Mapping QTL contributing to variation in posterior lobe morphology between strains of Drosophila melanogaster

Closely-related, and otherwise morphologically similar insect species frequently show striking divergence in the shape and/or size of male genital structures, a phenomenon thought to be driven by sexual selection. Comparative interspecific studies can help elucidate the evolutionary forces acting on genital structures to drive this rapid differentiation. However, genetic dissection of sexual trait divergence between species is frequently hampered by the difficulty generating interspecific recombinants. Intraspecific variation can be leveraged to investigate the genetics of rapidly-evolving sexual traits, and here we carry out a genetic analysis of variation in the posterior lobe within D. melanogaster. The lobe is a male-specific process emerging from the genital arch of D. melanogaster and three closely-related species, is essential for copulation, and shows radical divergence in form across species. There is also abundant variation within species in the shape and size of the lobe, and while this variation is considerably more subtle than that seen among species, it nonetheless provides the raw material for QTL mapping. We created an advanced intercross population from a pair of phenotypically-different inbred strains, and after phenotyping and genotyping-by-sequencing the recombinants, mapped several QTL contributing to various measures of lobe morphology. The additional generations of crossing over in our mapping population led to QTL intervals that are smaller than is typical for an F2 mapping design. The intervals we map overlap with a pair of lobe QTL we previously identified in an independent mapping cross, potentially suggesting a level of shared genetic control of trait variation. Our QTL additionally implicate a suite of genes that have been shown to contribute to the development of the posterior lobe. These loci are strong candidates to harbor naturally-segregating sites contributing to phenotypic variation within D. melanogaster, and may also be those contributing to divergence in lobe morphology between species.

亲缘关系密切、形态特征整体相似的昆虫物种，其雄性生殖结构的形状和/或大小往往存在显著差异，这一现象被认为由性选择驱动。种间比较研究有助于阐明作用于生殖结构、推动其快速分化的进化动力。然而，由于难以获得种间重组体，对物种间性性状分化进行遗传解析的研究常受限于此。可借助种内变异来解析快速演化性性状的遗传基础，本研究便针对黑腹果蝇（D. melanogaster）的后叶变异开展了遗传分析。后叶是黑腹果蝇及另外3个近缘物种生殖弓上长出的雄性特异性突起，对交配行为不可或缺，且在不同物种间呈现出极为显著的形态分化。该物种内部的后叶形状与大小也存在丰富变异，尽管这类变异相较于物种间的差异要细微得多，却仍是数量性状基因座（QTL）定位的宝贵研究材料。本研究从一对表型差异显著的近交系中构建了高级互交群体，对重组体进行表型分型与测序分型后，定位到了多个与后叶形态多项检测指标相关的QTL。本研究的定位群体经过多代重组交换，使得定位得到的QTL区间相较于传统F₂定位设计的区间更为狭窄。我们本次定位得到的QTL区间，与此前在独立定位杂交实验中发现的两个后叶QTL存在重叠，这或许表明该性状变异存在一定程度的共有遗传调控机制。本次定位得到的QTL还关联到了一系列已被证实参与后叶发育的基因。这些基因座是携带自然分离位点的强力候选位点，这些位点可导致黑腹果蝇种群内的表型变异，同时也可能是造成不同物种间后叶形态分化的遗传基础。