Regulation of numbers of symbioticChlorellaby density-dependent division

Abstract

There is much evidence that green hydra digestive cells control cell division of theirChlorellasymbionts so that the symbionts divide only at host cell division. However, it is not clear how the population size of the symbionts is determined, although repeated measurements show that in constant culture conditions the mean number of symbionts per cell also remains constant. In this paper, simple density-dependent compensatory models were tested by simulating large numbers of host cell divisions by using computer modelling techniques. Stability of the mean number of algae per cell was achieved over a wide range of values simply by altering the value of the boundary between division of all symbionts in a cell and density-dependent division. Changes in the boundary between density-dependent division and total inhibition of symbiont division had little effect on the mean number of symbionts per cell, but instead altered variance and the shape of the distribution. Correlation between operation of the mathematical models and possible regulatory mechanisms operating within the symbiosis are discussed.