Nielsen Claus
Biosystematics, The Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, Copenhagen, Denmark.
R Soc Open Sci. 2018 Jul 25;5(7):180042. doi: 10.1098/rsos.180042. eCollection 2018 Jul.
The trochophora larva, which is so well known from the marine plankton, is central to our understanding of the evolution of a large branch of the bilaterians. Two theories for this larval type have been prevalent, the trochaea theory and the theory proposed by Ivanova-Kazas. The embryology, or more precisely the cell-lineage, of these larvae seems to be central for our understanding of their origin, but important details have been missing. According to the trochaea theory, a circumblastoporal ring of blastomeres differentiates to follow the convoluted shape of the conspicuous ciliary bands of the larvae, with prototroch and metatroch around the mouth, forming a filtering system, and telotroch around the anus. According to the Ivanova-Kazas theory, the blastomeres with the ciliary bands develop through specialization of rings of cells of the general ciliation in a lecithotrophic larva. Now, a new cell-lineage study of the gastropod has shown that the ring of cells at the edge of the blastopore develops into the band of cells carrying prototroch and metatroch, characteristic of the trochophora. This gives strong support to the trochaea theory.
担轮幼虫在海洋浮游生物中广为人知，它对于我们理解两侧对称动物一个大分支的进化至关重要。关于这种幼虫类型有两种流行的理论，即担轮动物说和伊万诺娃 - 卡扎斯提出的理论。这些幼虫的胚胎学，或者更确切地说是细胞谱系，似乎是我们理解其起源的关键，但重要细节一直缺失。根据担轮动物说，卵裂球的围胚孔环分化，以追随幼虫明显的纤毛带的复杂形状，口周围有原担轮和中担轮，形成一个过滤系统，肛门周围有端担轮。根据伊万诺娃 - 卡扎斯理论，带有纤毛带的卵裂球是通过卵黄营养幼虫中一般纤毛化的细胞环特化发育而来的。现在，一项对腹足纲动物的新细胞谱系研究表明，胚孔边缘的细胞环发育成携带原担轮和中担轮的细胞带，这是担轮幼虫的特征。这为担轮动物说提供了有力支持。
