Speciation and gene flow between snails of opposite chirality
Speciation and gene flow between snails of opposite chirality
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DOI：
10.1371/journal.pbio.0030282
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发表时间：
2005-09-01
期刊：
影响因子：
9.8
通讯作者：
Clarke, B
中科院分区：
文献类型：
--
作者：
Davison, A;Chiba, S;Clarke, B
蜗牛的左右不对称很有趣，因为手性相反的个体要么无法交配，要么交配起来很困难，因此可能会在繁殖上彼此隔离。因此，我们研究了日本陆地蜗牛属Euhadra的手性进化，以了解手性的变化是否促进了物种的形成。特别是，我们旨在了解母系手性遗传对生殖隔离和基因流动的影响。我们发现Euhadra的线粒体DNA系统发育与单一的、相对古老的左缘物种进化是一致的，这表明要么是最近的“单基因物种形成”，要么是无法交配的手性变种之间的基因流动。为了弄清新的手性形态可能进化的条件，以及单基因物种形成是否可能发生，我们开发了一个与任何母体效应基因相关的数学模型。该模型表明，生殖特征位移可以促进新的手性形态的进化，倾向于抵消积极的频率依赖选择，否则将推动更常见的手性形态固定。因此，这暗示了蜗牛手性变异产生的一般机制。在包含两种手性变体的种群中，可能出现两种不同的情况。首先，由于母系手性遗传，即使没有手性交配，变种之间的基因流动也是实质性的。在第二种情况下，生殖隔离是可能的，但不稳定，如果手性内交配偶尔产生具有相反手性的后代，也会导致基因流动。总之，这些结果表明，手性逆转的物种形成只有在复杂的生物地理过程中才有意义，因此通常必须涉及其他因素。为了了解生殖性状位移和基因流动在手性进化中的作用，有必要对两种手性形态共存的种群进行研究。
Left-right asymmetry in snails is intriguing because individuals of opposite chirality are either unable to mate or can only mate with difficulty, so could be reproductively isolated from each other. We have therefore investigated chiral evolution in the Japanese land snail genus Euhadra to understand whether changes in chirality have promoted speciation. In particular, we aimed to understand the effect of the maternal inheritance of chirality on reproductive isolation and gene flow. We found that the mitochondrial DNA phylogeny of Euhadra is consistent with a single, relatively ancient evolution of sinistral species and suggests either recent "single-gene speciation'' or gene flow between chiral morphs that are unable to mate. To clarify the conditions under which new chiral morphs might evolve and whether single-gene speciation can occur, we developed a mathematical model that is relevant to any maternal-effect gene. The model shows that reproductive character displacement can promote the evolution of new chiral morphs, tending to counteract the positive frequency-dependent selection that would otherwise drive the more common chiral morph to fixation. This therefore suggests a general mechanism as to how chiral variation arises in snails. In populations that contain both chiral morphs, two different situations are then possible. In the first, gene flow is substantial between morphs even without interchiral mating, because of the maternal inheritance of chirality. In the second, reproductive isolation is possible but unstable, and will also lead to gene flow if intrachiral matings occasionally produce offspring with the opposite chirality. Together, the results imply that speciation by chiral reversal is only meaningful in the context of a complex biogeographical process, and so must usually involve other factors. In order to understand the roles of reproductive character displacement and gene flow in the chiral evolution of Euhadra, it will be necessary to investigate populations in which both chiral morphs coexist.