Formation of Diapause Cyst Shell in Brine Shrimp, Artemia parthenogenetica, and Its Resistance Role in Environmental Stresses

Artemia has attracted much attention for its ability to produce encysted embryos wrapped in a protective shell when subject to extremely harsh environmental conditions. However, what the cyst shell is synthesized from and how the formative process is performed remains, as yet, largely unknown. Over 20 oviparous specifically expressed genes were identified through screening the subtracted cDNA library enriched between oviparous and ovoviviparous Artemia ovisacs. Among them, a shell gland-specifically expressed gene (SGEG) has been found to be involved in the cyst shell formation. Lacking SGEG protein (by RNA interference) caused the cyst shell to become translucent and the chorion layer of the shell to become less compact and pultaceous and to show a marked decrease of iron composition within the shell. The RNA interference induced defective diapause cysts with a totally compromised resistibility to UV irradiation, extremely large temperature differences, osmotic pressure, dryness, and organic solvent stresses. In contrast, the natural cyst would provide adequate protection from all such factors. SGEG contains a 345-bp open reading frame, and its consequentially translated peptide consists of a 33-amino acid residue putative signal peptide and an 81-amino acid residue mature peptide. The results of Northern blotting and in situ hybridization indicate that the gene is specifically expressed in the cells of shell glands during the period of diapause cyst formation of oviparous Artemia. This investigation adds strong insight into the mechanism of cyst shell formation of Artemia and may be applicable to other areas of research in extremophile biology.Salt lakes on plateaus, are widely known as “seas of death,” because they represent one of the most hostile environments on the earth in terms of extreme salinity, high pH, anoxia, large temperature differences, and intermittent dry conditions. Hardly any animal can survive such extremes. However, one notable exception lies in the shape of a small crustacean, Artemia.Artemia, also called the brine shrimp, is an ancient species that first appeared ∼400 million years ago (1). To cope with harsh and complex habitats such as salt lakes, Artemia are able, when the circumstances become adverse, to release their offspring into a dormant, encysted state, rather than simply releasing swimming nauplius, to ensure survival. Such adverse conditions include environments where the Artemia may experience high salinity, low oxygen levels, short days, or conditions of extreme temperature variation (2, 3). These dormant cysts will keep diapause until the state is terminated by activation (triggered by factors such as desiccation, dehydration, cold or chemical treatment), at which point they resume development when appropriate and stable environmental conditions have arisen (4–7).The diapause cysts, with their greatly reduced metabolic activity, contain embryos existing as late gastrulae and are composed of ∼4000 cells that are arrested at the G₂/M phase with a complete turning off of RNA and protein synthesis (8, 9). Previous studies indicate that the resistance and resumption ability of Artemia cysts have several causes. In addition to the arrested cell cycle, it has been noted that large amounts of two molecular chaperone proteins, namely p26 and artemin, are synthesized (10–12), and a high concentration of trehalose is also accumulated (13–15). Moreover, a complicated enzyme system is also involved in the diapause and resumption mechanism, including AMP-activated protein kinase (16) and p90 ribosomal S6 kinase regulatory pathway (17–19).In addition to falling into diapause, Artemia themselves secrete a rigid noncellular shell to cope with the extreme environmental stresses before they release the diapause cysts. The complex noncellular cyst shell consists of two main regions; the outer region, secreted by the shell gland, is of hypochlorite-soluble chorion, whereas the hypochlorite-resistant inner region is formed by blastoderm cells and comprises the embryonic cuticle (5, 20, 21). The shell glands, which are composed of clusters of secretory cells, are situated at the ovisac and open into the uterus. There are many dark brown secretory granules, which probably contain chorion material and pigments such as hematin formed in the cells of the shell glands at the point where the oocytes emerge in the ovaries during the reproductive period. These are secreted out at the second day after the oocytes enter the uterus. Therefore the shell glands vary from dark brown to white, even to colorless, as reproductive cycles differ (22, 23).Microphotographs shot by Sugumar and Munuswamy (24) reveal that both the chorion and the embryonic cuticle have an exquisite structure (21). Chorion consists of two distinct layers. First, a compact outer covering is over the cyst with many radially aligned aeropyles penetrating through. This is known as the cortical layer. Second, in a cavernous region below the cortical layer is the alveolar layer, which may act as a float for the newly laid cysts. A thin supra cortical layer, probably consisting of cuticulin, covers the outer surface of the cortical layer. The embryonic cuticle, which is impermeable to nonvolatile solutes, is otherwise composed of a broad multilamellar region as a fibrous layer sandwiched between the outer and inner cuticular membranes and constructed as a tripartite structure. This forms an area of relative independence from the external environment and serves to maintain the homeostasis of inorganic ions (2). The molecular formulation of the cyst shell is complex, and details remain unclear, although it is known that the cyst shell does contain chitin, lipoprotein, hematin, and some metal elements (25–27).Besides preventing mechanical damage (28), the cyst shell also plays an important role in protecting the embryo within from other lethal environmental stresses. Previous experimental data have confirmed the protective capabilities of the cyst shell. Tanguay et al. (29) indicated that the hatching rate of intact cysts is significantly higher than the decapsulated ones after ultraviolet irradiation treatment. Hematin, the hemopigment of the cyst shell, is also demonstrated to have a light-screening function (27). Clegg (30) indicated that the cyst shell plays a critical role in desiccation tolerance, because the rate of dehydration of decapsulated cysts is much higher than intact ones in the dehydration study, and rapid water loss significantly reduces the hatching level of dehydrated cysts. Liu et al. (31) also found that intact cysts have better thermotolerance than decapsulated ones in both dry and water heating studies.In our experiments, through the in vivo gene knockdown by RNA interference, a shell gland-specifically expressed gene (SGEG) has been found to be involved in the cyst shell formation. The formed cyst shell has been demonstrated to play an important role in resistance to UV irradiation, large temperature differences, osmotic pressure, dryness, and organic solution stresses.