Zygoparity in Characidae - the first case of internal fertilization in the teleost cohort Otomorpha

Abstract Most teleosts are externally fertilizing, with internal fertilization occurring as a relatively rare event. Until now, Euteleosteomorpha is the only teleost cohort known to undergo internal fertilization. In the teleost cohort Otomorpha, it has been recorded the presence of sperm in the ovaries of some species of Characiformes and Siluriformes, but no fertilized eggs have been found so far in the female reproductive tract. It has been presumed that oocytes can be released into the water with associated spermatozoa and only there becomes fertilized, and the term insemination has been used to characterize the strategy adopted by these fish. Here, we present the discovery of the first case of internal fertilization in the teleost cohort Otomorpha, in Compsura heterura (Characiformes: Characidae). In the course of spawning, the eggs form the perivitelline space and the animal and vegetative poles within the ovaries, evidencing oocyte fertilization. The newly spawned eggs then continue to form the animal and vegetative poles and increase the perivitelline space. These eggs are in the zygotic stage. These data indicate that fertilized eggs are only retained for a short period, providing evidence that C. heterura is a zygoparous fish.

摘要 绝大多数真骨鱼类（teleosts）行体外受精，体内受精仅为相对罕见的演化事件。迄今为止，新真骨鱼总群（Euteleosteomorpha）是已知唯一被报道可发生体内受精的真骨鱼类支系。在耳形总目（Otomorpha）的真骨鱼类中，已有记录显示部分脂鲤目（Characiformes）和鲶形目（Siluriformes）物种的卵巢内存在精子，但迄今尚未在雌性生殖道内发现受精卵。此前学界推测，卵母细胞需被排放至水体中并与精子结合方能完成受精，学界以“授精（insemination）”一词指代这类鱼类所采用的繁殖策略。本研究首次在耳形总目脂鲤目（Characiformes）脂鲤科（Characidae）的异饰脂鲤（Compsura heterura）中发现了体内受精的案例。在产卵过程中，卵母细胞于卵巢内形成围卵腔、动物极与植物极，该现象可证明卵母细胞已完成受精。随后新产出的卵会继续形成动物极与植物极，并进一步扩大围卵腔，此时这些卵处于合子阶段。本研究数据表明，受精卵仅在雌性体内短暂存留，这证明异饰脂鲤为合子滞留型鱼类。