Week 3: Deuterostomes

Characteristics of Deuterostomes

• Deuterostomy (blastopore = anus)

• radial cleavage

• Enterocoely (coeloms from archenteron)

• Tripartite body (3 coeloms→ Protocoel, mesocoel, metacoel)

• Gill pores

Enterocoely

coelum buds off from central cavity

Former phyla of Deuterostomia (now known to be protostomes)

1. Chaetognath/ Arrow worm

   • Radial cleavage

   • Trimery

   • Deuterostomy

   • Enterocoely

2. Brachiopods/ lamp shells

   • radial cleavage

   • trimery

   • enterocoely

3. Pogonophoran/ Vestimentiferan/ Siboglinidae

   • Spiral cleavage

   • Trimery?

   • Enterocoely?

   • (Schizocoely in Pogonophores)

Possible Phyla of Deuterstomia

1. (Xen)Ambulacraria

   • Hemichordata

   • Echinodermata

   • Xena coelomorpha

2. Chordata

   • Cephalochordata

   • Tunicata

   • Vertebrata

Chordate Synapomorphies

1. Notochord

2. dorsal, hollow nerve cord

3. postnatal tail

4. pharyngeal slits

Tunicata - Sea Squirts

• No notochord/ nerve cord/ tail (attached to substrate/ rocks)

• ^ Body plan after metamorphosis → larvae have the right characteristics since they are mobile

Cephalochordata- Amphioxus

• filter feeders but don’t attach to substrate

• share many chordate synapomorphies

• Morphology leads cephalochordates to be more closely related to vertebrates because they look more like fish

How are tunicates more closely related to chordates than cephalochordates?

• Tunicates more closely related if you look at AA sequences→ now in a clade called olfactores

• apparatus used to smell well developed in both groups

• Tunicates have evolved a sessile, filter feeding lifestyle and a radically new, simple, adult bodyplan

• Tunicates have lost complexity.

• But their genomes are quite different from other chordates. They show chromosomal rearrangements and have lost many genes found in amphioxus and vertebrates

• Genome of Cephalochordates have a genome closer to mammals → depending on the data different conclusions on how closely related animals are

→ we may come up with different conclusions based on the data we have

Ambulacraria

• Hemichordates

• Echinoderms

• Show penta-radiality in adult life stage

How are hemichordates and echinoderms linked?

• dna

• shared used of isoleucine codon

  • in mt codon

  • Amino acid code AUA codes for the same thing in ophiuroids and sea stars→ not universal → codes for isoleucine instead of methionine

• larval morphology

Placement of xenacoelomorpha

• sister to protostomia or sister to ambulacraria

• used to be placed with platyhelminthes (protostomes)→ both have a highly simplified body plan