Bilateria And Bilaterian Phylogeny Flashcards
Types of characters used to compare morphologies of different bilateria (5)
- Symmetry
- Germ layers
- Body cavities
- Segmentation
- How the embryo cleaves
Bilateral symmetry
Similar anatomical parts are arranged on opposites side of a median axis so that only one plane can divide the body into identical halves
(Animal has a right and left side)
Radial symmetry
Similar parts arranged regularly around a central axis
Animal packs left and right sides
How many germ layers do most animals have and what are they?
3
Ectoderm
Mesoderm
Endoderm
What is a germ layer
Any of three primary cell layers formed in earliest stages of embryonic development
What is an acoelomate?
An animal that doesn’t possess a body cavity
What is a coelomate?
Organisms with a fluid filled cavity between the gut wall and the outer body wall
Coelom
Body cavity lined with an epithelial cell layer within the mesoderm
Two ways an embryo can cleave?
Radial cleavage
Spiral cleavage
Radial embryo cleavage
Cell division occurs at right angles to previous divisions resulting 4 blastomeres being situated directly on top of 4 other blastomeres.
Spiral embryo cleavage
Cells of early embryo divide and spiral around a pole to pole axis of the embryo so that 4 blastomeres sit in the grooves of 4 other blastomeres
Potential issues with using those 5 morphological characters to understand phylogeny (3)
A lineage may have had a character (eg segmentation) but then lost it
Convergent evolution could have occurred and the characters might be polyphyletic
Not looking at many characters for a reliable comparison
What was the first gene used to create a phylogenetic tree for invertebrates?
18S rRNA
Why was 18S rRNA the first gene use to create an invertebrate phylogenetic tree?
Conserved function in every species
2000 nucleotides long (short)
What problem that occurred when comparing morphologies does DNA sequencing overcome?
Number of characters compared is no longer an issue because a large amount of DNA is analysed
What problems that occurred when comparing characters of morphology are still an issue despite DNA sequencing?
Convergent evolution
Loss of a character (for example point mutations which altered bases and then altered them back to what they were previously would go undetected)
Long branch attraction
What is long branch attraction
A systematic error where distantly related lineages are incorrectly thought to be closely related
How does long branch attraction arise?
XX
When the amount of molecular or morphology l change accumulated in a lineage is large enough that it makes it appear similar to a distantly related lineage simple because both have undergone such a large amount of change (and therefore by chance appear similar????)
Lophophore
Feeding organ that is a ring of tentacles
Trochophore larvae
Small translucent free swimming lava characteristic of annelids and most mollusc groups
Stome
Mouth/opening like a mouth
What is an issue with the prebilaterian phylogeny?
We still don’t know the exact order that the four main prebilaterian lineages branched off
