Genetic Basis for Arthropod Limb Diversification

Abstract

Changes in the morphological character of appendages are essential to arthropod diversification and adaptation to a variety of living conditions. For instance, the fruit fly Drosophila melanogaster possesses cylindrical, uniramous (unbranched) walking legs that are well suited for terrestrial life, while Triops longicaudatus possesses paddle-like, multiramous (multibranched) limbs adapted to aquatic life. Comparative studies of limb patterning between different species of arthropods suggest that most animals utilize a conserved set of genes to construct a limb. How is this common set of genes used to produce morphological divergence? This question is addressed here by examining the spatiotemporal expression patterns of genes shown to establish distinct domains along the proximodistal (P/D) axis in arthropod species with legs morphologically distinct from Drosophila legs.In this dissertation, I investigate the role of the limb patterning genes, dachshund (dac) and homothorax (hth), in patterning the appendages of the crustacean Triops longicaudatus. I examine the spatiotemporal relationships of the expression of these two essential limb-patterning genes individually and simultaneously with two previously reported leg patterning genes, extradenticle (exd) and Distal-less (Dll). I discovered that Triops dac and hth, as expected, are expressed during leg development. I verified a cell-to-cell association between HTH and nuclear-EXD (n-EXD), a spatial relationship that had only been conjectured to exist outside of Drosophila. This spatial relationship represents an ancient unchangeable constraint on limb patterning. HTH expression reported here in addition to previously reported EXD and DLL expressions suggests a common, early subdivision of the leg into broad proximal and distal domains. However, the reiterated stripes of DAC expression found along the ventral axis do not support establishment of an intermediate leg domain but instead suggest that the ventral branches of the Triops limb are generated by a mechanism of segmentation not previously observed in other arthropod limbs. Additionally, I present a record of my attempts aimed at functional determination of genes believed to specify, pattern, or modify branchiopod appendages. Finally, I demonstrated that dac functions in the dung beetle leg to properly segment the tarsus as well as producing a structural modification, such as spiked protrusions.