Dinoflagellates, a major group (>3000 species) of protists; members are a significant group of phytoplanktons, the intracellular symbiotic microalgae (zooxanthallae) of corals (and many other invertebrates) and the major causatic agents of harmful algal blooms; they are also rich source of DHA and secondary metabolites, including many toxins and are infamous for causing most types of seafood poisoning syndromes (e.g. ciguatera). They also form the group alveolates with the ciliates (e.g. Tetrahymena) and the apicomplexans (e.g. the malarial Plasmodium).
Dinoflagellates have some of the largest known genomes (∞100 times human diploid genome) but counter-intuitively are nucleosomeless and have some of the lowest chromosomal protein-to-DNA ratio; they embody the fourth way of extant genome packaging. Dinoflagellates are thus rich sources of investigative problems in fundamental cell biology. Research in my lab focuses on two important areas of dinoflagellate cell biology:
Nucleosomeless Chromosomes. At high concentrations, DNAs can form liquid crystalline phases. Previous studies demonstrated Liquid Crystalline Chromosomes (LCCs) in dinoflagellates. LCCs encode some of the largest-known eukaryotic genomes (up to 80 times human genome size) but counter-intuitively are apparently nucleosomeless and have the lowest known chromosomal protein-to-DNA ratios in extant life-forms. Biology of LCCs will be of general interests in understanding how large genome-sized DNAs can be organized and condensed; at the same time replicated and transcribed.
Cell Size Regulation
Coordination between cellular growth and cell cycle progression are crucial to all cells requiring the integration of ongoing information on growth rates and sizes. The progressive expression of a positive cell-cycle regulator or progressive dilution of a negative cell-cycle regulator are conceptually indirect “sizers”(Cf..Zeuthen), potentially coupling with protein synthesis and mediating a threshold switch. Cell-cycle dependent gradient of a mitotic activator introduces an element of location with threshold. Cyclic ADP-ribose, a secondary product of metabolism additionally coupled to an element of metabolic rate and energy status to cell-cycle progression and cell division mode switching (Lam et al., 2009). dCel1, cell-cycle regulated expression of a wall-associated cellulase provided a further link between a restricting cell wall and cell-cycle progression (Kwok and Wong 2010); and that with lipid and carbohydrate biosynthesis(Kwok and Wong 2003,2006). It is unlikely that fundamental parameters like cell size would be monitor by a few “sizers” without an actual element of cell size; we are continuing our quest in this direction.
Molecular Cell Biology of Life-Cycle Transitions
Life cycles of most dinoflagellates are complex, characteristically with multiple life-cycle stages through the alternation of asexual and sexual reproduction, and consisting of motile and non-motile cells. Life-cycles of unicellular protists are also intimately related to their cell-cycles. Many species of dinoflagellates are also known to carry out multiple fission, a type of asexual reproduction in which a cell grow to a large size before dividing multiple times within the mother cell wall, before breakup into daughter cells.
Cellulosic Thecal Plates (CTPs) of Internal Cell Wall
Cell wall are generally extracellular; dinoflagellate cell walls are unique in being intracellular and can comprise of multiple layers, including the cellulosic pellicle and the cellulosic thecal plates (CTPs), internal to the outermost plasma membrane. CTPs can form highly decorated “armor-plates” in flattened cortical vesicles in thecate dinoflagellates. Major remodeling of their cell wall occur during life-cycle and cell-cycle transitions and we are interested in the underlying epigenetic regulation, in the apparently nucleosomeless background.
With the advent of genomics, the advancement of cell biological techniques, the global prominence of coral breaching and algal blooms, it is now urgent and strategic to study this important and interesting group of protists !!!
There are many other reasons why it is cool to study dinoflagellates !
- Kwok A.C. M. and Wong J.T.Y. (2010) Activities of a walled-bound cellulase is coupled to and is required for cell cycle progression in a dinoflagellate. Plant Cell 22:1281-1298
- Chow, M.H., Yan, K.T.H., Bennett, M.J. and Wong J.T.Y. (2010) Liquid crystalline chromosomes: birefringence and DNA condensation. Eukaryotic Cell 9:1577-1587. (selected for Front Cover)
- Leung, S.K.and Wong, J.T.Y. (2009) The replication of plastid minicircles involves rolling circle intermediates. Nucleic Acids Research 37: 1991-2002.
- Kwok, A.C.M., Mak, C.C.M., Wong, F.T.W. and Wong J.T.Y. (2007) A novel method for preparing spheroplasts from cells with an internal cellulosic cell wall. Eukaryotic Cell 6:563-567. (selected for Front Cover)
- Chan Y.H. and Wong J.T.Y. (2007) Concentration-dependent organization of DNA by the dinoflagellate histone-like protein HCc3. Nucleic Acids Research; 35:2573-2583.