ABSTRACT Rotifers that engage in cyclical parthenogenesis produce two types of eggs: subitaneous eggs that hatch as clonal females and meiotic eggs that hatch as haploid males, or if fertilized, as females after a period of diapause (resting eggs). The ultrastructure of resting eggshells is known for some motile species, but there are limited data on subitaneous eggshells, and no data on any eggshells of sessile rotifers. Here, we investigated the ultrastructure of the subitaneous eggshell of the sessile rotifer Stephanoceros millsii and its potential origins of secretion, the maternal vitellarium and embryonic integument. We also explored secretory activity in the larval and adult integuments to determine whether activity changes during ontogeny. The eggshell consists of a single layer with two sublayers: an external granular sublayer apparently derived from the maternal vitellarium, and an internal flocculent sublayer secreted by the embryonic integument that may form a hatching membrane or glycocalyx. Secretory activity remains high in both the larva and adult and appears to be the source of the thickening glycocalyx. Altogether, the subitaneous eggshell of S. millsii is the thinnest among monogonont rotifers. Thin eggshells may have evolved in response to the added protection provided by the mother’s extracorporeal tube.

中文翻译：

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无柄轮虫 Stephanoceros millsii (Monogononta) 的皮下蛋壳的精细结构与个体发育过程中外皮囊泡运输的观察

摘要 参与周期性孤雌生殖的轮虫产生两种类型的卵：作为克隆雌性孵化的皮下卵和作为单倍体雄性孵化的减数分裂卵，或者如果受精，在滞育一段时间后作为雌性（休眠卵）孵化。静息蛋壳的超微结构对于一些运动物种是已知的，但关于皮下蛋壳的数据有限，也没有关于无柄轮虫任何蛋壳的数据。在这里，我们研究了无柄轮虫 Stephanoceros millsii 的皮下蛋壳的超微结构及其潜在的分泌来源、母体 vitellarium 和胚胎外皮。我们还探索了幼虫和成虫体被的分泌活动，以确定活动在个体发育过程中是否发生变化。蛋壳由单层和两个子层组成：外部颗粒亚层显然来自母体卵黄体，以及由胚胎外皮分泌的内部絮状亚层，可形成孵化膜或糖萼。幼虫和成虫的分泌活性仍然很高，似乎是糖萼增厚的来源。总而言之，S. millsii 的下层蛋壳是单角轮虫中最薄的。薄蛋壳可能是由于母亲的体外管提供的额外保护而进化出来的。millsii 是单角轮虫中最瘦的。薄蛋壳可能是由于母亲的体外管提供的额外保护而进化出来的。millsii 是单角轮虫中最瘦的。薄蛋壳可能是由于母亲的体外管提供的额外保护而进化出来的。