Appendage patterning in the primitively wingless hexapods Thermobia domestica (Zygentoma: Lepismatidae) and Folsomia candida (Collembola: Isotomidae).

Arthropod appendages are among the most diverse animal organs and have been adapted to a variety of functions. Due to this diversity, it can be difficult to recognize homologous parts in different appendage types and different species. Gene expression patterns of appendage development genes have been used to overcome this problem and to identify homologous limb portions across different species and their appendages. However, regarding the largest arthropod group, the hexapods, most of these studies focused on members of the winged insects (Pterygota), but primitively wingless groups like the springtails (Collembola) or silverfish and allies (Zygentoma) are underrepresented. We have studied the expression of a set of appendage patterning genes in the firebrat Thermobia domestica and the white springtail Folsomia candida. The expressions of Distal-less (Dll) and dachshund (dac) are generally similar to the patterns reported for pterygote insects. Modifications of gene regulation, for example, the lack of Dll expression in the palp of F. candida mouthparts, however, point to changes in gene function that can make the use of single genes and specific expression domains problematic for homology inference. Such hypotheses should therefore not rely on a small number of genes and should ideally also include information about gene function. The expression patterns of homothorax (hth) and extradenticle (exd) in both species are similar to the patterns of crustaceans and pterygote insects, but differ from those in chelicerates and myriapods. The proximal specificity of hth thus appears to trace from a common hexapod ancestor and also provides a link to the regulation of this gene in crustaceans.