Bilateria And Bilaterian Phylogeny

Question 1

Types of characters used to compare morphologies of different bilateria (5)

Answer 1

1. Symmetry
2. Germ layers
3. Body cavities
4. Segmentation
5. How the embryo cleaves

Question 2

Bilateral symmetry

Answer 2

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)

Question 3

Radial symmetry

Answer 3

Similar parts arranged regularly around a central axis

Animal packs left and right sides

Question 4

How many germ layers do most animals have and what are they?

Answer 4

3
Ectoderm
Mesoderm
Endoderm

Question 5

What is a germ layer

Answer 5

Any of three primary cell layers formed in earliest stages of embryonic development

Question 6

What is an acoelomate?

Answer 6

An animal that doesn’t possess a body cavity

Question 7

What is a coelomate?

Answer 7

Organisms with a fluid filled cavity between the gut wall and the outer body wall

Question 8

Coelom

Answer 8

Body cavity lined with an epithelial cell layer within the mesoderm

Question 9

Two ways an embryo can cleave?

Answer 9

Radial cleavage

Spiral cleavage

Question 10

Radial embryo cleavage

Answer 10

Cell division occurs at right angles to previous divisions resulting 4 blastomeres being situated directly on top of 4 other blastomeres.

Question 11

Spiral embryo cleavage

Answer 11

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

Question 12

Potential issues with using those 5 morphological characters to understand phylogeny (3)

Answer 12

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

Question 13

What was the first gene used to create a phylogenetic tree for invertebrates?

Answer 13

18S rRNA

Question 14

Why was 18S rRNA the first gene use to create an invertebrate phylogenetic tree?

Answer 14

Conserved function in every species

2000 nucleotides long (short)

Question 15

What problem that occurred when comparing morphologies does DNA sequencing overcome?

Answer 15

Number of characters compared is no longer an issue because a large amount of DNA is analysed

Question 16

What problems that occurred when comparing characters of morphology are still an issue despite DNA sequencing?

Answer 16

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

Question 17

What is long branch attraction

Answer 17

A systematic error where distantly related lineages are incorrectly thought to be closely related

Question 18

How does long branch attraction arise?

XX

Answer 18

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????)

Question 19

Lophophore

Answer 19

Feeding organ that is a ring of tentacles

Question 20

Trochophore larvae

Answer 20

Small translucent free swimming lava characteristic of annelids and most mollusc groups

Question 21

Stome

Answer 21

Mouth/opening like a mouth

Question 22

What is an issue with the prebilaterian phylogeny?

Answer 22

We still don’t know the exact order that the four main prebilaterian lineages branched off