Genome-wide analysis of cell cycle-regulating genes in the symbiotic dinoflagellate Symbiodinium minutum

ABSTRACTA critical relationship exists between reef-building corals and their photosynthetic endosymbionts. As important as this relationship is for reef health, it is exquisitely delicate—exposure to temperatures only marginally above the average summer maximum can cause corals to bleach, expelling their resident algae. Interestingly, several studies indicate that failure of corals to properly regulate symbiont cell divisions at high temperatures may cause bleaching. This needs to be further investigated, but first, it is necessary to decipher the molecular mechanisms controlingl the cell division cycle in these organisms. As a first step toward this goal, we identified key cell cycle-regulating genes in the recently published genome of the symbiotic dinoflagellate Symbiodinium minutum. We then correlated expression of these genes with cell cycle phase in diurnally growing S. minutum in culture. Of particular interest, this approach allowed us to identify cyclins and cyclin-dependent kinases that are involved in the G1/S transition—a likely point for coral cells to exert control over algal cell divisions.

A diverse host thrombospondin-type-1 repeat protein repertoire promotes symbiont colonization during establishment of cnidarian-dinoflagellate symbiosis

The mutualistic endosymbiosis between cnidarians and dinoflagellates is mediated by complex inter-partner signaling events, where the host cnidarian innate immune system plays a crucial role in recognition and regulation of symbionts. To date, little is known about the diversity of thrombospondin-type-1 repeat (TSR) domain proteins in basal metazoans or their potential role in regulation of cnidarian-dinoflagellate mutualisms. We reveal a large and diverse repertoire of TSR proteins in seven anthozoan species, and show that in the model sea anemone Aiptasia pallida the TSR domain promotes colonization of the host by the symbiotic dinoflagellate Symbiodinium minutum. Blocking TSR domains led to decreased colonization success, while adding exogenous TSRs resulted in a ‘super colonization’. Furthermore, gene expression of TSR proteins was highest at early time-points during symbiosis establishment. Our work characterizes the diversity of cnidarian TSR proteins and provides evidence that these proteins play an important role in the establishment of cnidarian-dinoflagellate symbiosis.