The African Great Lakes have long been recognized as an excellent location to study speciation. Most famously, cichlid fishes have radiated in Lake Tanganyika and subsequently spread into Lake Malawi and Lake Victoria, where they again radiated. Other taxa have diversified in these lakes, such as catfish, ostracods, gastropods, and Monegenean gill parasites of cichlids. However, these radiations have received less attention, and the process leading to their speciation in this unique region remains to be further explored. Here I present evidence that suggests a radiation of Macrostomum flatworms has occurred in the African Great Lakes region, offering a good opportunity for such investigations. Recent field work has revealed a monophyletic clade of 16 Macrostomum flatworms that have, to date, only been collected from Lake Tanganyika. Additionally, a species collected from Lake Malawi was found nested within this clade. Molecular phylogenetic analysis, largely based on transcriptome data, suggests that this clade underwent rapid speciation, possibly due to a large habitat diversity in the lake. I also observed significant differences in the sperm morphology of these flatworms compared to those of species found outside Lake Tanganyika and Lake Malawi. These included the elongation of an anterior structure, a reduction in the size of the lateral sperm bristles, and changes in relative proportions. I propose functional hypotheses for these changes in sperm design, and formally describe Macrostomum gracilistylum sp. nov from Lake Malawi and its sister species Macrostomum crassum sp. nov., Macrostomum pellitum sp. nov., Macrostomum longispermatum sp. nov., and Macrostomum schäreri sp. nov., from Lake Tanganyika. The available evidence is consistent with the hypothesis that Macrostomum flatworms have radiated in Lake Tanganyika and subsequently spread to Lake Malawi. However, whether this represents a bona fide adaptive radiation still needs to be determined. Therefore, the African Great Lakes are promising targets for further research into flatworm diversity and speciation.

中文翻译：

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支持非洲大湖地区自由生活的扁虫的辐射以及五种新的大口虫物种的描述


非洲五大湖长期以来一直被认为是研究物种形成的绝佳地点。最著名的是慈鲷鱼在坦噶尼喀湖辐射，随后扩散到马拉维湖和维多利亚湖，并在那里再次辐射。这些湖泊中的其他类群已经多样化，例如鲶鱼、介形类、腹足类和慈鲷的莫内吉尼亚鳃寄生虫。然而，这些辐射受到的关注较少，导致它们在这个独特区域形成物种的过程仍有待进一步探索。在这里，我提供的证据表明非洲大湖地区已经发生了大口扁虫的辐射，为此类调查提供了良好的机会。最近的实地工作揭示了 16 种巨口扁虫的单系进化枝，迄今为止，这些扁虫仅从坦噶尼喀湖采集到。此外，还发现从马拉维湖采集的一个物种嵌套在这个分支中。主要基于转录组数据的分子系统发育分析表明，该进化枝经历了快速的物种形成，可能是由于湖中栖息地的多样性。我还观察到这些扁虫的精子形态与坦噶尼喀湖和马拉维湖以外发现的物种相比存在显着差异。这些包括前部结构的伸长、侧面精子刚毛尺寸的减小以及相对比例的变化。我针对精子设计的这些变化提出了功能假设，并正式描述了 Macrostomum gracilistylum sp。来自马拉维湖的 11 月及其姐妹物种 Macrostomum crassum sp.十一月，Macrostomum pellitum sp。十一月，Macrostomum longispermatum sp。 11月，和Macrostomum schäreri sp。十一月，来自坦噶尼喀湖。 现有证据与大口扁虫在坦噶尼喀湖辐射并随后传播到马拉维湖的假设相一致。然而，这是否代表真正的自适应辐射仍需要确定。因此，非洲五大湖是进一步研究扁虫多样性和物种形成的有希望的目标。