Molecular developmental evidence for a subcoxal origin of pleurites in insects and identity of the subcoxa in the gnathal appendages

Abstract

Pleurites are chitinous plates in the body wall of insects and myriapods. They are believed to be an adaptation to locomotion on land but their developmental and evolutionary origins are unclear. A widely endorsed explanation for their origin is through toughening pre-existing parts of the body wall; in contrast, the subcoxal theory suggests pleurites derive from a redeployment of the proximal-most section of the leg, the subcoxa. Here, by studying expression of appendage patterning genes in embryos and larvae of the beetle Tribolium castaneum, we provide the first molecular evidence for the existence of a cryptic subcoxal segment in developing legs. We follow this structure during development and show that the embryonic subcoxa later forms the pleurites of the larva as predicted by the subcoxal theory. Our data also demonstrate that subcoxal segments are present in all post-antennal appendages, including the first molecular evidence of a two-segmented mandible with a subcoxal segment in insects.

Dual evolutionary origin of insect wings supported by an investigation of the abdominal wing serial homologs in Tribolium

“…Since insect pleural plates are thought to have evolved from the most proximal segment of ancient legs, it is intriguing to speculate that the formation of some of the pleural plates (including the pleural wing serial homologs) is also suppressed by the same mechanism in the abdominal segments. The leg-derived nature of pleural plates in Tribolium has recently been supported from the developmental perspective (42), providing further support to the idea that the formation of pleural wing serial homologs is usually suppressed in the abdomen.…”

Exploring the origin of insect wings from an evo-devo perspective

“…The pleural plates are thought to have stemmed from ancestral proximal leg segments that have fused into the body wall of extant insects (B) [39]. Recent molecular analyses also support the proximal leg segment origin of the pleural plates [22,40,41].…”

Embryological evidence substantiates the subcoxal theory on the origin of pleuron in insects

“…A recent molecular developmental study by Coulcher et al . 4 is noteworthy. They distinguished five domains of the gene serrate homolog ( Tc-ser ) expression in the embryonic appendages (limbs) of the red flour beetle Tribolium castaneum , which correspond to five joints separating six segments, i.e., subcoxa, coxa, trochanter, femur, tibiotarsus, and pretarsus.…”

“…Therefore, as Coulcher et al . 4 mentioned, the pleuron plays an important role in providing mechanical support for limbs and wings, driven by highly developed muscles, as well as in providing elastic joints or hinges against the trunk.…”