An Ultrastructural Investigation into the Role of Calcium in Oosphere-initial Development in Saprolegnia diclina

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Abstract
Prior to formation of oosphere initials in septum-delimited oogonia of S. diclina, the volume density (volume fraction) of central vacuoles containing characteristic clusters of electron-opaque, granular material increased from 6.0 to 54.2% whereas dense-body vesicles decreased in volume density from 30.2 to 9.0%. During this time, dense-body vesicles were reduced in number by 50%, their mean diameter decreased from 0.861 to 0.697 .mu.m and the percentage of vesicles with more than 1 dense-body granule increased from 5.4 to 20.2. The volume density of lipid bodies did not change, while that of the cytoplasmic matrix decreased from 40.0 to 18.2%. Volume densities of mitochondria and nuclei remained constant relative to the peripheral oogonial protoplasm until central vacuoles had enlarged, but then decreased as oosphere initials formed. Small cytoplasmic vesicles were associated both with Golgi dictyosomes and with dense-body vesicles having blebbed membranes. Similar small vesicles were present close to both central vacuole membranes and oogonial walls. Oogonia grown in Ca-deficient medium showed a high level of oogonium and oosphere abortion. Aborted oogonia showed extreme protoplast disorganization. In unaborted oogonia grown under Ca-deficient conditions, central vacuoles contained crystalline inclusions and some oogonia did not form secondary wall layers. Organelles were of normal appearance but Ca deficiency increased the volume densities of mitochondria and nuclei at early stages of oogonial development and of dense-body vesicles and peripheral vacuoles at later stages; the volume density of lipid bodies was reduced at all stages. The numerical density of dense-body vesicles progressively increased during oogonial development and was greater at all developmental stages than in oogonia grown under conditions of Ca sufficiency. At late stages of oogonial development, Ca deficiency reduced both the mean diameter of dense-body vesicles and the fraction of them having 2 or more dense-body granules.

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