r- and K-tactics in the evolution of protist developmental systems: cell and genome size, phenotype diversifying selection, and cell cycle patterns.

I outline the significance for protist evolution of the r-, K-selection spectrum,, and of my earlier theory that the most fundamental way organisms adapt to this spectrum is by evolutionary variations in their cell volumes, cell growth rates and genome sizes. Then I introduce the concept of phenotype diversifying selection; this refers to those selective forces which favour an increase in the number of phenotypes produced during a single life cycle by an organism's genotype and epigenetic system. These ideas are then used to discuss the evolution of protist development, with special reference to modifications of the cell cycle whose evolutionary causes and consequences can be related to K-selection for large size and r-selection for rapid reproduction. The significance of multiple fission, syncytia, multicellularity, nuclear dimorphism plus polyploidy, and reversible polyploidy, is treated in detail. Predictions are made of the effects of these different developmental patterns on genome size and the distribution and amounts of nucleoskeletal RNA and heterochromatin. I suggest that heterochromatin exists primarily because of phenotype diversifying selection for differing nuclear volumes. The possibility of applying these ideas to other cell properties like mitotic or cytokinetic mechanisms is also briefly discussed.