The molecular basis of arthropod appendage diversity

Arthropods have an incredible diversity of limbs that are modified for walking, chewing, cleaning, mating, grasping, sensing, and more. Understanding the relationships and evolutionary histories of different limbs is a central task, but their sheer diversity makes this a daunting if not impossible task using morphology alone. Here, the in situ expression patterns and CRISPR-Cas9 phenotypes for the five best-studied leg-patterning genes - Distal-less, Sp6-9, dachshund, extradenticle, and homothorax - are described for all limbs of the crustacean Parhyale. Crustaceans are well-suited for this task because their limbs are more diverse than those of other arthropods, and each individual possesses a wide range of limb types that are relevant to many other arthropods, living and extinct. These results will a) provide a template for understanding the genetic basis of limb construction in arthropods more generally based on the strong phenotypes that can be obtained with CRISPR-Cas9, and b) contribute to our understanding of the evolution and affinities of highly modified legs like mouthparts and genitalia using molecular methods to complement previous morphological and embryological approaches.