BG18

Question 1

Karl Von Baer

Answer 1

compared embryos of vertebrates

- idea that animals share common juvenile stages when they diverge.

Question 2

Haekle biogenetic law

Answer 2

argued embryos of different species would diverge with age if evolution worked by adding new parts to the end of ontogeny.
- fraudulent diagrams.

Question 3

Aquatic elephants and sea cows?

Answer 3

aristotle thought elephant semi aqautic and used trunk to snorkel.
- molecular and fossil data show elephants and sea cows share a semi acquatic CA.

Question 4

recapitulated features of elephant foetuses

Answer 4

they have nephrostomes

• ciliated ducts that connect body cavity to glomeruli of mesenphros (an embryonic kidney)
• normally found in aquatic mammals
• used for osmoregulation of body cavity fuels.
• elephant trunk develops early, implies ancestor had strunk? haekle

Question 5

disproving the biogenic law

Answer 5

embryos can evolve themselves.

Question 6

frog example disproving biogenic law

Answer 6

• most amphibians undergo indirect development
  they have a larvae, a tadpole etc.
  however the puerto rican tree frog E. coqui egg hatches to a frog.

Question 7

flatworm example disproving biogenic law

Answer 7

some animals have evolved radically different embryogenesis
- most lophotrochozoans have regular spiral cleavage but triclad flatworms such as planaria have anarchic cleavage
= devoid of proper blastula or gastrula

Question 8

Von Baers third law

Answer 8

morphological divergence increases with embryonic/foetal age

- funnel model

Question 9

1980 hour glass model

Answer 9

noticed that early embryos can be very diverse, but that in some taxa they go through a conserved stage and diverge again.
conserved stage = phylotype

Question 10

phylotype in vertebrates

Answer 10

somite formation or tail bud formation.

Question 11

phylotype in insects

Answer 11

germ band stage, when mesoderm is segmented

Question 12

testing the models in C. elegans

Answer 12

tested used transcriptomic data of related species for 10 stages of development.
- expression data dimensionally reduced using PCA.
euclidian distances calcualted between species and average expression distance among species plotted for each stage.

• house glass model fits data
• min distance around 6-7 at ventral closure of blastopore = end of gastrulatioon

Question 13

testing the model - drosophila

Answer 13

similar study with 5 drosophila spp.

• estimated the age of d. melanogaster genes (when they arose in metazoan history), then see when expressed in flies development, before estimating average age of genes expressed at each stage.
• on average genes expressed very early and very late in development were found to be younger than those expressed in the middle: hourglass model

Question 14

zootype

Answer 14

single conserved stage in all animals.

marked by hox gene expression

Question 15

zootype experiment

Answer 15

compare c. elegans and xenopud gene expression during development.

• orthologs matched
• stage 6-8 ventral closure in worms expressed many of the same genes that are expressed in tail bud formation and neurulation in the frog.
• broadly conserved time of development: zootype.

Question 16

orthology groups expressed during proposed zootype stage

Answer 16

helix loop helix tfs
locomoter related genes (muscle)
hox genes.

Question 17

why do phylotype stages exist

Answer 17

1. developmental constraints
   (phylogenetic stage is highly buffered against mutations that affect it, any mutation that affects process ast this stage doesnt manifest itself int the embryo since the embryo compensates for it.
2. purifying selection
   (phylotypic stage is vulnerable to new mutations; mutations are mostly deleeterious and will be selected against

Question 18

evidence for why there is a phylotype (hamsters)

Answer 18

noted hamster vulnerable to tetragens agent that causes embryo malformation at neurulation/tail bud/pharyngeal stage day 6-9
- argued mutations would show same pattern = favour purifying selection

Question 19

evidence for why there is a phylptype c. elegans

Answer 19

ost vulnerable to RNAi at around ventral close stage

- favours purifying selection.