Institut für Immungenetik, Charité-Universitätsmedizin Berlin, Thielallee 73, 14195 Berlin, Germany.
BMC Evol Biol. 2010 Oct 18;10:313. doi: 10.1186/1471-2148-10-313.
The digestive tract of many metazoan invertebrates is characterized by the presence of caeca or diverticula that serve secretory and/or absorptive functions. With the development of various feeding habits, distinctive digestive organs may be present in certain taxa. This also holds true for sea urchins (Echinodermata: Echinoidea), in which a highly specialized gastric caecum can be found in members of a derived subgroup, the Irregularia (cake urchins, sea biscuits, sand dollars, heart urchins, and related forms). As such a specialized caecum has not been reported from "regular" sea urchin taxa, the aim of this study was to elucidate its evolutionary origin.
Using morphological data derived from dissection, magnetic resonance imaging, and extensive literature studies, we compare the digestive tract of 168 echinoid species belonging to 51 extant families. Based on a number of characters such as topography, general morphology, mesenterial suspension, and integration into the haemal system, we homologize the gastric caecum with the more or less pronounced dilation of the anterior stomach that is observed in most "regular" sea urchin taxa. In the Irregularia, a gastric caecum can be found in all taxa except in the Laganina and Scutellina. It is also undeveloped in certain spatangoid species.
According to our findings, the sea urchin gastric caecum most likely constitutes a synapomorphy of the Euechinoidea. Its occurrence in "regular" euechinoids is linked to the presence of an additional festoon of the anterior stomach in ambulacrum III. Both structures, the additional festoon and the gastric caecum, are absent in the sister taxon to the Euechinoidea, the Cidaroida. Since the degree of specialization of the gastric caecum is most pronounced in the predominantly sediment-burrowing irregular taxa, we hypothesize that its evolution is closely linked to the development of more elaborate infaunal lifestyles. We provide a comprehensive study of the origin and evolutionary plasticity of a conspicuous digestive tract structure, the gastric caecum, in a major taxon of the extant invertebrate macrozoobenthos.
许多后生动物的消化道的特点是存在盲囊或憩室，它们具有分泌和/或吸收功能。随着各种进食习惯的发展，某些类群可能存在独特的消化器官。这同样适用于海胆（棘皮动物门：海胆纲），在衍生的亚群不规则类群（不规则海胆、海饼干、沙钱、心形海胆和相关形式）中可以发现高度特化的胃盲囊。由于这种特化的盲囊尚未在“规则”海胆类群中报道，因此本研究旨在阐明其进化起源。
使用来自解剖、磁共振成像和广泛文献研究的形态学数据，我们比较了属于 51 个现存科的 168 种海胆物种的消化道。基于一些特征，如地形、一般形态、系膜悬挂和纳入血系统，我们将胃盲囊与在大多数“规则”海胆类群中观察到的或多或少明显扩张的前胃等同起来。在不规则类群中，除了在 Laganina 和 Scutellina 中外，所有类群都可以发现胃盲囊。在某些星虫类物种中，它也没有发育。
根据我们的发现，海胆胃盲囊很可能是真海胆目的一个synapomorphy。它在“规则”真海胆中的出现与在第三步带中前胃的额外褶皱有关。这两个结构，即额外的褶皱和胃盲囊，都不存在于真海胆目的姊妹类群 Cidaroida 中。由于胃盲囊在主要以沉积物为食的不规则类群中的特化程度最高，我们假设它的进化与更复杂的穴居生活方式的发展密切相关。我们对一个显著的消化道结构——胃盲囊在现存无脊椎动物大型底栖动物的一个主要类群中的起源和进化可塑性进行了全面研究。
