One longstanding puzzle about malaria parasites is why they have evolved tightly synchronised cell cycles, in which each generation of parasites bursts out of infected red blood cells at the same time. Synchronicity in malaria infections is so well known that it has been used to diagnose which species a patient has. Across 150 species of malaria, cell cycles take a multiple of 24 hours, strongly suggesting that parasites have their own circadian clock or use information about their in-host or abiotic environments to ‘tell the time'. But why and how malaria parasites maintain their synchronicity remains a mystery.
Throughout their infections, malaria parasites must produce specialised sexual stages to enable them to transmit to mosquito vectors. The conventional wisdom is that timing and synchronicity decisions allow the maturation of sexual stages to coincide with vector biting patterns. However, from an evolutionary perspective they also seem unlikely to be true: it is precisely the synchronous bursting that elicits a rapid and short-lived immune response (paroxysm) and this response sterilizes gametocytes, which blocks their ability to transmit to vectors for some time. Furthermore, vector species have a variety of activity patterns, mature sexual stages are present around the clock in infections, and different species of malaria parasite vary in their synchronicity.
Some in vitro data suggest that parasites may even be able to alter their growth rate because faster and more synchronous development occurs in response to host melatonin and co-culture with other parasites at high density. Our observations of rodent malarias suggest that synchronicity breaks down throughout infections as parasite density increases. These data indicate that synchronicity is an important trait for malaria parasites but it is unclear what the fitness benefits of this co-ordination actually are.