Deep homology influences localized postembryonic metamorphosis in male genitalia of the flat-backed millipede Pseudopolydesmus serratus (Myriapoda, Diplopoda, Polydesmida, Polydesmidae)

In the arthropod subphylum Myriapoda, localized modification of walking legs is taxonomically widespread. Male millipedes (class Diplopoda) exemplify this phenomenon, with diverse postembryonic walking leg transformations including clasping organs, reduced or hyper-enlarged leg pairs, and intromittent copulatory genitalia called gonopods. Gonopods are fully formed in adult males, but usually develop internally through several stadia. The clear developmental continuity throughout the transformation of walking legs into gonopods invites comparison between the two types of appendages. We hypothesized that, using a criterion of topology, muscles of the gonopods could be identified as serial homologs of those in the walking legs. µCT x-ray imaging was used to illustrate the appendicular skeletal elements and muscles of the flat-backed millipede Pseudopolydesmus serratus. Special attention was paid to the origin and insertion points of muscles as potential points of topological continuity preserved through development. Additionally, a series of juvenile male Pseudopolydesmus stadia was imaged using µCT to investigate the structural transformation of the future gonopod appendage pair. Topological continuity of muscles between walking legs and gonopods could be established for only a few muscles. Moreover, the developmental series revealed near-complete obliteration of appendicular muscles before adult gonopod formation begins, disproving the hypothesis that gonopod muscles are physically retained serial homologs of those in the walking legs. Therefore, the few structural similarities between muscles in walking legs and gonopods are more likely due to the structure being preserved from the ancestral node of gonopod-bearing millipedes (deep homology) than to developmental continuity within the individual.