Lophotrochozoans, embracing such taxa as brachiopods and molluscs, represent one of the three major groups of bilaterians.They have pivotal roles in understanding the origins and evolution of animal body plans because, after Ediacaran, they left an abundant and continuous fossil record of the shells, which consitute integral parts of their body plans.To gain insight into the origins and evolution of those skeletons, it would be essential to unveil the genetic and molecular mechanisms of their formation processes, which may be decomposed into the following three mutually interwined sub-processes:（1） differentiation along a gene cascade,（2） morphogenesis, and（3） biomineralization.The transcription factor gene engrailed（en） is known to be expressed in the margin of the region where the initial formation of the shell takes place in the species of all the molluscan classes so far examined.Thus, en likely resides near the origin of the cascade for shell formation.In the brachiopod Lingula, en is expressed in larval mantle lobe, suggesting a role in shell development homologous to molluscs.However,comparisons of amino acid sequences and the upstream genomic region organizations of en and microsynteny around en among lophotrochozoans indicated that the role of en in shell formation evolved independently in brachiopods and molluscs by gene co-option.Molluscan shells are diverse, but they share one and the same underlying rule of growth, that is, to form a logarithmic spiral.Exact molecular mechanisms to form shells of a logarithmic spiral are still unknown, but it has been shown that the signal transduction gene dpp has a role in the production of a coiled shell through formation of a laterally asymmetric gradient of the Dpp concentration in the shell forming tissues.Shell formation is finally achieved by the precipitation of biominerals, or biomineralization, which is believed to be controlled mainly by the proteins that become entombed in the shell matrices.Combined transcriptomic and proteomic analyses of shell matrix proteins（SMPs） revealed presence of literally hundreds of SMPs in a given molluscan or brachiopod species.SMPs are diversified in sequence and in protein repertoire even between closely related species, indicating generally high rates of evolution for SMPs.Many SMPs have a domain structure, and some domains appear to have been conserved ever since Cambrian.However, phylogenetic analyses of those seemingly conserved domains, such as the carbonic anhydrase domain, indicated a more recent and independent deployment among different bivalve lineages.In future, the roles of the gene products involved in shell formation have to be more precisely accessed so as to understand the mechanisms and the dynamic evolutionary processes of shell formation in fossiliferous lophotrochozoans.
类型: 国际会议
作者: Kazuyoshi ENDO,Keisuke SHIMIZU
来源: 第一届亚洲古生物学大会—暨中国古生物学会成立90周年纪念 2019-11-17
年度: 2019
分类: 基础科学
专业: 生物学,地质学
单位: University of Tokyo, Department of Earth and Planetary ScienceUniversity of Exeter, College of Life and Environmental Sciences
分类号: Q915
页码: 175-176
总页数: 2
文件大小: 167k
下载量: 1
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