The functional evolution of collembolan Ubx on the regulation of abdominal appendage formation

Background: Folsomia candida , a tiny soil-dwelling arthropod within the Collembola, represents a basal hexapod lineage, serving as an outgroup to Insecta. Notably, it features three distinct abdominal appendages: the ventral tube in the first abdominal segment (A1), the retinaculum in A3, and the furca in A4. These structures stand out morphologically from the limbs in the thorax. The specification of these abdominal appendages falls under the responsibility of Collembolan Ubx and AbdA. However, despite their role in appendage specification, these genes are unable to repress the expression of the appendage marker gene Dll . The genetics toolkit involved in appendage formation during collembolan embryogenesis and the molecular mechanisms through which Ubx and AbdA regulate Dll transcription, subsequently shaping appendage development, remain elusive. Results: : To address these gaps, we explored the developmental transcriptomes of F. candida and identified 36 candidate genes implicated in appendage formation, including Ubx (FcUbx). Through rigorous experiments, the electrophoretic mobility shift assays and dual luciferase assays, we unveiled that FcUbx exhibits binding and repression capacities on Drosophila Dll akin to the longest isoform of Drosophila melanogaster Ubx (DmUbx_Ib). Additionally, the regulatory mechanism of the C-terminus of FcUbx on Dll repression mirrors that of the crustacean Artemia franciscana Ubx (AfUbx). These findings collectively demonstrate that the function of collembolan Ubx occupies an intermediate position between that of Insecta and Crustacea. Conclusion: Our study not only sheds light on appendage formation in F. candida but also contributes to the reconstruction of the functional evolutionary trajectory of Ubx on abdominal appendages, spanning from crustaceans to insects. We propose a model that collembolan Ubx might exert its repression function in abdominal segments in a context-specific manner.