Oysters, the common name for a number of different bivalve molluscs, are the worldwide aquaculture species and also play vital roles in the function of ecosystem As invertebrate, oysters have evolved an integrated, highly complex innate immune system to recognize and eliminate various invaders via an array of orchestrated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, as well as encapsulation and phagocytosis of the circulating haemocytes The hematopoietic tissue, hematopoiesis, and the circulating haemocytes have been preliminary characterized, and the detailed annotation of the Pacific oyster Crassostrea gigas genome has revealed massive expansion and functional divergence of innate immune genes in this animal Moreover, immune priming and maternal immune transfer are reported in oysters, suggesting the adaptability of invertebrate immunity Apoptosis and autophagy are proved to be important immune mechanisms in oysters This review will summarize the research progresses of immune system and the immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, neuropeptides, GABAergic and nitric oxidase system, which possibly make oysters ideal model for studying the origin and evolution of immune system and the neuroendocrine-immune regulatory network in lower invertebrates

The oyster immunity.

When taking a blood meal on a person infected with malaria, female Anopheles gambiae mosquitoes, the major vector of human malaria, acquire nutrients that will activate egg development (oogenesis) in their ovaries. Simultaneously, they infect themselves with the malaria parasite. On traversing the mosquito midgut epithelium, invading Plasmodium ookinetes are met with a potent innate immune response predominantly controlled by mosquito blood cells. Whether the concomitant processes of mosquito reproduction and immunity affect each other remains controversial. Here, we show that proteins that deliver nutrients to maturing mosquito oocytes interfere with the antiparasitic response. Lipophorin (Lp) and vitellogenin (Vg), two nutrient transport proteins, reduce the parasite-killing efficiency of the antiparasitic factor TEP1. In the absence of either nutrient transport protein, TEP1 binding to the ookinete surface becomes more efficient. We also show that Lp is required for the normal expression of Vg, and for later Plasmodium development at the oocyst stage. Furthermore, our results uncover an inhibitory role of the Cactus/REL1/REL2 signaling cassette in the expression of Vg, but not of Lp. We reveal molecular links that connect reproduction and immunity at several levels and provide a molecular basis for a long-suspected trade-off between these two processes.

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The major yolk protein vitellogenin interferes with the anti-plasmodium response in the malaria mosquito Anopheles gambiae.

Amino acids are vital for all living organisms. During early fish ontogeny, they are important fuel molecules, signalling factors and major substrates for the synthesis of a wide range of bioactive molecules and proteins. Because the majority of fish eggs are cleidoic, i.e. closed free-living systems following ovulation and activation, early development of fish depends on the maternal provision of amino acids during oogenesis. While more than 600 proteins have been identified in the growing oocytes of fish, the major vehicles for supplying amino acids to the growing oocyte before ovulation are the vitellogenins, of which many genes and multiple forms are known. Here we review the importance of amino acids for the intermediary metabolism of fish embryos and larvae, where amino acids have been shown to be the preferred catabolic substrate. Subsequently, we address the specialization of the lysosomal pathway involved in the uptake and degradation of yolk proteins. This latter pathway is specifically modified in the germline to facilitate the long-term storage of egg yolk proteins. In marine teleosts, the degradative pathway may be activated before fertilization during oocyte maturation to release free amino acids for oocyte hydration and the acquisition of egg buoyancy. In other species, including freshwater fish, a more latent activation of acid hydrolases occurs after fertilization during the four phases of yolk resorption. The developmental contributions of the yolk syncytial layer, vitelline circulation and liver are essential components of the amino acid supply during fish ontogeny.

Requirement for amino acids in ontogeny of fish

Vitellogenin, a multivalent sensor and an antimicrobial effector.

Vitellogenins (Vg) genes code for the major egg yolk protein precursor in insects and many other oviparous species. In insects, the Vg gene is expressed extra-ovarially in the fat body in sex-, tissue- and stage-specific manners. During the reproductive phase, the Vg mRNA is expressed in large quantities, which is then translated, secreted into hemolymph and ultimately taken up by the developing oocytes through receptor-mediated endocytosis. Once sequestered, the Vgs are stored as vitellin (Vn), the main nutritional reserve for the developing embryo. The regulation of Vg genes is directly under the control of hormones at the transcriptional level. Hormones involved in Vg gene transcription are juvenile hormone (JH), ecdysteroids and some neuropeptides. The overall understanding that has emerged is that the insects can be classified, based on the system of hormonal regulation of Vg gene transcription, into three groups: (i) insects (like most of hemipterans) that use only JH for Vg gene transcription; (ii) insects (like dipterans) that need both JH and ecdysteroids for Vg regulation; and (iii) insects like lepidopterans that require JH, ecdysteroids and additional hormones to regulate their reproductive biology. However, why insect species diverge in using different hormones to govern their reproductive physiology remains unclear. The present contribution focuses on the current status of knowledge regarding the regulation of Vg genes in insects. Besides a brief information on biochemical and molecular features, the role of Vg genes as a target of endocrine disruptors will be addressed. Also, the molecular mechanism of Vg gene regulation will be discussed.

Regulation of vitellogenin genes in insects

Background 
Vitellogenin (Vg), a major reproductive protein, has been associated with infection-resistant response in fish. However, the underlying mechanisms by which Vg is involved in anti-infectious response are not understood.
Methodology/Results 
By both protein-microbe interaction analysis and enzyme-linked immunosorbent assay as well as phagocytosis test, we demonstrate for the first time that fish Vg acts as a pattern recognition molecule with multiple specificities that can recognize bacteria as well as fungus rather than self components from fish, and functions as an opsonin that can enhance macrophage phagocytosis.
Conclusions 
This study shows that fish Vg plays an integrative function in regulating immunity via its pleiotropic effects on both recognizing pathogen-associated molecular patterns and promoting macrophage phagocytosis. It also supports the notion that factors normally involved in control of female reproduction are associated with immunity in organisms that rely on Vg for oocyte development.

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Zhao-Jie Li

Papers

Vitellogenin functions as a multivalent pattern recognition receptor with an opsonic activity.

Characterization of a pattern recognition molecule vitellogenin from carp (Cyprinus carpio).