Evolution and Development of the Onychophoran Head and Nervous System

Abstract: Onychophorans are closely allied to the arthropods and possess a body organisation more similar to Middle Cambrian fossils than to recent arthropods. This means that onychophorans in some respects can be regarded as a model for the last common ancestor to both the Arthropoda and the Onychophora. This thesis mainly deals with the morphology of the head region of the Onychophora, but developmental investigations of the expression of a key regulatory gene, engrailed, are also carried out. The innervation of the head was found to differ from that reported in earlier investigations. The nerves that support the mouth were found to originate from three different regions of the brain. That innervation pattern suggests that present day onychophorans with a ventrally placed mouth, have evolved from an ancestor with a terminal mouth. Furthermore it was confirmed that the onychophoran structure with the unfortunately chosen term labrum is not homologous to the structure in arthropods that bears this name. Instead it is a muscular outgrowth from the pharynx. The embryological investigations gave further support for an ancestral and terminal mouth. The two most anterior oral lips are first located on the dorso-frontal side of the head, and later migrate to their final position at the ventral side. This phenomenon also explains their somewhat unexpected innervation from the dorsum. It was also established that the eye originates at a position posterior to the antenna. This is reversed compared to the condition in arthropods, were the eye is innervated from the protocerebrum and the first antenna from the dutocerebrum, and implies that the eye and antenna are not serially homologous between the two groups. A structure in the onychophoran head that has gained little attention is the hypocerebral organ, also termed infracerebral organ. It has been suggested as a corpora allata analog by earlier workers. Its ultrastructure was investigated, and great similarities to the corpora allata of the stick insect Carausius morosus were found. However, the lack of innervation of the hypocerebral organ of Onychophora poses a problem since the corpora allata of insects is controlled by nerves. Instead, cellular strands were found that connected the hypocerebral organ with the brain, and it is possible that these strands act as an alternative communication. The expression pattern of the segment polarity gene engrailed was found to be different from that reported in an earlier account of onychophorans. Engrailed was expressed in a subset of developing neurons in the brain anlage and in the ectoderm and mesoderm of the limb buds. The engrailed positive cells in the brain anlage were located in the area were the first commissure will form. This indicates that engrailed might have a function in axon guidance, as has been reported in other organisms. Later embryonic stages showed expression in the neuropile of the brain. There were no indications of this gene acting in determination of segment polarity. This suggests that there may be at least two copies of engrailed in onychophorans.