Ultrastructural studies on the aquatic ascomycetes Annulatascus velatisporus and A-triseptatus sp nov.

Annulatascus species are common freshwater ascomycetes in the tropics. In t his paper, the type species, Annulatascus velatisporus, and a new species, A. triseptatus, are described and illustrated at light and electron microsc ope levels. Annulatascus triseptatus differs from A. velatisporus in having three-septate ascospores surrounded by a thin sheath. The asci of both spe cies have bilamellate ascus walls and a bipartite apical ring. The upper pa rt of the apical ring differentiates from the inner ascus wall layer and th e lower part elongates downwards during maturation. The mesosporium is the first formed ascospore wall layer followed by the episporium, which is cove red with verruculose ornamentations. The mucilaginous sheath of the ascospo res is fibrillar or amorphous depending on the fixation methods employed, a nd appears to be derived from the episporial verruculose ornamentations.