Rapid increase of host defence against brood parasites in a recently parasitized area: the case of village weavers in Hispaniola
Rapid increase of host defence against brood parasites in a recently parasitized area: the case of village weavers in Hispaniola
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在最近被寄生的地区，宿主对寄生寄生虫的防御能力迅速增强：伊斯帕尼奥拉岛乡村织布工的案例
DOI：
10.1098/rspb.1999.0727
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
G. Sorci
中科院分区：
文献类型：
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作者：
M. Robert;G. Sorci
雀形目寄生鸟的育雏宿主对外来卵的识别和排斥能力通常各不相同。两个主要的假设已经提出来解释持久性的受体个体的物种，利用了育雏寄生虫。进化滞后假说假定，一些宿主还没有进化出辨别外来卵的能力。一旦通过突变出现了识别寄生卵的能力，由于排斥的选择优势，它就会传播开来。然后选择寄生虫产生更多的模仿卵，以逃避宿主的歧视，最终导致寄生虫和宿主之间的军备竞赛。进化均衡假说是基于这样的假设，即在没有寄生的情况下，由于识别错误，拒绝行为是代价高昂的。当寄生率波动或持续较低时，受体宿主可以持续存在。Cruz和Wiley提供了在没有寄生的情况下拒绝成本的间接证据，他们报告了世纪前从非洲引入到伊斯帕尼奥拉岛（西印度群岛）的乡村织布鸟（Ploceus cucullatus）的低拒绝率。在非洲，该物种被金龟子杜鹃寄生，并表现出高度的卵歧视。由于没有育雏寄生虫是目前在伊斯帕尼奥拉岛，克鲁兹和威利建议，拒绝选择反对在没有寄生，由于识别成本。因此，在伊斯帕尼奥拉岛引进乡村织工，为检验一种适应性的衰退提供了一个独特的机会。在过去的世纪里，闪亮的燕八哥（Molothrus bonariensis）已经扩大了它的活动范围，从南美洲入侵了西印度群岛的大部分地区。它于1972年在伊斯帕尼奥拉岛首次被发现，并开始利用村庄的织布工作为宿主。考虑到闪亮的燕八哥大大降低了织布工的繁殖成功率，我们应该期待与克鲁兹和威利16年前报道的结果相比，现在的拒绝率更高。与这一预测一致，我们发现燕八哥模型蛋的拒绝率很高（89.3%，95% CI = 81.1 - 97.5%），非模拟织纹虫模型卵的中等排斥水平（67.5%，95%CI = 52.5 ~ 82.5%）和相当低水平的模拟织纹虫模型卵排斥反应（25%，95%CI = 4 ~ 46%）。因此，人工燕八哥卵的排斥率在16年内从13.8%（95%CI = 5至22.6%）增加到89.3%。为了检查这种宿主抗性的快速增加是否与遗传微进化变化相容，我们建立了一个种群动力学模型，其中，作为上限，抗性由后代以1的概率遗传。这个简单的模型表明，所观察到的排斥率的变化是兼容的遗传微进化的转变，只有在最有利的情况下，拒绝传播。放宽这些假设中的一个或几个（例如，高寄生率，没有拒绝成本）大大延长了拒绝传播所需的时间。我们认为，遗传和学习过程可能参与了观察到的变化。
Passerine hosts of brood parasitic birds usually vary in their ability to discriminate and reject alien eggs. Two main hypotheses have been suggested to explain the persistence of acceptor individuals in species that are exploited by brood parasites. The evolutionary lag hypothesis postulates that some hosts have not yet evolved the ability to discriminate against alien eggs. Once the ability to recognize the parasitic egg has appeared by mutation, it spreads because of the selective advantage of rejection. Parasites are then selected to produce more mimetic eggs, in order to escape host discrimination, which eventually ends up in an arms race between the parasite and the host. The evolutionary equilibrium hypothesis is based on the assumption that rejection behaviour is costly in the absence of parasitism, because of recognition errors. Acceptor hosts can persist when parasitism rate fluctuates or is consistently low. Indirect evidence for costs of rejection in the absence of parasitism has been provided by Cruz and Wiley, who reported low rejection rate for a population of village weavers (Ploceus cucullatus) introduced from Africa to Hispaniola (West Indies) more than a century ago. In Africa the species is parasitized by Chrysococcyx cuckoos and shows high levels of egg discrimination. Since no brood parasite was present in Hispaniola, Cruz and Wiley suggested that rejection was selected against in the absence of parasitism due to recognition costs. Introduction of village weavers in Hispaniola, therefore, provided a unique opportunity to test the decline of an adaptation. During the past century the shiny cowbird (Molothrus bonariensis) has expanded its range–invading most of the West Indies from South America. It was first observed in Hispaniola in 1972, and it started to exploit village weavers as a host. Given that shiny cowbirds substantially reduce the reproductive success of weavers, we should expect higher rejection rates nowadays compared to those reported by Cruz and Wiley 16 years ago. In agreement with this prediction, we found a high rejection rate of cowbird model eggs (89.3%, 95% CI = 81.1 to 97.5%), moderate levels of rejection of non–mimetic weaver model eggs (67.5%, 95% CI = 52.5 to 82.5%) and rather low levels of rejection of mimetic weaver model eggs (25%, 95% CI = 4 to 46%). The rejection rate of artificial cowbird eggs has therefore increased from 13.8% (95% CI = 5 to 22.6%) to 89.3% in 16 years. To check whether this rapid increase in host resistance is compatible with a genetic microevolutionary change, we built a population dynamics model where, as an upper bound, resistance is inherited by the progeny with a probability of one. This simple model shows that observed changes of rejection rate are compatible with a genetic microevolutionary shift only under the most favourable scenario for rejecters to spread. Relaxing one or several of these assumptions (e.g. high parasitism rate, absence of rejection costs) considerably lengthens the period needed for rejecters to spread. We suggest that both genetic and learning processes might be involved in the observed changes.