Admixture and environment shape population genetic and phytochemical variation across a conifer hybrid zone

Ancestry variation in hybrid zones can reflect the causes and genetic basis of reproductive isolation and result in novel phenotypic variation with the potential for extended ecological effects. Junipers (Juniperus) are foundational tree species in many semi-arid landscapes of western North America and often hybridize in zones of secondary contact. Such hybridization can be ecologically significant in foundational tree species, due to the strong genetic control and ecological consequences of plant chemistry. We generated genetic and phytochemical data to analyze hybridization among Juniperus grandis, J. occidentalis, and J. osteosperma in western Nevada and its impact on plant chemistry. We used population genomic data (9,125 SNPs; 326 individuals; 25 populations) to quantify patterns of genetic variation across populations and species and to characterize ancestry variation in hybrids. While populations within species showed little genetic differentiation, the parental species formed distinct, monophyletic lineages with clear phenotypic and ecological differences. Hybrids occupied intermediate environments, contained ancestry from all three parents, and were mainly F1 or backcross hybrids. Phytochemical data (GC-MS; 163 terpenoid compounds) were likewise analyzed to understand the consequences of hybridization for plant chemistry. The parental species and hybrids displayed distinct phytochemical profiles, with hybrids often characterized by a combination of transgressive and intermediate chemical concentrations. Our results illustrate that geography and environment shape hybrid ancestry for a syngameon involving three Juniperus species, and that admixture generates novel phytochemical variation likely to have ecological consequences.

杂交带中的祖先成分变异，可反映生殖隔离的成因与遗传基础，同时可催生全新的表型变异，进而具备引发广泛生态效应的潜力。刺柏属（Juniperus）植物是北美西部诸多半干旱生境中的核心建群树种，且常在次生接触带发生杂交。鉴于植物化学性状受强烈的遗传调控且会带来显著生态后果，这类杂交在核心建群树种中往往具有重要的生态学意义。本研究通过获取遗传与植物化学数据，对内华达州西部的大果刺柏（Juniperus grandis）、西部刺柏（Juniperus occidentalis）以及硬叶刺柏（Juniperus osteosperma）三者间的杂交现象及其对植物化学性状的影响展开分析。本研究利用群体基因组数据（9125个单核苷酸多态性位点[Single Nucleotide Polymorphism，SNPs]、326个个体、25个种群），量化了不同种群与物种间的遗传变异模式，并对杂交个体的祖先成分变异进行了表征。尽管同一物种内的种群间遗传分化程度极低，但亲本物种形成了具有显著表型与生态差异的独立单系谱系。杂交个体所处生境处于中间类型，其基因组包含来自三个亲本的祖先成分，且主要为F1代或回交杂种。本研究同时对植物化学数据（气相色谱-质谱联用仪[Gas Chromatography-Mass Spectrometry，GC-MS]检测得到的163种萜类化合物）展开分析，以解析杂交对植物化学性状的影响。亲本物种与杂交个体具有截然不同的植物化学谱型，其中杂交个体通常兼具超亲型与中间型的化学物质浓度特征。本研究结果表明，地理与环境因素共同塑造了包含三个刺柏属物种的杂交复合群的杂种祖先成分，且遗传混合可催生全新的植物化学变异，进而可能产生相应的生态效应。