Department of Invertebrate Zoology, Faculty of Biology, Saint-Petersburg State University, St-Petersburg, Russia.
Zoological Institute RAS, St-Petersburg, Russia.
Evol Dev. 2024 Jul;26(4):e12462. doi: 10.1111/ede.12462. Epub 2023 Oct 27.
Orthonectida is an enigmatic group of animals with still uncertain phylogenetic position. Orthonectids parasitize various marine invertebrates. Their life cycle comprises a parasitic plasmodium and free-living males and females. Sexual individuals develop inside the plasmodium; after egress from the host they copulate in the external environment, and the larva, which has developed inside the female infects a new host. In a series of studied orthonectid species simplification of free-living sexual individuals can be clearly traced. The number of longitudinal and transverse muscle fibers is gradually reduced. In the nervous system, simplification is even more pronounced. The number of neurons constituting the ganglion is dramatically reduced from 200 in Rhopalura ophiocomae to 4-6 in Intoshia variabili. The peripheral nervous system undergoes gradual simplification as well. The morphological simplification is accompanied with genome reduction. However, not only genes are lost from the genome, it also undergoes compactization ensured by extreme reduction of intergenic distances, short intron sizes, and elimination of repetitive elements. The main trend in orthonectid evolution is simplification and miniaturization of free-living sexual individuals coupled with reduction and compactization of the genome.
栉水母动物门是一个神秘的动物类群，其系统发育位置仍不确定。栉水母动物门寄生在各种海洋无脊椎动物上。它们的生命周期包括寄生的质体和自由生活的雄性和雌性。有性个体在质体中发育；从宿主中逸出后，它们在外部环境中交配，在雌性体内发育的幼虫感染新宿主。在一系列研究的栉水母物种中，可以清楚地追踪到自由生活的有性个体的简化。纵向和横向肌纤维的数量逐渐减少。在神经系统中，简化更为明显。构成神经节的神经元数量从 Rhopalura ophiocomae 的 200 个急剧减少到 Intoshia variabili 的 4-6 个。外周神经系统也逐渐简化。形态简化伴随着基因组的减少。然而，基因组不仅失去了基因，而且通过极端减少基因间距离、缩短内含子大小和消除重复元件来实现紧凑化。栉水母动物门进化的主要趋势是自由生活的有性个体的简化和小型化，以及基因组的减少和紧凑化。
