Developmental system drift in the patterning of the arthropod tarsus

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Genetics

Background:

• Arthropod leg patterning is largely understood through insect models, assuming conserved gene regulatory networks.
• Chelicerates show distinct appendage patterning, but functional data for key genes are limited.
• The homeobox gene clawless (often abbreviated as cll) is known to specify the claw-bearing pretarsus in insects.

Purpose of the Study:

• To investigate the function of the clawless gene in the harvestman, Phalangium opilio.
• To compare the role of clawless in harvestman leg development with its known function in insects.
• To infer the ancestral function of clawless in chelicerates and panarthropods.

Main Methods:

• RNA interference (RNAi) was used to knock down clawless expression in Phalangium opilio.
• Developmental and patterning defects in the harvestman leg were analyzed post-knockdown.
• Homologs of clawless were surveyed across diverse chelicerate species.

Main Results:

• Knockdown of clawless in harvestmen disrupted tarsal growth and patterning, affecting the proximal tarsal boundary.
• No effect on claw formation was observed after clawless knockdown.
• Truncation of the tarsus was associated with defective tarsomere formation.
• Survey of clawless homologs suggests a tarsal-patterning role in the chelicerate common ancestor.

Conclusions:

• The function of clawless in leg patterning has diverged between insects and chelicerates.
• Harvestman clawless plays a role in tarsal development, distinct from its insect counterpart.
• Panarthropod appendage patterning shows significant developmental system drift, with conserved genes exhibiting novel functions.