Parker G A, Ball M A, Chubb J C
Department of Evolution, Ecology and Behaviour, Institute of Integrative Biology, University of Liverpool, Liverpool, UK.
J Evol Biol. 2015 Feb;28(2):292-304. doi: 10.1111/jeb.12576. Epub 2015 Jan 27.
We review how trophically transmitted helminths adapt to the special problems associated with successive hosts in complex cycles. In intermediate hosts, larvae typically show growth arrest at larval maturity (GALM). Theoretical models indicate that optimization of size at GALM requires larval mortality rate to increase with time between infection and GALM: low larval growth or paratenicity (no growth) arises from unfavourable growth and mortality rates in the intermediate host and low transmission rates to the definitive host. Reverse conditions favour high GALM size or continuous growth. Some support is found for these predictions. Intermediate host manipulation involves predation suppression (which decreases host vulnerability before the larva can establish in its next host) and predation enhancement (which increases host vulnerability after the larva can establish in its next host). Switches between suppression and enhancement suggest adaptive manipulation. Manipulation conflicts can occur between larvae of different ages/species a host individual. Larvae must usually develop to GALM before becoming infective to the next host, possibly due to trade-offs, e.g. between growth/survival in the present host and infection ability for the next host. In definitive hosts, if mortality rate is constant, optimal growth before switching to reproduction is set by the growth/morality rate ratio. Rarely, no growth occurs in definitive hosts, predicted (with empirical support) when larval size on infection exceeds growth/mortality rate. Tissue migration patterns and residence sites may be explained by variations in growth/mortality rates between host gut and soma, migration costs and benefits of releasing eggs in the gut.
我们回顾了经营养传播的蠕虫如何适应复杂生命周期中与连续宿主相关的特殊问题。在中间宿主体内，幼虫通常在幼虫成熟时出现生长停滞（GALM）。理论模型表明，GALM时大小的优化要求幼虫死亡率随感染与GALM之间的时间增加而上升：中间宿主体内不利的生长和死亡率以及向终末宿主的低传播率导致幼虫生长缓慢或滞育（无生长）。相反的条件有利于GALM时的大尺寸或持续生长。这些预测得到了一些支持。中间宿主操控包括捕食抑制（在幼虫能够在其下一个宿主体内定殖之前降低宿主易感性）和捕食增强（在幼虫能够在其下一个宿主体内定殖之后增加宿主易感性）。抑制和增强之间的转换表明是适应性操控。在一个宿主个体内不同年龄/物种的幼虫之间可能会发生操控冲突。幼虫通常必须发育到GALM阶段才对下一个宿主具有感染性，这可能是由于权衡取舍，例如当前宿主体内的生长/存活与对下一个宿主的感染能力之间的权衡。在终末宿主体内，如果死亡率恒定，转向繁殖前的最佳生长由生长/死亡率比率决定。很少情况下，终末宿主体内不发生生长，当感染时的幼虫大小超过生长/死亡率时可预测到这种情况（并有实证支持）。组织迁移模式和驻留部位可以通过宿主肠道和躯体之间生长/死亡率的差异、迁移成本以及在肠道中释放卵的利弊来解释。
