In chordates, obligate photosynthetic symbiosis has been reported exclusively in some colonial ascidians of the family Didemnidae. The vertical transmission of the symbionts is crucial in establishing the obligate symbiosis between the cyanobacteria and the host ascidians. The results of comparative surveys on the morphological processes of cyanobacterial transmission suggest the occurrence of convergent evolution of the vertical transmission in the host species harboring symbionts in the cloacal cavity. In Trididemnum species harboring cyanobacterial cells in the tunic, the symbiont cells are transported by the tunic cells to the tunic of embryos brooded in the tunic of the parent colony. The present study examined whether the mode of symbiont transmission is the same in host species harboring the symbionts in the tunic, regardless of host genera, or whether non‐Trididemnum hosts have a different vertical transmission mode. Our results showed that the vertical transmission process in Lissoclinum midui was almost the same as in the Trididemnum species, supporting the occurrence of convergent evolution in the two distinct didemnid genera, that is, Trididemnum and Lissoclinum. High plasticity of the embryogenic process in didemnid ascidians may be important in developing the mechanism of vertical transmission; this assumption may also explain why the obligate cyanobacterial symbiosis has been exclusively established in didemnid ascidians among chordates.

中文翻译：

---

蓝藻专性共生菌Prochloron在殖民地海鞘中的垂直传播模式的收敛演化

在脊索动物中，仅在双翅目科的一些殖民地海生动物中发现专性光合共生。共生体的垂直传播对于在蓝细菌和宿主海鞘之间建立专性共生至关重要。对蓝细菌传播的形态过程进行的比较调查结果表明，在泄殖腔中存在共生菌的宿主物种中，垂直传播发生了趋同演化。在Trididemnum中在外膜中带有蓝细菌细胞的物种中，共生细胞被外膜细胞转运到在亲代外壁中孵出的胚的外壁。本研究研究了共生体传播方式在容纳中山药共生体的寄主物种中是否相同，而与寄主属无关，或者非三角Tri寄主是否具有不同的垂直传播方式。我们的研究结果表明，在垂直传输过程Lissoclinum米堆是几乎相同的Trididemnum物种，支撑在两个不同的didemnid属，即，趋同进化的发生Trididemnum和Lissoclinum。双孢子虫海藻的胚发生过程的高可塑性可能对发展垂直传播的机制很重要。这个假设也可以解释为什么专性蓝藻共生是在脊索动物的双足类海鞘中唯一建立的。