Similar genetic routes are independently targeted for mimetic color convergence in bumble bees

In the bumble bee subgenus Pyrobombus, a largely North American lineage, several major groups mimic each other across Western American mimicry complexes, with black forms attained in Pacific coastal regions and red forms attained in the Rocky mountain distributions of these taxa. These species have intervening taxa in the phylogeny that have different patterns and thus could involve different loci or sorting of ancestral variation. We perform genome sequencing and a genome wide association analysis in males of both color forms in B. Vancouverensis to identify gene elements initially driving the variation and enable comparison in loci involved in the same polymorphisms in co-mimicking species. This species presents additional challenges to comimics such as B. melanopygus as the color is continuous between the two forms, suggesting regulation may be multigenic. We also examine the genes driving differential coloration across three sets of comimicking pairs (B. vancouverensis red and black form, B. melanopygus red and black form, sister species pair B. huntii (red) and B. vosnesenskii (black)) using RNA-seq differential gene expression analysis across developmental stages spanning the start (P15 pupation), middle (0hr of eclosion), and late stages (12 hr after eclosion) of coloration. With this system, we examine how gene expression of red/black epidermal segments shifts across the lineage, to determine (1) which genes are changing to generate these phenotypes, (2) whether the same genes or different genes of the same gene networks are implicated in repeated phenotypes, and (3) how expression and roles of core regulatory genes shift across the clade. Two sister group lineages that are black form were also analyzed for their transcriptomes: B. impatiens (P15, 0hr, 12hr) and B. bimaculatus (P15, 0hr), to assess evolution of gene expression patterns.

作为主要分布于北美的演化支，火熊蜂亚属（Pyrobombus）内的多个主要类群在北美西部的拟态复合体中彼此拟态：该类群的黑色型分布于太平洋沿岸区域，红色型则分布于这些类群的落基山脉分布区。这些类群的系统发育树中存在居间类群，其体色模式存在差异，因此可能涉及不同的基因座或祖先变异的分选过程。本研究对温哥华熊蜂（Bombus vancouverensis）两种体色型的雄性个体开展基因组测序与全基因组关联分析，以鉴定最初驱动体色变异的基因元件，并为比较共拟态物种中参与同一多态性的基因座提供参照。与暗足熊蜂（Bombus melanopygus）这类共拟态类群相比，温哥华熊蜂存在额外的研究挑战：其两种体色型之间的体色呈现连续渐变特征，提示其体色调控可能为多基因调控。本研究还针对三组共拟态类群对开展基于RNA-seq的差异基因表达分析，以解析驱动体色差异的基因：分别为温哥华熊蜂的红色型与黑色型、暗足熊蜂的红色型与黑色型，以及姐妹类群对亨氏熊蜂（Bombus huntii，红色型）与弗氏熊蜂（Bombus vosnesenskii，黑色型）；分析涵盖体色形成的三个发育阶段：起始阶段（P15蛹期）、中间阶段（羽化后0小时）以及晚期阶段（羽化后12小时）。依托该研究体系，我们将探究红色与黑色表皮区段的基因表达在该演化支内的变化模式，以明确三个核心科学问题：(1) 哪些基因的表达变化产生了上述体色表型；(2) 重复出现的体色表型是由同一类基因还是同一基因网络内的不同基因所介导；(3) 核心调控基因的表达模式与功能在该演化支内如何演化。本研究还对两种黑色型的姐妹群类群开展转录组分析：即明亮熊蜂（Bombus impatiens，涵盖P15蛹期、羽化后0小时、羽化后12小时三个阶段）与双斑熊蜂（Bombus bimaculatus，涵盖P15蛹期、羽化后0小时两个阶段），以评估体色相关基因表达模式的演化历程。