Laboratoire d'Ecologie Alpine, CNRS-UMR 5553, Université Joseph Fourier, BP 53, 38041 Grenoble cedex 09, France.
BMC Genomics. 2009 Nov 21;10:551. doi: 10.1186/1471-2164-10-551.
Genome scans are becoming an increasingly popular approach to study the genetic basis of adaptation and speciation, but on their own, they are often helpless at identifying the specific gene(s) or mutation(s) targeted by selection. This shortcoming is hopefully bound to disappear in the near future, thanks to the wealth of new genomic resources that are currently being developed for many species. In this article, we provide a foretaste of this exciting new era by conducting a genome scan in the mosquito Aedes aegypti with the aim to look for candidate genes involved in resistance to Bacillus thuringiensis subsp. israelensis (Bti) insecticidal toxins.
The genome of a Bti-resistant and a Bti-susceptible strains was surveyed using about 500 MITE-based molecular markers, and the loci showing the highest inter-strain genetic differentiation were sequenced and mapped on the Aedes aegypti genome sequence. Several good candidate genes for Bti-resistance were identified in the vicinity of these highly differentiated markers. Two of them, coding for a cadherin and a leucine aminopeptidase, were further examined at the sequence and gene expression levels. In the resistant strain, the cadherin gene displayed patterns of nucleotide polymorphisms consistent with the action of positive selection (e.g. an excess of high compared to intermediate frequency mutations), as well as a significant under-expression compared to the susceptible strain.
Both sequence and gene expression analyses agree to suggest a role for positive selection in the evolution of this cadherin gene in the resistant strain. However, it is unlikely that resistance to Bti is conferred by this gene alone, and further investigation will be needed to characterize other genes significantly associated with Bti resistance in Ae. aegypti. Beyond these results, this article illustrates how genome scans can build on the body of new genomic information (here, full genome sequence and MITE characterization) to finally hold their promises and help pinpoint candidate genes for adaptation and speciation.
基因组扫描正成为研究适应和物种形成遗传基础的一种越来越受欢迎的方法，但仅凭自身，它们往往无法识别选择针对的特定基因或突变。由于目前正在为许多物种开发大量新的基因组资源，这种缺点有望在不久的将来消失。在本文中，我们通过对埃及伊蚊进行基因组扫描，为这个令人兴奋的新时代提供了一个先睹为快的机会，目的是寻找参与抵抗苏云金芽孢杆菌以色列亚种（Bti）杀虫毒素的候选基因。
使用约 500 个基于转座子的分子标记对 Bti 抗性和 Bti 敏感菌株的基因组进行了调查，并对显示最高菌株间遗传分化的基因座进行测序并映射到埃及伊蚊基因组序列上。在这些高度分化的标记附近，确定了几个 Bti 抗性的候选基因。其中两个基因，一个编码钙粘蛋白，另一个编码亮氨酸氨肽酶，在序列和基因表达水平上进行了进一步研究。在抗性菌株中，钙粘蛋白基因显示出与正选择作用一致的核苷酸多态性模式（例如，高频突变相对于中频突变的过剩），以及与敏感菌株相比显著的低表达。
序列和基因表达分析都表明，正选择在该钙粘蛋白基因在抗性菌株中的进化中起作用。然而，Bti 抗性不太可能仅由该基因赋予，需要进一步研究以鉴定与埃及伊蚊 Bti 抗性显著相关的其他基因。除了这些结果，本文还说明了基因组扫描如何利用新的基因组信息（在这里是全基因组序列和转座子特征）来实现其承诺，并有助于确定适应和物种形成的候选基因。
