Klementz Benjamin C, Brenneis Georg, Laumer Ethan M, Neu Sophie M, Harvey Mark S, Sharma Prashant P
Department of Integrative Biology, University of Wisconsin-Madison, Madison, WI, USA.
Unit Integrative Zoologie, Department Evolutionsbiologie, Universität Wien, Vienna, Austria.
Arthropod Struct Dev. 2025 Jul;87:101446. doi: 10.1016/j.asd.2025.101446. Epub 2025 Apr 30.
A major theme in the evolution of Arthropoda is the origin and diversification of jointed appendages. One appealing framework for the evolution of arthropod appendage diversity has long been that a small network of homologous genes in the panarthropod ancestor established and subdivided the proximo-distal (PD) appendage axis, with lineage-specific modifications of these genes' expression domains resulting in novel types of appendages. A corollary of this idea is the inference that each segment in the arthropod leg can be directly homologized to other such segments, based on anatomical or developmental genetic landmarks. Here, we explore the evolution of leg segments in Chelicerata, a group which exhibits marked diversity in leg architecture and number of leg segments, and thereby poses a greater challenge to the exercise of assigning segmental homologies. Focusing on the controversial nomenclature of leg segments in Pycnogonida (sea spiders), we identify potential markers of positional homology in different parts of the sea spider and arachnid PD axis, using comparative gene expression data. Nevertheless, we identify caveats to the use of transcription factor expression domains as landmarks for inference of positional homology, highlighting cases where datasets conflict in homology assignment. We postulate that the utility of gene expression data for inferring homologies is a function of phylogenetic distance.
节肢动物进化的一个主要主题是分节附肢的起源和多样化。长期以来，一个关于节肢动物附肢多样性进化的有吸引力的框架是，泛节肢动物祖先中的一小群同源基因建立并细分了附肢的近端-远端（PD）轴，这些基因表达域的谱系特异性修饰导致了新型附肢的出现。这个观点的一个推论是，基于解剖学或发育遗传学标志，可以推断节肢动物腿部的每个节段都能直接与其他此类节段同源。在这里，我们探讨了螯肢动物纲腿部节段的进化，该类群在腿部结构和腿部节段数量上表现出显著的多样性，因此对进行节段同源性的判断提出了更大的挑战。以海蜘蛛（海蛛纲）腿部节段存在争议的命名法为重点，我们利用比较基因表达数据，确定了海蜘蛛和蛛形纲动物PD轴不同部位位置同源性的潜在标志物。然而，我们也指出了将转录因子表达域用作推断位置同源性标志的一些注意事项，强调了数据集中在同源性分配上存在冲突的情况。我们推测，基因表达数据用于推断同源性的效用是系统发育距离的一个函数。
