Kewalo Marine Laboratory, Pacific Biosciences Research Center, University of Hawaii, 41 Ahui Street, Honolulu, HI 96813, USA.
Evodevo. 2011 Jun 6;2(1):13. doi: 10.1186/2041-9139-2-13.
Bryozoa is a clade of aquatic protostomes. The bryozoan life cycle typically comprises a larval stage, which metamorphoses into a sessile adult that proliferates by asexual budding to form colonies. The homology of bryozoan larvae with other protostome larvae is enigmatic. Bryozoan larvae exhibit blastemic tissues that contribute to build the adult during morphogenesis. However, it remains unclear if the cells of these tissues are pre-determined according to their future fate or if the cells are undifferentiated, pluripotent stem cells. Gene expression studies can help to identify molecular patterning of larval and adult tissues and enlighten the evolution of bryozoan life cycle stages.
We investigated the spatial expression of 13 developmental genes in the larval stage of the gymnolaemate bryozoan Bugula neritina. We found most genes expressed in discrete regions in larval blastemic tissues that form definitive components of the adult body plan. Only two of the 13 genes, BnTropomyosin and BnFoxAB, were exclusively expressed in larval tissues that are discarded during metamorphosis.
Our results suggest that the larval blastemas in Bugula are pre-patterned according to their future fate in the adult. The gene expression patterns indicate that some of the bryozoan blastemas can be interpreted to correspond to homologous adult tissues of other animals. This study challenges an earlier proposed view that metazoan larvae share homologous undifferentiated "set-aside cells", and instead points to an independent origin of the bryozoan larval stage with respect to other lophotrochozoans.
苔藓动物是水生原口动物的一个分支。苔藓动物的生命周期通常包括幼虫阶段，幼虫经历变态发育为固着的成年个体，然后通过无性出芽进行增殖，形成群体。苔藓动物幼虫与其他原口动物幼虫的同源性是神秘的。苔藓动物幼虫具有胚质组织，这些组织有助于在形态发生过程中构建成年个体。然而，目前尚不清楚这些组织的细胞是根据其未来命运预先确定的，还是未分化的多能干细胞。基因表达研究有助于确定幼虫和成年组织的分子模式，并阐明苔藓动物生命周期阶段的进化。
我们研究了 Gymnolaemata 苔藓动物 Bugula neritina 幼虫阶段 13 个发育基因的空间表达。我们发现大多数基因在形成成年体式图明确成分的胚质组织的离散区域中表达。在 13 个基因中，只有 BnTropomyosin 和 BnFoxAB 这两个基因仅在变态发育过程中被丢弃的幼虫组织中表达。
我们的结果表明，Bugula 的幼虫胚质是根据其在成年后的未来命运预先形成模式的。基因表达模式表明，一些苔藓动物胚质可以被解释为对应于其他动物的同源成年组织。这项研究挑战了早期提出的多细胞动物幼虫具有同源未分化“预留细胞”的观点，而是指出苔藓动物幼虫阶段相对于其他担轮动物具有独立的起源。
