Damian-Serrano A, Hughes M, Sutherland K R
University of Oregon, Department of Biology, Institute of Ecology and Evolution. 473 Onyx Bridge, 5289 University of Oregon, Eugene OR 97403-5289, USA.
P.O. Box 4979, Kailua Kona HI 96745, USA.
Integr Org Biol. 2023 Sep 27;5(1):obad037. doi: 10.1093/iob/obad037. eCollection 2023.
Salps are marine pelagic tunicates with a complex life cycle, including a solitary and colonial stage composed of asexually budded individuals. These colonies develop into species-specific architectures with distinct zooid orientations, including transversal, oblique, linear, helical, and bipinnate chains, as well as whorls and clusters. The evolutionary history of salp colony architecture has remained obscured due to the lack of an ontology to characterize architectures, as well as a lack of phylogenetic taxon sampling and resolution of critical nodes. We (1) collected and sequenced eight species of salps that had never been sequenced before, (2) inferred the phylogenetic relationships among salps, and (3) reconstructed the evolutionary history of salp colony architecture. We collected salp specimens via offshore SCUBA diving, dissected tissue samples, extracted their DNA, amplified their 18S gene, and sequenced them using Sanger technology. We inferred the phylogeny of Salpida based on 18S using both Maximum Likelihood and Bayesian approaches. Using this phylogeny, we reconstructed the ancestral states of colony architecture using a Bayesian ordered Markov model informed by the presence and absence of specific developmental mechanisms that lead to each architecture. We find that the ancestral salp architecture is either oblique or linear, with every other state being derived. Moreover, linear chains have evolved independently at least three times. While transversal chains are developmentally basal and hypothesized to be ancestral, our phylogenetic topology and reconstructions strongly indicate that they are evolutionarily derived through the loss of zooid torsion. These traits are likely critical to multijet locomotory performance and evolving under natural selection. Our work showcases the need to study the broader diversity of salp species to gain a comprehensive understanding of their organismal biology, evolutionary history, and ecological roles in pelagic ecosystems.
樽海鞘是具有复杂生命周期的海洋浮游被囊动物，其生命周期包括由无性出芽个体组成的单体和群体阶段。这些群体发育成具有特定物种结构的形态，其游动孢子的方向各不相同，包括横向、倾斜、线性、螺旋状和双羽状链，以及螺旋和簇状。由于缺乏用于描述结构的本体论，以及缺乏系统发育分类群抽样和关键节点的分辨率，樽海鞘群体结构的进化历史一直模糊不清。我们（1）收集并测序了8种以前从未测序过的樽海鞘物种，（2）推断了樽海鞘之间的系统发育关系，（3）重建了樽海鞘群体结构的进化历史。我们通过近海水肺潜水收集樽海鞘标本，解剖组织样本，提取其DNA，扩增其18S基因，并使用桑格技术进行测序。我们使用最大似然法和贝叶斯法基于18S推断了海樽纲的系统发育。利用这个系统发育关系，我们使用贝叶斯有序马尔可夫模型重建了群体结构的祖先状态，该模型由导致每种结构的特定发育机制的存在与否提供信息。我们发现，祖先的樽海鞘结构要么是倾斜的，要么是线性的，其他所有状态都是衍生的。此外，线性链至少独立进化了三次。虽然横向链在发育上是基础的，并且被假设为祖先状态，但我们的系统发育拓扑结构和重建结果强烈表明，它们是通过游动孢子扭转的丧失而在进化上衍生出来的。这些特征可能对多喷射运动性能至关重要，并在自然选择下进化。我们的工作表明，需要研究更广泛的樽海鞘物种多样性，以全面了解它们的生物体生物学、进化历史以及在浮游生态系统中的生态作用。
