Department of Zoology, University of Otago, Dunedin, New Zealand.
Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Canada.
Parasit Vectors. 2019 Jan 4;12(1):5. doi: 10.1186/s13071-018-3241-6.
Division of labour has evolved in many social animals where colonies consist of clones or close kin. It involves the performance of different tasks by morphologically distinct castes, leading to increased colony fitness. Recently, a form of division of labour has been discovered in trematodes: clonal rediae inside the snail intermediate host belong either to a large-bodied reproductive caste, or to a much smaller and morphologically distinct 'soldier' caste which defends the colony against co-infecting trematodes. We review recent research on this phenomenon, focusing on its phylogenetic distribution, its possible evolutionary origins, and how division of labour functions to allow trematode colonies within their snail host to adjust to threats and changing conditions. To date, division of labour has been documented in 15 species from three families: Himasthlidae, Philophthalmidae and Heterophyidae. Although this list of species is certainly incomplete, the evidence suggests that division of labour has arisen independently more than once in the evolutionary history of trematodes. We propose a simple scenario for the gradual evolution of division of labour in trematodes facing a high risk of competition in a long-lived snail host. Starting with initial conditions prior to the origin of castes (size variation among rediae within a colony, size-dependent production of cercariae by rediae, and a trade-off between cercarial production and other functions, such as defence), maximising colony fitness (R) can lead to caste formation or the age-structured division of labour observed in some trematodes. Finally, we summarise recent research showing that caste ratios, i.e. relative numbers of reproductive and soldier rediae per colony, become more soldier-biased in colonies exposed to competition from another trematode species sharing the same snail, and also respond to other stressors threatening the host's survival or the colony itself. In addition, there is evidence of asymmetrical phenotypic plasticity among individual caste members: reproductives can assume defensive functions against competitors in the absence of soldiers, whereas soldiers are incapable of growing into reproductives if the latter's numbers are reduced. We conclude by highlighting future research directions, and the advantages of trematodes as model systems to study social evolution.
分工在许多群居动物中进化，其中群体由克隆或近亲组成。它涉及到形态不同的群体执行不同的任务，从而提高群体的适应性。最近，在吸虫中发现了一种分工形式：蜗牛中间宿主内的克隆雷迪虫要么属于体型较大的生殖群体，要么属于体型较小且形态明显不同的“士兵”群体，后者可以保护群体免受共感染的吸虫的侵害。我们回顾了最近关于这种现象的研究，重点关注其系统发育分布、可能的进化起源以及分工如何运作，使蜗牛宿主内的吸虫群体能够适应威胁和变化的条件。迄今为止，已在三个科的 15 个物种中记录到分工：Himasthlidae、Philophthalmidae 和 Heterophyidae。尽管这些物种的列表肯定不完整，但证据表明，分工在吸虫的进化历史中已经不止一次独立出现。我们提出了一个简单的情景，即在蜗牛宿主寿命长、竞争风险高的情况下，吸虫分工逐渐进化。从分工起源之前的初始条件开始（群体内雷迪虫的大小变异、雷迪虫产生尾蚴的大小依赖性以及尾蚴产生与其他功能（如防御）之间的权衡），最大限度地提高群体适应性（R）可以导致形成群体或观察到一些吸虫中的年龄结构分工。最后，我们总结了最近的研究，表明在面临同一蜗牛中的另一种吸虫竞争的情况下，群体中的生殖雷迪虫和士兵雷迪虫的比例变得更偏向于士兵，并且对威胁宿主生存或群体本身的其他压力源也会做出反应。此外，还有证据表明个体群体成员之间存在不对称的表型可塑性：在没有士兵的情况下，生殖体可以承担防御功能以抵御竞争者，而如果后者的数量减少，士兵则无法发育成生殖体。最后，我们强调了未来的研究方向，以及吸虫作为研究社会进化的模型系统的优势。
