McFadden C S, Donahue R, Hadland B K, Weston R
Department of Biology, Harvey Mudd College, Claremont, California 91711, USA.
Evolution. 2001 Jan;55(1):54-67. doi: 10.1111/j.0014-3820.2001.tb01272.x.
The soft coral genus Alcyonium is among the most reproductively diverse invertebrate taxa known: The genus includes species that vary both in mode of reproduction (including broadcast spawners, internal brooders, and external brooders) and sexual expression (gonochores, hermaphrodites, and a unisexual parthenogen). Such diversity offers a unique opportunity to examine associations between reproductive and morphological traits in a phylogenetic context. We used an approximately 900-bp sequence of the nuclear ribosomal gene complex spanning the internal transcribed spacer (ITS) regions to construct a molecular phylogeny for 14 European and North American species of Alcyonium onto which we mapped the known distribution of reproductive and morphological traits. The phylogeny suggests that hermaphroditism or parthenogenesis has evolved independently at least twice in this genus, and always in internally brooding species. Broadcast spawning and external brooding only occur in species with large colony size, whereas all species with small colony size brood their larvae internally. Internal brooding and small size appear to be ancestral in this genus; if this is the case, an association between broadcast spawning and large colony size has evolved independently in at least two clades. This tendency of small adults to brood their larvae while large adults broadcast spawn them into the plankton has been observed in a variety of solitary invertebrate taxa, but to date has not been documented in any other colonial invertebrates. Moreoever, it has been suggested that organisms with a colonial growth form should not experience the allometric constraints on brood space that have been proposed to explain the association between adult size and mode of reproduction in solitary organisms. Unlike many other colonial groups, however, module (polyp) size is strongly correlated with colony size in Alcyonium, and constraints on brooding may be imposed by module, rather than colony, allometry. The very close genetic relationship (< 1% sequence divergence) and shared polymorphisms among A. digitatum (a large, gonochoric broadcast spawner), A. siderium, and A. sp. A (intermediate-sized and small hermaphroditic, internal brooders) suggest that evolutionary transitions between broadcast spawning and brooding and between gonochorism and hermaphroditism can occur easily and rapidly in this group.
软珊瑚属（Alcyonium）是已知的繁殖方式最为多样的无脊椎动物类群之一：该属包括繁殖方式（包括体外产卵者、体内育幼者和体外育幼者）和性表达（雌雄异体、雌雄同体和单性孤雌生殖）都有所不同的物种。这种多样性为在系统发育背景下研究繁殖性状与形态性状之间的关联提供了独特的机会。我们使用了一段跨越核糖体基因复合体内部转录间隔区（ITS）区域的约900个碱基对的序列，为14种欧洲和北美的软珊瑚属物种构建了分子系统发育树，并在其上标注了已知的繁殖和形态性状分布。系统发育树表明，雌雄同体或孤雌生殖在该属中至少独立进化了两次，并且总是出现在体内育幼的物种中。体外产卵和体外育幼仅发生在群体规模较大的物种中，而所有群体规模较小的物种都在体内育幼其幼虫。体内育幼和小体型似乎是该属的祖先特征；如果是这样，那么体外产卵与大群体规模之间的关联在至少两个进化枝中是独立进化的。在各种独居无脊椎动物类群中都观察到了小体型成年个体在体内育幼其幼虫，而大体型成年个体将卵产入浮游生物中的这种趋势，但迄今为止在任何其他群体无脊椎动物中都没有记录。此外，有人提出具有群体生长形式的生物体不应受到为解释独居生物体成年体型与繁殖方式之间的关联而提出的育幼空间的异速生长限制。然而，与许多其他群体不同，软珊瑚属中模块（珊瑚虫）大小与群体大小密切相关，并且育幼的限制可能是由模块而非群体的异速生长造成的。指状软珊瑚（A. digitatum，一种大型、雌雄异体的体外产卵者）、铁色软珊瑚（A. siderium）和A. sp. A（中型和小型雌雄同体的体内育幼者）之间非常近的遗传关系（序列差异<1%）和共享多态性表明，在这个类群中，体外产卵与育幼之间以及雌雄异体与雌雄同体之间的进化转变可以很容易且迅速地发生。
