Rise of animals 5 Flashcards
How do differences in regulatory gene complements correspond to major differences in morphology in both arthropods and echinoderms?`
Changes in the expression patterns of two hox genes in different crustaceans correlate well with the modification of their thoracic limbs into feeding appendages
Primitive crustaceans had a rather uniform series of thoracic segments so changes in the hox gene expression seems to have been involved with the specialisation of anterior appendages for feeding these changes appear to have independently taken place within different groups
What hox gene difference is seen in barnacles?
They have lost the hox gene abdominal A which may explain their loss of abdominal segments
What is Ubx?
This is a gene found in flies which represses wing growth producing little balancing structures known as halters though in butterflies this gene controls the shape and colour of the wings
Where has it been seen that Hox genes can evolve new activities?
Insects and crustaceans evolved from a common ancestor, in insects the hox gene Ubx represses two how genes involved in limb development while in crustaceans it turns off one of the ses genes but not the other
This difference in function is a result in the changes in the structure of the gene in an ancestral insect
It results in limb suppression in insects but not crustaceans explaining my insects have fewer appendages
What is the body plan of echinoderms?
These organisms possess one of the most highly derived body plans of all metazoan phyla with radial symmetry, a calcitic exoskeleton and a water vascular system
As they are deuterostomes they must have evolved from a bilaterian ancestor and the embryos of these groups are bilaterally symmetrical with a few bilaterally symmetrical gene expression pattersn being seen in the larvae
What is stereom
All echinoderm crown and stem groups have stereom, a calcitic endoskeleton tissue, crown group echinoderms appear in the fossil record about 485 MYA while stem groups appear about 525 MYA
Genes controlling development of this structure must have evolved in the early Cambrian as sea urchins and starfish share many hox gene features and these are thought to have diverged at 490 MYA and remained in the echinoderm genome ever since
What has detailed study of the hox genes revealed about Urbilateria?
That all or most of the hox genes found in living animals were present in the Urbilateria with the evolution of these genes since this time mainly being change in function and gene loss rather than gene duplication
When is the major period of hox gene duplication thought to have occurred?
This is thought to have preceded the bilaterian animals and it is important to note that the evolution of phylum specific body plans did not require the evolution of new developmental genes but novel gene regulatory circuitry for instance Pax6 controls eye development in drosophila and mice but is also fuctional in nematodes which lack eyes
This also demonstrates that the presence of a particular gene only shows that the biochemical properties of that protein have been conserved not that its function within a certain morphological structure has been
What does the molecular phylogeny tell us about the ecdysozoans during the Cambrian?
Many of the taxa which are abundant as Cambrian fossils are related as ecdyosozoa
Molecular relationships tell us that nematodes and priapulids sit at the base of the tree and that onychophorans are seen as thesister group to the tardigrades and euarthropods
What is the new tree that has been produced which combines data from the fossil record and molecular analysis?
The cambrian ‘weird wonders’ including hallucigenia, kerygmachela, opabinia and anomalocaris as branches off the lineage that led to the euarthropods
This was able to be done through the discovery that kerygmachela had gill filaments on its lateral lobes making it an intermediate between the onychophorans and opabania and anomalocaris as anomalocaris had gilled lobes but many euarthropod features
This allows lobopodians to be seen as a paraphyletic succession of froms which progressively evolved lateral lobes, segmentation, a ventral mouth and stalked eyes, cuticular appendages and finally a sclerotized cuticle
What happens to the tardigrades under the new phylogeny?
They become placed within the lobopodians however their exact position is uncertain as these organisms are tiny and all their organ systems are reduced so their highly derived characteristics make it difficult to compare their morphology with that of larger animals
Molecular analysis do however consistently place them closer to euarthropods than onycophorans
How does locomotion in lobopods compare to that of jointed legs in the euarthropods?
Locomotion in the soft-bodied onychoporans works through a hydrostatic system where contraction of the peripheral muscles acts against the incompressible fluid haemocoel in the body and lobopods to flex them while euarthropods use a lever system where skeletal muscles inside appendages work to flex hinges between the segmented tergites of the exoskeleton with peripheral muscle being reduced or absent and the haemocoel playing virtually no role in movement
How would the transition from lobopods in onychoporans to jointed legs in the euarthropods occurred?
This was solved through the discovery of Pamdelurion which had a soft body, haemocoel and peripheral muscles like an onychphoran but also had well developed internal skeletal muscles which braced the trunk
Side to side contraction of these peripheral muscles when braced by alternate contractions of the skeletal muscle would have cause the trunk of this organism to bend making locomotion in this organism a functional intermediate between onychophorans and euarthopods
What was the selective pressure to evolve the mixed locomotory function seen in Pamdelurion?
It has been speculated that the skeletal muscle may have evolved to partition the haemocoel as Pamdelurion was a big animal similar to anomalocaris with a tail and lateral, flap like lobes while the more basal forms were all small
This leads to the conclusion that a large size means there is a demand to control the movement of fluid within the haemocoel driving the evolution of the internal skeletal muscles which would later form the basis for locomotion in euarthropods
How can the onychophoran-arthropod transition be looked at in terms of developmental mechanisms?
The euarthropod taxa shares and almost identical set of hox genes that pattern the body axis so the disparity seen in the Cambrian and recent members of this group has evolved from an ancient and conserved set of hox genes
The increase in segment diversity within the arthropod clade is correlated with changes un the relative domains of hox gene expression along the main body axis
