寄生虫の有害性に対するタイミングの影響

タイトル: Complex interactions in the life cycle of a simple parasite shape the evolution of virulence

概要: Evolutionary expectations about the virulence of parasites (i.e., the parasite-induced mortality rate of the host) often focus solely on the within-host transmission stage, overlooking the time spent between hosts and variations in transmission cycles. Moreover, parasite growth rate within the host is closely linked to virulence. We here suggest that a simplified view of transmission and parasite evolution makes it difficult to predict how virulence will evolve. We illustrate our ideas with a parasite with a simple life cycle, the microsporidian Vavraia culicis, which infects the mosquito Anopheles gambiae. We selected the parasite over six host generations for early or late host transmission, corresponding to shorter or longer time within the host. Selecting for late transmission increased their exploitation of the host, resulting in higher virulence and a shorter life cycle with rapid infective spore production, comparatively to selection for early transmission. In response, hosts infected with late-selected spores shortened their life cycle and shifted to earlier reproduction. Notably, higher virulence was negatively correlated with the endurance of infective spores outside the host, demonstrated in a parallel study using these parasite lines. These findings emphasize the importance of considering the entire transmission cycle in studies of parasite evolution and raise concerns about how host density and social settings might influence virulence evolution. Significance statementClassical expectations on parasite evolution assume a trade-off between transmission rate and harm to the host (virulence), favoring parasites with intermediate virulence. However, recent studies challenge this idea, suggesting it is too simplistic. Here, by selecting the parasite Vavraia culicis for early or late transmission (or shorter and longer time within the host) in the host Anopheles gambiae we demonstrate that understanding virulence evolution requires considering the entirety and the type of transmission cycle and not merely the within-host stage. These findings highlight the importance of including all transmission stages in parasite evolution studies and demonstrate how certain social conditions might unintentionally select more virulent parasites.