Selector genes and the Cambrian radiation of Bilateria.

There is a significantly greater post-Cambrian decline in frequency of ordinal origination among serially constructed Bilateria, such as arthropods, than in nonserially constructed Bilateria. Greater decline in arthropod ordinal origination is not predicted by ecologic, diversity-dependent models of decline in the production of higher taxa. Reduction in ordinal origination indicates increased constraint on arthropod body-plan evolution. The dispersal of selector genes in the genomes of arthropods in conjunction with the retention of a simple regulatory hierarchy in development may have caused the increased constraint seen. Increased constraint would not be expected in those organisms that are not serially constructed and presumably have not retained the simple ancestral regulatory hierarchy in development of selector gene differentiation of serial elements. The hypothesis of differential constraint tested against the fossil record in this paper can be further tested by examination of the distribution of selector genes in the genomes of arthropods. IMAGES:

相较于非分节构造的两侧对称动物（Bilateria），节肢动物（arthropods）这类分节构造的两侧对称动物，其后寒武纪时期的目级起源频率降幅显著更高。节肢动物目级起源频率的更大降幅，无法通过针对高级分类单元产生衰退的生态依赖多样性模型进行预测。目级起源频率的下降表明，节肢动物身体构型演化所受的约束有所增强。节肢动物基因组内选择基因（selector genes）的扩散，结合其发育过程中保留的简单调控层级，可能是导致上述约束增强现象的原因。对于不具备分节构造、且推测未在串行结构分化相关选择基因的发育过程中保留祖先简单调控层级的生物而言，这种约束增强现象不会出现。本文通过化石记录验证的差异化约束假说，可通过检测节肢动物基因组内选择基因的分布情况得到进一步检验。图像资料：