Biology Department, Queens College and the Graduate Center of the City University of New York, Flushing, NY, USA.
Proc Biol Sci. 2010 Oct 22;277(1697):3113-21. doi: 10.1098/rspb.2010.0342. Epub 2010 May 19.
A pathogen can readily mutate to infect new host types, but this does not guarantee successful establishment in the new habitat. What factors, then, dictate emergence success? One possibility is that the pathogen population cannot sustain itself on the new host type (i.e. host is a sink), but migration from a source population allows adaptive sustainability and eventual emergence by delivering beneficial mutations sampled from the source's standing genetic variation. This idea is relevant regardless of whether the sink host is truly novel (host shift) or whether the sink is an existing or related, similar host population thriving under conditions unfavourable to pathogen persistence (range expansion). We predicted that sink adaptation should occur faster under range expansion than during a host shift owing to the effects of source genetic variation on pathogen adaptability in the sink. Under range expansion, source migration should benefit emergence in the sink because selection acting on source and sink populations is likely to be congruent. By contrast, during host shifts, source migration is likely to disrupt emergence in the sink owing to uncorrelated selection or performance tradeoffs across host types. We tested this hypothesis by evolving bacteriophage populations on novel host bacteria under sink conditions, while manipulating emergence via host shift versus range expansion. Controls examined sink adaptation when unevolved founding genotypes served as migrants. As predicted, adaptability was fastest under range expansion, and controls did not adapt. Large, similar and similarly timed increases in fitness were observed in the host-shift populations, despite declines in mean fitness of immigrants through time. These results suggest that source populations are the origin of mutations that drive adaptive emergence at the edge of a pathogen's ecological or geographical range.
病原体很容易突变以感染新的宿主类型，但这并不能保证其在新栖息地成功定植。那么，哪些因素决定了出现成功呢？一种可能性是病原体种群不能在新的宿主类型上维持自身（即宿主是汇），但从源种群的迁移允许适应性可持续性，并通过从源种群的遗传变异中采样有益突变，最终实现适应性出现。无论汇宿主是否真正新颖（宿主转移），还是汇是一个现有的或相关的、在不利于病原体持续存在的条件下茁壮成长的类似宿主种群（范围扩张），这种情况都适用。我们预测，由于源遗传变异对汇中病原体适应性的影响，在范围扩张下，汇适应应该比在宿主转移中更快发生。在范围扩张下，源迁移应该有利于汇中的出现，因为作用于源和汇种群的选择很可能是一致的。相比之下，在宿主转移期间，源迁移可能会破坏汇中的出现，因为宿主类型之间的选择或表现权衡没有相关性。我们通过在汇条件下对新型宿主细菌进行噬菌体种群的进化，同时通过宿主转移与范围扩张来操纵出现，从而验证了这一假设。对照组研究了当未进化的原始基因型作为移民时，汇适应情况。正如预测的那样，适应性在范围扩张下最快，而对照组没有适应性。尽管随着时间的推移，移民的平均适应度下降，但在宿主转移种群中观察到了较大、相似且时间相似的适应性增加。这些结果表明，源种群是驱动病原体生态或地理范围边缘适应性出现的突变的起源。
