Fahey Shireen J, Healy John M
School of Molecular and Microbial Sciences, University of Queensland, Brisbane, Qld. 4072 Australia.
J Morphol. 2003 Jul;257(1):9-21. doi: 10.1002/jmor.10086.
Comparative sperm ultrastructure within the molluscan nudibranch genus Halgerda (Discodorididae) was examined for the first time using transmission electron microscopy (TEM), based on 17 of the 35 known species. In addition, observations on two other discodorids are made to facilitate outgroup comparison with Halgerda, including one species of Discodoris (D. boholiensis) and Asteronotus cespitosus (currently accepted as the closest sister taxon to Halgerda). Comparison was also made with some genera of the Chromodorididae in view of sperm similarities. Spermatozoa of all species examined were of the complex, helical, elongate ( approximately 300-400 micro m) type characteristic of most heterobranch gastropods. These cells exhibit the following discrete regions (in anteroposterior sequence) : an acrosomal complex (composed of a rounded, membrane-bound vesicle and a column-like pedestal); a solid, helical nucleus; an elongate, helical midpiece (composed of an axoneme and associated nine coarse fibers, an enveloping mitochondrial derivative of matrix, and paracrystalline materials and glycogen helix); an annular complex; and a short glycogen piece. Of these regions, the midpiece is by far the longest, occupying over 90% of the total sperm length. Comparison with other members of the radula-bearing cryptobranch dorids reveals several sperm similarities to other genera in the clade, particularly those of other Discodorididae and also with the Chromodorididae. Comparison with previously studied genera reveals noteworthy sperm differences within the Discodorididae. The most notable differences are the internal structure of the acrosomal pedestal (long and homogeneous in Halgerda, Discodoris; short and homogeneous in Asteronotus; long and finely striated in Rostanga; oblong with angular electron-lucent striations in Jorunna) and the internal structure of the glycogen piece. The pronounced helical keels of most Halgerda and Discodoris nuclei contrast with the weakly helical nucleus of Asteronotus. Sperm features alone do not provide a means of defining the genus Halgerda or the family Discodorididae nor do they support the monophyletic status of the caryophyllidia-bearing dorids. Important sperm characters such as the acrosome, nucleus, and midpiece can often still be determined from specimens that have been initially fixed in formalin, then stored in ethanol for extended periods of time (i.e., museum material). Of all sperm features, the mitochondrial derivative of the midpiece is the most resistant to long-term fixation : the survival of acrosomal, nuclear, and axonemal components is variable, presumably a factor of prefixation autolysis, varied primary fixation times and temperatures, formalin quality, and duration of alcohol storage.
首次使用透射电子显微镜（TEM），基于已知35个物种中的17个，对软体动物裸鳃亚目海兔属（Discodorididae）内的精子超微结构进行了比较研究。此外，还对另外两种盘状海兔进行了观察，以便与海兔属进行外类群比较，其中包括一种盘状海兔（D. boholiensis）和丛生星耳海牛（目前被认为是与海兔属关系最近的姐妹分类单元）。鉴于精子的相似性，还与多彩海牛科的一些属进行了比较。所有被检查物种的精子均为大多数异鳃腹足类动物特有的复杂、螺旋状、细长（约300 - 400微米）类型。这些细胞呈现出以下离散区域（从前向后顺序）：顶体复合体（由一个圆形的、膜结合囊泡和一个柱状基座组成）；一个实心的、螺旋状的细胞核；一个细长的、螺旋状的中段（由一个轴丝和相关的九条粗纤维、一个包围基质的线粒体衍生物、以及类晶体物质和糖原螺旋组成）；一个环形复合体；以及一个短的糖原片段。在这些区域中，中段是迄今为止最长的，占精子总长度的90%以上。与其他有齿隐鳃海牛的成员进行比较，发现与该分支中的其他属有几个精子相似之处，特别是其他盘状海兔科的属，也与多彩海牛科相似。与先前研究的属进行比较，发现盘状海兔科内存在值得注意的精子差异。最显著的差异在于顶体基座的内部结构（海兔属、盘状海兔属中长且均匀；星耳海牛属中短且均匀；玫瑰海牛属中长且有精细条纹；乔伦海牛属中呈椭圆形且有角状电子透明条纹）以及糖原片段的内部结构。大多数海兔属和盘状海兔属细胞核明显的螺旋状龙骨与星耳海牛属微弱螺旋状的细胞核形成对比。仅精子特征并不能提供定义海兔属或盘状海兔科的方法，也不支持有叶鳃海牛的单系地位。重要的精子特征，如顶体、细胞核和中段，通常仍可从最初用福尔马林固定、然后在乙醇中长时间保存的标本（即博物馆标本）中确定。在所有精子特征中，中段的线粒体衍生物对长期固定最具抵抗力：顶体、细胞核和轴丝成分的存活情况各不相同，这可能是由于固定前的自溶、不同的初次固定时间和温度、福尔马林质量以及酒精保存时间等因素造成的。
