Variation in genome size—an ecological interpretation

Previous attempts to explore the significance of variation in genome size have involved comparisons with respect to life history1–4, taxonomic and evolutionary affiliations5–9 and geographical distribution10,11. Here we examine variation in the British flora. Large genomes are particularly associated with Mediterranean geophytes and grasses in which growth is confined to the cool conditions of winter and early spring. We suggest that large genomes have evolved under circumstances in which growth is limited by the effect of low temperature on rates of cell division and are part of a mechanism whereby growth at low temperature is achieved by rapid inflation of large cells formed during a preceding warm dry season. Where moisture supply allows growth to occur in the summer, temporal separation of mitosis and cell expansion confers no advantage and the longer mitotic cycle of large cells is likely to restrict rates of development; here the effect of natural selection has been to reduce cell and genome size.