Transition to Bilaterians

Question 1

what are placozoa?

Answer 1

another diploblast, non-bilaterian phylum

Question 2

what are key characteristics of bilaterians?

Answer 2

left side a mirror image of the right side

anterior head which leads forward leading to cephalization (ie in rag worms swimming and spawning)

Question 3

how have hox genes helped us to view the transition to bilaterians?

Answer 3

we can see hox gene clusters and A-P patterning
e.g drosophila - wings and antenna
we see that certain genes code for certain body parts

Question 4

how can we rebuild Urbilateria?

Answer 4

comparisons of developmental gene networks across the animals
we look at hox gene and Dpp gene expression
- how genes for A-P
- Dpp for D-V
sea anemone was a bilaterian before the cnidarian-bilaterian split, so do we have bilateria in the right place - maybe not
think it was a acelomate flatworm animal at the base of bilateria diversification

Question 5

what is a coelom and what can it help with?

Answer 5

a fluid filled cavity between the body wall and internal organs, lined by epithelium
allows independence of the body wall and gut for movement and feeding
space for organs, storage space
hydrostatic skeleton for locomotion and burrowing

Question 6

what is schizocoelic?

Answer 6

mesoderm hollows out - happens in annelids

Question 7

what is enterocoelic?

Answer 7

the embryonic mesoderm folds, rounding off to enclose coelomic pouches - happens in echinoderms

Question 8

what was the problem encountered when mapping the transition?

Answer 8

molecular data - the acoelomate, psudeocoelomate and eucoelomate phyla all mixed up together and so they’re not informative terms
found that flatworms aren’t basal but polypheletic (derived from more than one common ancestor)
are acoels a basal lineage or deuterostomes??

Question 9

what are xenacoelomorpha and how do they help map the transition?

Answer 9

flat worms which glide along ciliated epidermis
no through gut - sac like without anus, some have a translucent gut
acoelomates - solid with no body cavity
anterior concentration of nerves diffuse - not a true brain

possible branch between cnidarians and other bilaterians

Question 10

what are the hypotheses for the origin of triploblasts from diploblasts?

Answer 10

1. acoeloid-planuloid hypothesis - flatworm bilaterian ancestor via paedogenesis (retention of juvenile features) from a cnidarian planula larva
2. gastrea-trochae hypothesis - opening of the gastrovascular cavity of a cnidarian like ancestor, zipped closed in the middle leaving an anterior opening (anus) and posterior opening (mouth)
3. enterocoely hypothesis - a complex coelomate bilaterian ancestor with lumens of coelom having formed via pocketing of cnidarian-like coelenteron