Differentiation is a central aspect of the parasite life cycle and encompasses adaptation to both host and environment. If we accept that evolution cannot anticipate an organism’s needs as it enters a new environment, how do parasite differentiation pathways arise? The transition between vertebrate and insect stage African trypanosomes is probably one of the better studied and involves a cell-cycle arrested or ‘stumpy’ form that activates metabolic pathways advantageous to the parasite in the insect host. However, a range of stimuli and stress conditions can trigger similar changes, leading to formation of stumpy-like cellular states. We propose that the origin and optimisation of this differentiation program represents repurposing of a generic stress response to gain considerable gain-of-fitness associated with parasite transmission.

分化是寄生虫生命周期的一个核心方面，包括对宿主和环境的适应。如果我们接受进化无法预测生物体进入新环境时的需求，那么寄生虫分化途径是如何产生的？脊椎动物和昆虫阶段非洲锥虫之间的过渡可能是研究得较好的一种，它涉及细胞周期停滞或“粗壮”形式，其激活有利于昆虫宿主寄生虫的代谢途径。然而，一系列刺激和压力条件可以触发类似的变化，导致形成粗壮的细胞状态。我们建议这种分化程序的起源和优化代表了对一般应激反应的重新利用，以获得与寄生虫传播相关的可观的适应性增益。