated that the otolith glycoproteins were associated with daily ring formation. Both the polysaccharide and the oyster protein showed inhibitory activity to crystallization of calcium carbonate from its supersaturated solution, suggesting their association with calcification.

Aim
Molecular analysis of calcification mechanisms in marine invertebrates including hermatypic (reef-building) corals, and oogenesis in hermatypic corals. Taxonomy in hermatypic corals using molecular tools.
Results
1) The primary structure was determined in a protein present in the skeletal organic matrix of the hermatypic coral Galaxea fascicularis. The structure and function was characterized in crustocalcin (CCN), a crustacean protein localized in calcified areas of the exoskeleton. CCN induces nucleation of CaCO3 crystals in vitro. 2) Based on an analysis of an intergenic region in the mitochondrial DNA, the population of G. fascicularis in Okinawa was found to consist of two genetically differentiated groups. 3) Complementary DNAs encoding major egg proteins were cloned in two hermatypic coral species, and expression profiles of the cognate mRNAs were characterized.

The aim of our study is to identify essential hormones that allow fish to adapt to hyperosmotic seawater environments. We have shown that the natriuretic peptides play pivotal roles for seawater adaptation in euryhaline eels. The diversification of natriuretic peptide molecules well coincides with the evolution of the osmoregulatory mechanisms. We are currently pursuing such essential hormones further and have focused our attention to novel hormones, guanylin and adrenomedullin.

Almost two-thirds of the surface of our planet is the realm of the deep-sea, where darkness, low temperature and high hydrostatic pressure reign. Deep-sea animals survive these harsh and extreme environments through special adaptations, and sometimes thrive around hydrothermal vents. The actual state of the gas-bladders of the deepest dwelling fishes, bathymetrical distribution, foraging behaviors in the deserts and the mapping of the micro-distribution of the vent communities are favorite research items.