Origin And Homology Of Abdominal Appendages Of Larvae In Panorpidae

In addition to three pairs of thoracic legs,the eruciform larvae of the primitive holometabolous insects are characterized by several pairs of abdominal prolegs.During evolution,abdominal prolegs generally exhibit a considerable diversity in terms of number,position and morphology among different groups.Studying these differences can provide more evidence for exploring the evolution and phylogenetics of the holometabolous insects.However,the homology and origin of these prolegs across lineages are still controversial.The larvae of Mecoptera represented by Panorpidae are of eruciform type,and bear a pair of prominent compound eyes on both sides of the head.Mecoptera is one of the most primitive orders in holometabolous insects,representing the intermediate link between hemimetabola and holometabola and playing an important role in phylogenetic study of insect order.However,the homology and origin of the abdominal appendages of Mecopteran larvae has not been determined,and is still debatable based on morphological and anatomical data.Therefore,in order to solve the problem,they still need more convincing characters to verify,especially the molecular evidence.Based on the comparative embryology,we studied the embryonic development of appendages(especially the thoracic legs and prolegs)in detail using scanning electron microscopy(SEM),and compared the expression patterns of homologues of some important regulatory genes that pattern appendage development in the scorpionfly Panorpa liui(Hua,1997)by in situ hybridization during embryogenesis,to reveal the developmental mechanisms of prolegs,and to provide fundamental information to judge the homology and origin of the abdominal appendages in Panorpidae.These results can lay a foundation for the evolution and phylogenetics of the holometabolous insects and the main results are as follows:We compared the developmental position of the thoracic legs with abdominal appendages in the scorpionfly P.liui during embryogenesis to investigate whether they are serially homologous or not.At 86 h post oviposition,pairs of primary(lateral)swellings in line with the rudiments of the thoracic legs arose on the first eight abdominal segments.Shortly thereafter,they ceased to develop and incorporated into the sternum of the larval abdomen,leaving two small lobe-like remnants on the ventral surface of the abdominal segments.At 110 h,pairs of new swellings appeared mesial to the primary abdominal swellings on the ventral side of the first eight abdominal segments,just in a row with maxillary endites.Later,these medial swellings in the abdomen grew further and developed into cone-shaped,unsegmented prolegs.To investigate Barlet's hypothesis(1981)that the prolegs of Mecoptera are the telopodite remnant,we examined the expression pattern of Distal-less(Dll)in the scorpionfly P.liui during embryogenesis.Throughout embryonic development,Dll expression consistently appeared in the distal portions of all cephalic(including telopodite of maxially endites)and thoracic appendages,whereas it showed a dynamic expression on the abdominal segments.Dll transcripts appeared on the lateral swellings as a circle with a short-lived expression,but they were continuously expressed in the proleg primordium and eventually accumulated into the distal telopodites of prolegs.To investigate Du's hypothesis(2009)that the abdominal lateral swellings of Mecoptera represent the coxopodites of appendages,we examined the expression pattern of extradenticle(exd)in the scorpionfly P.liui during embryogenesis.exd expression was confined to the lateral swellings as a circle.In the thoracic segments,exd was expressed in the proximal part of the thoracic legs correspond to the coxae.To provide more evidence for Bitsch's hypothesis that scorpionfly prolegs are outgrowths of coxal origin,we examined the expression pattern of decapentaplegic(dpp)and wingless(wg)in the scorpionfly P.liui during embryogenesis.dpp expression was completely restricted to the distal tips of each thoracic leg.The abdominal segments re-expressed dpp transcripts,but the exact expression domains were located in the lateral-posterior compartment of each abdominal segment,ectoderm cells,not the prolegs.wg was expressed extending along the ventral sides of thoracic legs and prolegs to the distal tips.To further gain a better understanding of the molecular mechanism that patterning scorpionfly prolegs,we examined the expression pattern of ultrabithorax(Ubx)and abdominal-A(abd-A)in the scorpionfly P.liui during embryogenesis.During the early embryonic stages,Ubx/abd-A expression was located at the pleuropodia,A1 appendages,along the midventral line of the abdomen.At 110 h,Ubx was expressed in the distal region of the thoracic legs and A1,whereas abd-A expression was detected in the proleg primordium on the first eight segments.At 124 h,abd-A transcripts were restricted to a proximal ring in each abdominal proleg,with the most distal region of each proleg lacking.Based on the observation on the morphological features of embryonic appendages,and the expression patterns of several genes in relation to appendage development in the scorpionfly P.liui,we concluded that the lateral swellings of P.liui may be the coxopodites of appendages which are serially homologous with the thoracic legs,whereas the prolegs are the outgrowths derived from coxal endites of abdominal appendages,but not the telopodite remnant.