A rare recording of a Greenland shark (Somniosus microcephalus) – a long-lived species with enormously large cells – at Admiralty Inlet, Nunavut. Photo: Hemming1952.
Cell size is the result of a fundamental and strong selection pressure on energy use. ‘That is why water fleas have small cells and Greenland sharks have enormous ones.’
‘Cell size is the result of a double-edged sword and of the evolutionary trade-off between whether a species squanders resources or benefits from an energy-saving mode. This theory is not new, but it reveals a fundamental and little-known principle that governs the ecology and evolution of all living species. That is why it belongs in the textbooks’, says ecologist Wilco Verberk, recently appointed Professor of Functional Ecology in Nijmegen. Together with Polish evolutionary biologist Marcin Czarnoleski, he publishes in Trends in Ecology & Evolution (November issue, published online 2 October) a review of the Theory of Optimal Cell Size, which predicts that cell size in organisms – from prokaryotes to mammals – is governed by the same universal rules. ‘It is not just an interesting insight, but also offers illuminating perspectives on evolutionary adaptations and ecological interactions’, says Verberk.
Trade-off
‘All organisms have charged membranes to maintain a distinction between inside and outside. Maintaining differences in ion concentrations across these membranes costs a great deal of energy, and this creates a fundamental trade-off’, explains Verberk. ‘In a nutrient-poor environment, you can save energy by having large cells, which create a large volume with relatively little cell surface area. Whereas small cells have relatively much more surface area, allowing rapid uptake of nutrients and oxygen. It explains why fast-growing water fleas have small cells, whilst Greenland sharks – living in energy-saving mode – have enormous ones.’
The review shows that cell volume is strongly correlated with nucleus size and genome size. Verberk: ‘Due to evolutionary innovations, this relationship differs somewhat between, for example, bacteria, reptiles and birds, but within such broad groups genome size is a reliable indicator of cell size.’ A relationship that Verberk and colleagues have also substantiated through experiments with different populations of fruit flies and with diploid and triploid zebrafish. ‘A triploid zebrafish has one and a half times as much DNA, one and a half times larger cells, and one and a half times fewer cells, but otherwise looks perfectly normal.’
Esoteric
Verberk has himself been working for years on the relationship between water temperature and adaptations in freshwater organisms, and came into contact with the Polish research on cell size and energy metabolism at a conference. ‘At first it struck me as rather esoteric research’, says Verberk. ‘But later I began to see more and more pieces falling into place. For instance, it offers very interesting explanations for differences in flight capacity in birds, hibernation or aestivation, and even the diving behaviour of whales.’
The Theory of Optimal Cell Size explains why the water flea (Daphnia pulex) has small cells. (Photo: ImageSelect)
Gerard Jagers op Akkerhuis, an independent theoretical biologist based in Wageningen who was not involved in this review, regards the publication as ‘a fine integrative study on an underexposed topic’. It gives a clear picture of ‘the weighing of the metabolic advantages of large cells with abundant DNA against the disadvantages in terms of greater maintenance costs and reduced membrane-bound transport of substances’, says Jagers. ‘I also see an underlying connection with the Dynamic Energy Budget theory of systems biologist Bas Kooijman, which calibrates the uptake and excretion of nutrients and toxins to the surface areas of organisms and their metabolism to volumes.’