Pellmyr O, Leebens-Mack J, Huth C J
Department of Biology, Vanderbilt University, Nashville, Tennessee 37235, USA.
Nature. 1996 Mar 14;380(6570):155-6. doi: 10.1038/380155a0.
Interspecific mutualisms are regarded as having evolved from antagonistic or commensalistic interactions, with most mutualisms remaining facultative but some having coevolved into obligate reciprocal dependency. Underlying mutualism is an intrinsic conflict between the parties, in that each is under selection for increased exploitation of the other. Theoretical models suggest that this conflict is a source of evolutionary instability, and that evolution of 'cheating' by one party may lead to reciprocal extinction. Here we present phylogenic evidence for reversal of an obligate mutualism: within the yucca moth complex, distinct cheater species derived from obligate pollinators inflict a heavy cost on their yucca hosts by laying their eggs but not pollinating the yucca. Phylogenetic data show the cheaters to have existed for a long time. Coexisting pollinators and cheaters are not sister taxa, supporting predictions that evolution of cheating within a single pollinator is evolutionarily unstable. Several lines of evidence support a hypothesis that host shifts preceded the reversal of obligate mutualism. Host or partner shifts is a mechanism that can provide a route of evolutionary escape among obligate mutualists in general.
种间互利共生关系被认为是从对抗性或共生关系演化而来的，大多数互利共生关系仍然是兼性的，但有些已经共同进化为 obligate reciprocal dependency（可译为“专性互惠依赖”）。互利共生的潜在基础是双方之间的内在冲突，因为双方都在被选择以增加对另一方的利用。理论模型表明，这种冲突是进化不稳定的一个来源，一方的“欺骗”进化可能导致相互灭绝。在这里，我们展示了一种专性互利共生关系逆转的系统发育证据：在丝兰蛾复合体中，源自专性传粉者的不同欺骗性物种通过产卵但不为丝兰授粉，给它们的丝兰宿主带来了沉重代价。系统发育数据表明这些欺骗性物种已经存在了很长时间。共存的传粉者和欺骗性物种不是姊妹类群，这支持了这样的预测，即单个传粉者内部的欺骗进化在进化上是不稳定的。几条证据支持了一个假说，即在专性互利共生关系逆转之前就发生了宿主转移。宿主或伙伴转移是一种机制，总体上可以为专性互利共生者提供一条进化逃逸途径。
