Homology And Origin Of Larval Prolegs In Panorpidae (Mecoptera) Based On Distal-less

There is considerable diversity in the distribution of larval prolegs on insects, with a wide range of variation in both arrangement and number, which is considered as the functional adaptation to the environment. Thus, the study on homology and origin of prolegs is helpful for exploring the evolutionary process. However, it is still debatable for the homology and origin of larval prolegs.Mecoptera, one of the most primitive orders in Holometabola, possesses some unique characteristic, making it one of important groups in developmental study. Previous studies found that the positions of thoracic legs and prolegs are not consistent, and the proleg primordia appear later than thoracic leg primordia based on morphology and anatomy. These studies made mecopteran one of the model species on larval prolegs of insects, but lacking direct evidence. Therefore, it is necessary to solve the problem from the development mechanism with modern molecular methods.The developmental pattern of prolegs during embryonic stage of Panorpa dubia was studied using stereoscope and scanning electronic microscope. Then the molecular developmental mechanism was studied by in situ hybridization. And protocol about in situ hybridization in mecoptera was set.The developmental period of appendage is 70-110 h after oviposition. The thoracic leg primordia appear in 70 h after oviposition and the prolegs appear in 90 h after oviposition. A pair of swelling appears on the first eight abdominal segments, which stood in a line with the thoracic legs. Also, the position of prolegs is along the mid-ventral line, mesial to that of thoracic legs. In addition, the thoracic leg primordia are divided into 5 segments, whereas the proleg primordia are unsegmented and much smaller than thoracic leg primordia. The Distal-less orthologue(Dll) of P. dubia is cloned, the full-length cDNA is 2950 bp, 5’-UTR is 1073 bp, 3’-UTR is 1043 bp, and one single ORF is 834 bp, encoding 278 amino acids.In 100 h after oviposition, Dll is expressed in the most distal segment of thoracic legs and the distal tip of prolegs. No Dll expression is detected in the lateral position of the prolegs, where once occurred a pair of swelling.Based on embryonic observation and expression pattern of Dll, this study confirmed that the prolegs and thoracic legs of P. dubia are not homologous, and assumed the prolegs may derive from the basial endites of appendage in the abdominal segments. Also, the expression of Dll is not only in the distal part of appendages, but also in the protrude structure of appendages.