Population genetic data of a model symbiotic cnidarian system reveal remarkable symbiotic specificity and vectored introductions across ocean basins

The Aiptasia-Symbiodinium symbiosis is a promising model for experimental studies of cnidarian-dinoflagellate associations, yet relatively little is known regarding the genetic diversity of either symbiotic partner. To address this we collected Aiptasia from 17 localities throughout the world and examined the genetic diversity of both anemones and their endosymbionts. Based on newly-developed SCAR markers, Aiptasia consisted of two genetically-distinct populations, one Aiptasia lineage from Florida and a second network of Aiptasia genotypes found at other localities. These populations did not conform to the distributions of described Aiptasia species, suggesting that taxonomic re-evaluation is needed in light of molecular genetics. Associations with Symbiodinium further demonstrated the distinctions among Aiptasia populations. According to 18S-RFLP, ITS2-DGGE, and microsatellite flanker region sequencing, Florida anemones engaged in diverse symbioses predominantly with members of Symbiodinium Clades A and B, but also C, whereas anemones from elsewhere harboured only S. minutum within Clade B. Symbiodinium minutum apparently does not form a stable symbiosis with other hosts, which implies a highly-specific symbiosis. Fine-scale differences among S. minutum populations were quantified using six microsatellite loci. Populations of S. minutum had low genotypic diversity and high clonality (R=0.14). Furthermore, minimal population structure was observed among regions and ocean basins, due to allele and genotype sharing. The lack of genetic structure and low genotypic diversity suggest recent vectoring of Aiptasia and S. minutum across localities. This first ever molecular-genetic study of a globally-distributed cnidarian and its Symbiodinium assemblages reveals host-symbiont specificity and widely-distributed populations in an important model system.

海葵（Aiptasia）-虫黄藻（Symbiodinium）共生体系是研究刺胞动物-双鞭毛虫共生关系的极具应用前景的实验模型，然而目前学界对二者任一共生伙伴的遗传多样性仍缺乏深入认知。为厘清这一认知空白，本研究从全球17个采样点位采集了海葵样本，并系统分析了宿主海葵及其内共生体的遗传多样性特征。依托新开发的序列特征扩增区域标记（SCAR markers），研究结果显示海葵可划分为两个遗传分化显著的类群：其一为分布于佛罗里达的海葵专属谱系，其余采样点的海葵基因型则共同构成另一类群网络。这两类群的分布格局与已正式描述的海葵物种分布范围并不匹配，提示需结合分子遗传学证据对海葵的分类学地位进行重新审视。虫黄藻共生关系的分析进一步印证了不同海葵类群间的遗传分化。通过18S核糖体RNA基因限制性片段长度多态性（18S-RFLP）、ITS2变性梯度凝胶电泳（ITS2-DGGE）以及微卫星侧翼区测序三种技术手段，研究发现佛罗里达的海葵可形成多样化的共生组合，其共生伙伴主要隶属于虫黄藻Clade A与Clade B类群，同时也可与Clade C类群建立共生；而其余地区的海葵则仅与Clade B类群中的微小虫黄藻（Symbiodinium minutum，S. minutum）形成共生。研究发现微小虫黄藻无法与其他宿主建立稳定的共生关系，这表明海葵与该虫黄藻类群之间存在高度特异性的共生关联。研究借助6个微卫星位点对微小虫黄藻种群的精细遗传差异进行了量化分析，结果显示，微小虫黄藻种群的基因型多样性较低，且呈现显著的克隆繁殖特征（克隆多样性指数R=0.14）。此外，由于等位基因与基因型的跨区域共享，在不同采样区域乃至大洋盆地间仅检测到极微弱的种群遗传结构。种群遗传结构的缺失与较低的基因型多样性，提示海葵与微小虫黄藻近期曾通过媒介被广泛传播至全球各地。本研究作为全球范围内首个针对广布刺胞动物及其虫黄藻群落的分子遗传学研究，揭示了这一重要模型体系中宿主-共生体的特异性关联以及种群的广泛分布特征。