干旱区生态环境知识资源中心

The life cycle of the monogenean Pseudodiplorchis americanus which infects a desert amphibian (Scaphiopus couchii) has many unusual features: the most notable concern the infective stages which are nourished during development in utero by direct transfer of metabolites from the parent and which, after hatching, are very long lived and tolerate exposure to desiccation. This study examined the development and ultrastructural adaptations of the tegument of these larvae. A primitive epithelium which initially forms around the encapsulated embryo is replaced by a syncytial tegument. Invasion of the host via the nares involves migration above the water surface and the oncomiracidia remain infective and their cilia are still functional after drying. There are no obvious ultrastructural modifications but the tegument appears unusually tough: oncomiracidia do not disintegrate after death, like those of other monogeneans, and the tegument retains its integrity for several days. Tegumental vesicles are probably responsible for the secretion of a glycocalyx and the ciliated cells may be protected by secretions from their network of electron-dense strands. Asynchronous shedding of ciliated cells occurs 1-2 h post-infection so the parasite may not immediately be committed to the host it first locates. Exciliation involves coalescence of basal vacuoles to form a large cavity which enlarges under the ciliated cell, eventually causing the lateral septate desmosomes to rupture. P. americanus is unusual as the underlying basal lamina may be exposed to the environment during exciliation until the surrounding surface layer extends to form a continuous layer.