Center for Population Biology, University of California, Davis, 1 Shields Avenue, Davis, CA, 95616, USA.
California Academy of Sciences, 55 Music Concourse Drive, San Francisco, CA, 94118, USA.
BMC Genomics. 2018 May 18;19(1):368. doi: 10.1186/s12864-018-4768-9.
Larval developmental patterns are extremely varied both between and within phyla, however the genetic mechanisms leading to this diversification are poorly understood. We assembled and compared the developmental transcriptomes for two sea biscuit species (Echinodermata: Echinoidea) with differing patterns of larval development, to provide a resource for investigating the evolution of alternate life cycles. One species (Clypeaster subdepressus) develops via an obligately feeding larva which metamorphoses 3-4 weeks after fertilization; the other (Clypeaster rosaceus) develops via a rare, intermediate larval type-facultative feeding- and can develop through metamorphosis entirely based on egg provisioning in under one week.
Overall, the two transcriptomes are highly similar, containing largely orthologous contigs with similar functional annotation. However, we found distinct differences in gene expression patterns between the two species. Larvae from C. rosaceus, the facultative planktotroph, turned genes on at earlier stages and had less differentiation in gene expression between larval stages, whereas, C. subdepressus showed a higher degree of stage-specific gene expression.
This study is the first genetic analysis of a species with facultatively feeding larvae. Our results are consistent with known developmental differences between the larval types and raise the question of whether earlier onset of developmental genes is a key step in the evolution of a reduced larval period. By publishing a transcriptome for this rare, intermediate, larval type, this study adds developmental breadth to the current genetic resources, which will provide a valuable tool for future research on echinoderm development as well as studies on the evolution of development in general.
幼虫的发育模式在门和门之间以及门内都非常多样化，但是导致这种多样化的遗传机制还知之甚少。我们组装并比较了两种海胆（棘皮动物：海胆纲）的发育转录组，它们具有不同的幼虫发育模式，为研究替代生命周期的进化提供了资源。一种物种（Clypeaster subdepressus）通过必需摄食的幼虫发育，受精后 3-4 周变态；另一种（Clypeaster rosaceus）通过罕见的中间幼虫类型-兼性摄食-发育，并可以完全通过卵供应在不到一周的时间内进行变态。
总体而言，两个转录组非常相似，包含大量具有相似功能注释的直系同源物。然而，我们发现两个物种之间的基因表达模式存在明显差异。兼性浮游幼虫的 C. rosaceus 幼虫在更早的阶段开启基因，并在幼虫阶段之间的基因表达差异较小，而 C. subdepressus 则表现出更高程度的阶段特异性基因表达。
本研究是对具有兼性摄食幼虫的物种进行的首次遗传分析。我们的结果与已知的幼虫类型之间的发育差异一致，并提出了一个问题，即发育基因的早期启动是否是缩短幼虫期进化的关键步骤。通过发布这种罕见的中间幼虫类型的转录组，本研究为当前的遗传资源增加了发育广度，这将为棘皮动物发育的未来研究以及发育进化的一般研究提供有价值的工具。
