ULTRASTRUCTURE AND MORPHOGENESIS OF WHITE SPOT SYNDROME BACULOVIRUS (WSSV)

Since 1993 non-occluded baculoviruses, associated with a syndrome with high mortalities, have been reported in cultured penaeid shrimp from Asia and the Indo-Pacific region. Infections are typically accompanied by the presence of white spots on the cuticle. Numerous names were gi ven to the virus(es) in early reports on the disease, but the syndrome is increasingly known as White Spot Syndrome (WSS) and its viral agen t(s) as White Spot Syndrome Baculovirus (WSSV). The WSS virion is a st ocky rod-shaped particle with an apical envelope extension. The nucleo capsid is cylindrical with asymmetric ends, and has a superficial segm ented appearance. The pattern of degradation confirms that the nucleoc apsid is a cylinder formed by stacks of rings, which are in turn compo sed of 2 rows of regularly spaced subunits. WSSV replication takes pla ce in the nucleus and is first indicated by chromatin margination and nuclear hypertrophy. Viral morphogenesis begins by the formation of me mbranes de novo in the nucleoplasm and by the elaboration of segmented , empty, long tubules. These tubules break into fragments to form nake d empty nucleocapsids. After that, membranes envelop the capsids leavi ng an open extremity. The nucleoproteins, which have a filamentous app earance, enter the capsid through this open end. When the core is comp letely formed, the envelope narrows at the open end and forms the apic al tail of the mature virion.