Genetic diversity in the <i>Diplosoma listerianum</i> complex (Ascidiacea: Didemnidae) from the Western Atlantic

(Received 19 January 2021; revised 27 September 2021; accepted 27 September 2021) Diplosoma listerianum has been re-described with more than 40 currently invalid names, but recent data suggests that some of those names may be valid. At least four distinct clades were recently revealed by molecular analysis using mitochondrial DNA (mtDNA). Yet, very few samples were analyzed from the western Atlantic, of which none were from the extensive Brazilian coast. Here, we analyze genetic diversity in D. listerianum in light of morphological data. We sequenced 55 samples from Brazil, Panama, Mexico, and the United States and analyzed them together with published data, to test whether local populations are part of widespread, known, clades or are native species hidden within this complex. Phylogenetic analysis found eight clades, three of which were different from the ones recovered before. Former clade A was split into two larger clades: clade A1 occurs in the western Atlantic, on the Pacific side of Panama and in South Africa; clade A2 occurs worldwide, with representatives found on every continent. Species delimitation analyses (ABGD, bPTP and GMYC) found 16 MOTUs, which suggests that those large clades are very diverse and may comprise more than one species. Diversity was greatest in Panama (6 clades) and Mexico (3 clades). Larval morphological characters were examined and described here. However, for only a few of the sequenced samples could fully grown larvae be obtained and phenotypic variation was large. Thus, larval morphology could not be used to diagnose molecular clades. Diplosoma listerianum remains a species complex, with evidence of ongoing speciation, and high genetic diversity, especially in the tropical western Atlantic. Fast-growing colonies, rapid life cycles, and amenability to cultivation in the laboratory indicate that this species can be a model for studies in speciation.(2021年1月19日收稿；2021年9月27日修回；2021年9月27日接受) 利斯尔双列海鞘（Diplosoma listerianum）曾基于40余个当前已无效的分类名称被重新描述，但最新研究数据显示其中部分名称或许具备有效性。近期基于线粒体DNA（mtDNA）的分子分析已揭示至少4个独立演化支。然而，西大西洋区域的分析样本极为匮乏，且未包含分布广泛的巴西海岸相关样本。本研究结合形态学数据，对利斯尔双列海鞘的遗传多样性展开分析：我们对采集自巴西、巴拿马、墨西哥及美国的55份样本进行了测序，并结合已发表的公开数据开展联合分析，以验证当地种群是否属于已被广泛认知的演化支，抑或是隐藏于该复合类群中的本土物种。系统发育分析共识别出8个演化支，其中3个为此前未被报道的新分支。原演化支A被划分为两个更大的分支：演化支A1分布于西大西洋、巴拿马太平洋沿岸及南非区域；演化支A2则为全球性分布，各大洲均有其代表类群。物种界定分析（ABGD、bPTP及GMYC）共识别出16个分子操作分类单元（MOTU），表明上述大型演化支具有极高的多样性，可能包含多个未被定义的物种。遗传多样性以巴拿马（6个演化支）和墨西哥（3个演化支）最为突出。本研究对幼体形态特征进行了检视与描述，但仅能从少数测序样本中获取完全发育的幼体，且表型变异幅度较大，因此无法通过幼体形态学特征对分子演化支进行区分界定。利斯尔双列海鞘仍属于一个物种复合类群，存在持续的物种形成过程，且具有极高的遗传多样性，尤其在热带西大西洋区域。该类群的群体生长迅速、生命周期短，且易于在实验室中培养，表明其可作为物种形成研究的模式生物。