Extent and Mechanisms of Gene Flow Limitation between Divergent Populations

在紫花苜蓿和三叶草上邻近的豌豆蚜种群之间发现了明显的局部适应和遗传分化。然而，对于目前维持两种寄主上观察到的蚜虫遗传分化的选择和基因流之间的平衡，人们知之甚少。在这两种寄主上，移栽个体的表现都远低于寄主，这表明有很强的破坏性选择。豌豆蚜虫能够在不同作物的田地之间飞行，专业克隆人可以杂交并产生可存活的后代，因此迁徙和基因流动在物理上是可能的。究竟发生了多少基因流动？这项提案涉及的主要问题是(1)面对作物之间的自由基因流动，与寄主相关的分歧是否通过选择来维持？以及(2)如果如我们所料，基因流动受到限制，这是由什么生态和遗传机制造成的？首先，将使用直接和间接方法来确定寄主间基因流动的程度，并以同一作物田地之间的基因流动的测量为参照点。初步结果表明，这两种作物上的种群既不是完全孤立的，也不是泛发性的。然后将评估可能限制这些不同种群之间基因流动的各种机制，包括有翅(有翅)形式的栖息地选择、针对候鸟的选择、两种作物交配种群之间的物候分离，以及由于发育失败和/或缺乏对亲本环境的适应而导致的杂交后代适合度降低。这些实验的结果将提供一个详细的机制，通过这些机制，不同生境中邻近的种群保持遗传分化，尽管有可能进行杂交。这项研究为研究当地适应和种群分化的机制提供了一个独特的机会。这些实验还将提供一个基因流动限制的案例研究，这对物种形成和害虫适应特定作物的过程的研究具有意义。

Pronounced local adaptation and genetic divergence have been found between closely adjacent pea aphid populations on alfalfa and clover. However, little is known about the balance between selection and gene flow that currently maintains the observed genetic differentiation of aphids on the two hosts. On both hosts, the performance of transplanted individuals is much lower than that of residents, indicating strong disruptive selection. Pea aphids are capable of flying between fields of the different crops and specialist clones can interbreed and produce viable offspring, so migration and gene flow are physically possible. How much gene flow actually takes place? The major questions addressed in this proposal are (1) is the host-associated divergence maintained by selection in the face of free gene flow between crops? and (2) if, as we expect, gene flow is restricted, by what ecological and genetic mechanisms does this occur? First, the extent of inter- host gene flow will be determined using both direct and indirect methods, with measures of gene flow between fields of the same crop as a reference point. Preliminary results suggest that populations on the two crops are neither completely isolated nor panmictic. Various mechanisms that could act to limit gene flow between these divergent populations will be then evaluated, including habitat choice by alate (winged) forms, selection against migrants, phenological separations between mating population on the two crops, and reduced fitness of hybrids due to developmental failure and/or lack of adaptation to the parental environments. The results of these experiments will provide a detailed view of the mechanisms by which closely adjacent populations in different habitats remain genetically differentiated despite the potential to interbreed. This study represents a unique opportunity to study mechanisms of local adaptation and population divergence. These experiments will also provide a case study of gene flow restriction that has implications for the study of speciation, and the process by which insect pests adapt to specific crop plants.