Phenotypic variation within a species is often structured geographically in clines. In Drosophila americana , a longitudinal cline for body colour exists within North America that appears to be due to local adaptation. The tan and ebony genes have been hypothesized to contribute to this cline, with alleles of both genes that lighten body colour found in D. americana . These alleles are similar in sequence and function to the allele fixed in D. americana's more lightly pigmented sister species, Drosophila novamexicana . Here, we examine the frequency and geographic distribution of these D. novamexicana‐like alleles in D. americana . Among alleles from over 100 strains of D. americana isolated from 21 geographic locations, we failed to identify additional alleles of tan or ebony with as much sequence similarity to D. novamexicana as the D. novamexicana‐like alleles previously described. However, using genetic analysis of 51 D. americana strains derived from 20 geographic locations, we identified one new allele of ebony and one new allele of tan segregating in D. americana that are functionally equivalent to the D. novamexicana allele. An additional 5 alleles of tan also showed marginal evidence of functional similarity. Given the rarity of these alleles, however, we conclude that they are unlikely to be driving the pigmentation cline. Indeed, phenotypic distributions of the 51 backcross populations analysed indicate a more complex genetic architecture, with diversity in the number and effects of loci altering pigmentation observed both within and among populations of D. americana . This genetic heterogeneity poses a challenge to association studies and genomic scans for clinal variation, but might be common in natural populations.

中文翻译：

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美洲果蝇体色系的遗传结构。


一个物种内的表型变异通常是在地理上按谱系结构的。在美洲果蝇中，北美境内存在体色的纵向变化，这似乎是由于当地的适应。据推测，棕褐色和乌木基因对这种细胞系有贡献，在美洲果蝇中发现了这两个基因的等位基因，使体色变浅。这些等位基因在序列和功能上与美洲果蝇颜色较浅的姐妹物种新墨西哥果蝇中固定的等位基因相似。在这里，我们检查了这些D. novamexicana样等位基因在美洲 D. americana中的频率和地理分布。在从 21 个地理位置分离的 100 多个美洲龙胆品系的等位基因中，我们未能鉴定出与先前描述的D. novamexicana类似等位基因与D. novamexicana具有如此多的序列相似性的棕褐色或乌木等位基因。然而，通过对来自 20 个地理位置的 51 个美洲石斛菌株进行遗传分析，我们鉴定了在美洲石斛中分离的一种新的乌木等位基因和一种新的棕褐色等位基因，它们在功能上与D.novamexicana等位基因相同。另外 5 个tan等位基因也显示出功能相似性的边缘证据。然而，鉴于这些等位基因的稀有性，我们得出的结论是它们不太可能驱动色素沉着倾向。 事实上，所分析的 51 个回交群体的表型分布表明了更复杂的遗传结构，在美洲果蝇群体内部和群体之间观察到改变色素沉着的基因座的数量和效应具有多样性。这种遗传异质性对关联研究和临床变异的基因组扫描提出了挑战，但在自然群体中可能很常见。