CHARACTERISTICS OF SEXUAL AND ASEXUAL RESTING CYST (STATOSPORE) FORMATION IN DINOBRYON CYLINDRICUM IMHOF (CHRYSOPHYTA)1

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Abstract
The reproductive significance of siliceous cysts (statospores) produced by the common vernal chrysophyte Dinobryon cylindricum Imhof has been investigated under defined culture conditions. Three types of statospores have thus far been induced in culture: 1) uninucleate, asexual; 2) binucleate, asexual (potentially autogamic); 3) binucleate, sexual (zygotic). The production of each type of cyst responds differently to an array of nutrient deficiencies (P, N, vitamins, micrometals). An individual clone may be capable of participating in the production of all or only a subset of these types of resting cysts. All D. cylindricum statospores are morphologically identical regardless of their reproductive significance. Sexual reproduction leading to zygotic statospore formation is anisogamous, heterothallic, and involves a gametogenic hormone (erogen) that is apparently continuously released from female clones. Only a single bipolar mating group is documented here and clonal compatibility varies considerably within the mating group.The dynamics of encystment for each type of statospore has been determined relative to the growth of the vegetative cell population. Statospores may be produced either during the exponential phase (intrinsic encystment) or stationary phase (extrinsic encystment) of culture growth depending on the clones involved. The effect of both asexual and sexual resting cyst production on the net growth rate and dynamics of natural chrysophyte populations is discussed. Statospores appear to represent a more flexible reproductive strategy than the resistant zygospores produced by the other common groups of planktonic microalgae.