Matthias Galipaud and Hanna Kokko
Anyone who has ever snorkelled above a coral reef has probably wondered how old a coral structure on a reef is. This is not an entirely easy question to answer. Corals reproduce sexually by sending off gametes into ocean water, in the hope that they find each other and fuse, but coral ‘growth’ is also a form of reproduction: new polyps are produced directly by a parent polyp, without sex (fertilization) playing a role. Should a biologist think of asexual polyp additions as advancing the age of the entire coral? Is it appropriate to assign the age zero to the newly formed individual who arose from sex in the ocean? Such questions are not restricted to coral life cycles, but are of central importance to the question of ageing in general. Even humans, who cannot reproduce via budding, consist of cell lineages that age inevitably: various kinds of damage, including mutational load, accumulate in our tissues and also – albeit at a slower rate – in the gonads. Our review looks at how life manages the question of ageing, how sex may help rejuvenate cell lineages, and why sex is often associated with a stage in life where the future organism is a single cell. We also highlight that the mathematical treatment of this subject shows an intriguing analogy between old individuals (or old cell lines) and a concept of evolutionary ecology: source-sink populations, where some individuals live in areas that enable them to give rise to future generations (sources), and others inhabit places where reproduction is too poor to sustain itself (demographic sinks).