#6037 - Arthropods - Evolution of Invertebrates

Arthropods Introduction Arthropoda is a large phyla consisting of 3 sub-phyla: o Chelicerates which include horse-shoe crabs, spiders, scorpions, sea spiders, ticks and mites o Mandibulates which include crustaceans, myriapods and insects o Trilobites (which are now extinct) Features of arthropods: the key features of arthropods appeared around 600MYa o Jointed limbs o Segmented body that is divided into tagmata (fused segments, e.g., head, abdomen) o A cuticular exoskeleton o An open circulatory system with a dorsally positioned contractile heart o Complex mouthparts o A dorsal brain which passes around the gullet in a post-oral commissure Evolution of the brain Early annelid ancestor o Archicerebrum connected with eyes o Prosocerebrum is located caudal to the stomodaeum Early myriapod form o Prosocerebrum moves forward o Fuses with the archicerebrum o The dueterocerebrum fuses partially with the prosocerebrum Chelicerate / insect bauplan o Dueterocerebrum fused with prosocerebrum o Tritocerebrum fused partially with dueterocerebrum o Retainment of post-oral commissure Evolution of arthropods Arthropods evolved from annelids, around 600 million years ago. The diagram shows a genetic analysis of hox genes and it can be seen that most of the hox genes of insects and annelids are the same, showing they pre-date the insect/annelid divergence. All insect hox genes pre-date the crustacean/insect divergence. The easy availability of molecular sequencing data means that arthropod evolution is an incredibly hot topic. Key adaptations and their consequences o Pronounced tagmatization which led to the possibility of adding jointed limbs and segments devoted to differed functions o Chitin in exoskeleton which offered protection from predators in the environment and provided points at which muscles could be anchored to the body, which again led to the evolution of jointed appendages and/or wings. However, a drawback of chitinisation is that if the animal wishes to grow larger, the animal must be able to develop the ability to moult and the hormonal systems in place that control moulting. Or you can have holometabolous insects that have different life stages, or hermit crabs that parasitise shells. o Jointed appendages led to very rapid movement o Complex mouthparts which led to a diverse range of feeding habit, including technique and diet, evolved from compression of original annelid segments o Compound eyes and ocelli Phylogeny Are arthropods monophyletic? Do they have a common ancestor that they don't share with another organism? Yes. However, the position of the trilobites is debated. Don't kow if more related to chelicerates and mandibulata. The onycophorans may be basal arthropods suggesting that chelicerates monophyletic, and in this version the hexapods are closer to the myriapods to crustaceans. However, there is reason to suggest that this may not be the case. There were four versions of phylogeny that were postulated, with different evidence for each, with the relatedness of myriapods, hexapods and chelicerates all under debate. A study in Nature took 337 morphological characters and the study suggested a strong affinity between hexapods and crustaceans, and that they are separate from myriapods and chelicerates. They were