Department of Evolutionary Biology, Unit for Integrative Zoology, University of Vienna, Althanstraße 14, 1090, Vienna, Austria.
Department of Functional and Evolutionary Ecology, Unit for Bio-Oceanography and Marine Biology, University of Vienna, Althantraße 14, 1090, Vienna, Austria.
Sci Rep. 2021 Feb 11;11(1):3575. doi: 10.1038/s41598-021-82122-6.
Hox genes are key developmental regulators that are involved in establishing morphological features during animal ontogeny. They are commonly expressed along the anterior-posterior axis in a staggered, or collinear, fashion. In mollusks, the repertoire of body plans is widely diverse and current data suggest their involvement during development of landmark morphological traits in Conchifera, one of the two major lineages that comprises those taxa that originated from a uni-shelled ancestor (Monoplacophora, Gastropoda, Cephalopoda, Scaphopoda, Bivalvia). For most clades, and bivalves in particular, data on Hox gene expression throughout ontogeny are scarce. We thus investigated Hox expression during development of the quagga mussel, Dreissena rostriformis, to elucidate to which degree they might contribute to specific phenotypic traits as in other conchiferans. The Hox/ParaHox complement of Mollusca typically comprises 14 genes, 13 of which are present in bivalve genomes including Dreissena. We describe here expression of 9 Hox genes and the ParaHox gene Xlox during Dreissena development. Hox expression in Dreissena is first detected in the gastrula stage with widely overlapping expression domains of most genes. In the trochophore stage, Hox gene expression shifts towards more compact, largely mesodermal domains. Only few of these domains can be assigned to specific developing morphological structures such as Hox1 in the shell field and Xlox in the hindgut. We did not find traces of spatial or temporal staggered expression of Hox genes in Dreissena. Our data support the notion that Hox gene expression has been coopted independently, and to varying degrees, into lineage-specific structures in the respective conchiferan clades. The non-collinear mode of Hox expression in Dreissena might be a result of the low degree of body plan regionalization along the bivalve anterior-posterior axis as exemplified by the lack of key morphological traits such as a distinct head, cephalic tentacles, radula apparatus, and a simplified central nervous system.
Hox 基因是关键的发育调控因子，参与动物个体发生过程中形态特征的建立。它们通常沿着前后轴以交错或共线性的方式表达。在软体动物中，身体计划的范围非常广泛，目前的数据表明它们参与了双壳类动物发育过程中标志性形态特征的形成，双壳类动物是由一个单壳祖先起源的两个主要谱系之一（单板纲、腹足纲、头足纲、掘足纲、双壳纲）。对于大多数支系，特别是双壳类动物，关于整个个体发生过程中 Hox 基因表达的数据非常匮乏。因此，我们研究了斑马贻贝 Dreissena rostriformis 的发育过程中的 Hox 表达，以阐明它们在多大程度上有助于形成特定的表型特征，就像其他双壳类动物一样。软体动物的 Hox/ParaHox 补体通常由 14 个基因组成，其中 13 个存在于双壳类动物的基因组中，包括 Dreissena。我们在这里描述了 9 个 Hox 基因和 ParaHox 基因 Xlox 在 Dreissena 发育过程中的表达。Dreissena 的 Hox 表达在原肠胚阶段首次被检测到，大多数基因的表达域广泛重叠。在担轮幼虫阶段，Hox 基因的表达向更加紧凑的、主要是中胚层的区域转移。只有少数这些区域可以分配到特定的发育形态结构，例如在壳域中的 Hox1 和在后肠中的 Xlox。我们没有发现 Dreissena 中 Hox 基因空间或时间交错表达的痕迹。我们的数据支持这样一种观点，即 Hox 基因的表达已经在各自的双壳类动物支系中独立地、并且在不同程度上被特化到谱系特异性结构中。Dreissena 中非共线性的 Hox 表达模式可能是由于双壳类动物前后轴上的身体计划区域化程度较低所致，例如缺乏明显的头部、头触手、齿舌器官和简化的中枢神经系统等关键形态特征。
