The cell cycle of symbiotic Chlorella. III. Numbers of algae in green hydra digestive cells are regulated at digestive cell division.

Regression analysis of the relationship between the size of interphase and mitotic digestive cells of green hydra, and the numbers and total volume of the symbiotic Chlorella algae they contain showed a partial correlation only, suggesting that numbers of algae per cell are not regulated by limiting them to a specific proportion of the host cell, and that the variation observed in numbers of algae per cell is not due to variation in host cell size. After hydra were fed, which stimulates algae and digestive cells to divide at the same time, numbers of algae per cell were higher in prophase than in interphase cells, and numbers increased as mitosis proceeded. In excised regenerating peduncles algae divide before digestive cells, and at the onset of digestive cell division mitotic cells were found to contain almost twice the number of algae as before excision. Thus, almost all of the algal cell division necessary to maintain a constant population size was associated with digestive cell division. Analysis of variance in numbers of algae in telophase mother and daughter cells suggested that the proportion of algae dividing as a result of host cell mitosis was greater in digestive cells with few algae than in those with many algae. The fact that the mechanism controlling the proportion of algae dividing in host cells is expressed at host cell division and is manifested in the daughter cells may contribute to wide variation in numbers of algae per cell.