L22: Deuterostomes and evolution of the Chordates

Traits of Deuterostomes

– left right Bilateral symmetry

– coelom

– radial cleavage during embryonic development

– Blastopore turns into anus

– Two phyla (echinodermata and chordata)

What are the two phyla of Deuterostomia?

– Echinodermata

– Chordata

Summary:

Echinodermata (3)

– 7000 species

– Most are Marine, sessile

– DNA data indicates very closer related to the Chordates

What species are included in Echinodermata? (6)

– Starfish

– Sand dollars

– Sea urchins

– Sea lilies

– See cucumber

– Brittle stars

Why is there so much interest in sequencing the sea urchin genome?

– Share a common ancestor with humans

– Most close to humans

Summary:

Sea urchin

– 7000 genes in common with humans

– Including the genes for Parkinson's, Alzheimer's, Huntington's muscular dystrophy

Echinodermata:

Skeleton

– Endoskeleton made up of interlocking CaCO3 (calcerous) plates and spines

– Covered by thin epithelium

Echinodermata:

sea urchin endoskeleton

– Tightly fused

Echinodermata:

Starfish endskeleton

– Loosely joined

Echinodermata:

Sea cucumber endoskeleton

– Microscopic

Echinodermata:

Vascular system (3)

– Unique water system

– Network of water filled canals

– Branch to form tube feet: for locomotion and feeding

Echinodermata:

sexual reproduction

– Releases gametes into the water

– Spawns

Echinodermata:

Asexual reproduction

– Autotomy

– Shed arm turns into a new individual (comet)

What are the five classes of Echinodermata?

-Asteroidea

-Echinoidea

-Ophiuroidea

-Crinoidea

-Holothuroidea

Echinodermata:

Asteroidea

-starfish

-sea daises

Echinodermata:

Echinoidea

-sea urchins

Echinodermata:

Ophiuroidea

-brittle stars

Echinodermata:

Crinoidea

-sea lilies

-feather stars

Echinodermata:

Holothuroidea

-sea cucumbers

what are the subphylums of Chordata?

-Vertebrata

-Cephalochordata

-Urochordata

What species are included in subphylum vertebrata? (5)

– Fish

– Amphibians

– Reptiles

– Bird

– Mammals

What species are included in some subphylum Cephalochordata?

-Amphioxus spp

What species are included in subphylum Urochordata?

-sea squirts

Summary:

Chordata

– Earliest coordinate fossil appeared in the Cambrian period (530 mya)

– the four common features of chordates will appear somewhere during the life cycle

What are the four common features that define Chordates?

– Notochord

– pharyngeal slits

– Dorsal, hollow nerve cord

– Muscular, anal tail

chordates:

notochord (7)

-slender rod

– Develops from the mesoderm

– Lies dorsal to the coelom

– Lies beneath and parallel to the CNS

- consist of a core of cells and fluid, surrounded by fibrous sheath

– Mechanical properties of an elastic rod

– Allows locomotion through lateral undulations

Notochord in fish and terrestrial vertebrates (3)

– Present during embryonic development

– Replaced by variable column

– Persists as the nucleus pulposus of the intervertebral discs

chordates:

pharyngeal slits

–Longitudinal series of openings and walls of pharynx

pharyngeal slits:

primitive chordates

such as Amphioxus

used for feeding

pharyngeal slits:

Terrestrial vertebrates

-present in embryo

-lost during embryogenesis

pharyngeal slits:

Aquatic vertebrates

– Gills develop adjacent to pharyngeal slits

– allow for flow of water across gills

summary:

Dorsal, hollow NC (4)

– derived from ectoderm

– Lies above the gut

– Is hollow

– surrounds the fluid filled neurocoel

summary:

muscular, post-anal tail

-Posterior elongation of the body beyond the anus

– Consists of segmental musculature and the notochord

Summary:

Subphylum Cephalochordata

–Resemble the earliest chordates

– Fossil date back to the Cambrian

– Occur worldwide in temperate and tropical seas

– Anatomically simple

What is the living example of a Cephalochordate?

Branchiostoma - amphioxus

Summary:

Branchiostoma - amphioxus

-shape is blade like

– adult possesses all four chordate features

– posterior end buried in sand

– Interior end exposed to water

– Filter feeder

– Ciliated pharynx

– Mucous nets across slits

Cephalochordata:

Digestive system

– possible precursor of the vertebrate organ

– Midgut caecum: liver, pancreas

Cephalochordata

Circulatory system (5)

– Has same general pattern as that vertebrates

– Ventral and dorsal aorta

– afferent & efferent vessels

– Capillary networks

- no heart

Cephalochordata

Brain

– Shares homology with vertebrae brain

– not differentiated

– Cluster of cells at an anterior end of nerve cord

What is the similarity between Cephalochordate and vertebrate brains?

– Same gene expression that organize the forebrain, midbrain, and hindbrain

summary:

subphylum Urochordata

-tunicates

-all marine

-Class Ascidiacea

-4 chordate traits only in larvae

-w/ verts evolved from common ancestor

Urochordata:

Class Ascidiacea

-ascidians: sea squirts

-sessile adults

-planktonic larvae

Urochordata CNS:

sensory vesicle (2)

-ocellus: photoreceptive organ

-otolith: gravity sensitive

Urochordata CNS:

visceral ganglion (2)

-sends nerve tracts to musculature

-receives sensory nerves from tail

Urochordata CNS:

nerve cord

-ciliated ependymal cells around neurocoel

Urochordata CNS:

neural crest cells (4)

-in verts, derived from nerve cord

-differentiate into different cells → structures (teeth, bone, neurons)

-also present in ascidian larva (pigment cells)

-in aquatic craniates, pharyngeal slits gave rise to gill slits (mm, nerves, active pumping H20)

who did the craniates evolve from?

-chordates

chordates with a head

-more complex movt and feeding

-unique characteristic: neural crest

what is the most basal craniate?

Hagfish

summary:

Craniata - hagfish

-cartilaginous skull

-lack jaws and vertebrae

-retain the notochord in the adult

-30 species (all marine)

-feed on detritis

-produce mucous as a defense

hagfish (craniata)

when did vertebrates appear?

-cambrian period: craniates gave rise to vertebrates

what are the most basal vertebrates?

-lampreys

summary:

vertebrata - lampreys (6)

-cartilaginous skeleton

-lack jaws but have rasping tongue & teeth

-retain notochord in adult

-primitive vertebrate with enclosed notochord

-35 species

-marine & freshwater

what was one of the most significant steps in early vertebrate evolution?

-development of jaws in primitive fishes

summary:

vertebrates - gnathostomes

-transition from suspension feeding of the agnathans to selective feeding

-paired pectoral & pelvic fins

-2 lines of evolution: Chondrichthyes and Osteichthyes

gnathostomes:

Chondrichthyes summary (4)

-cartilaginous fish

-lack a swim bladder

-placoid scales

-internal fertilization: oviparous & viviparous

what groups are within class chondrichthyes?

-sharks

-rays

-700 species

chondrichthyes:

skeleton

-cartilaginous fish

-cartilaginous vertebral column replaces notochord

chondrichthyes:

swim bladder (3)

-no swim bladder

-large liver contains a proportion of oil

-pectoral fins provide lift

chondrichthyes:

placoid scales

-dentine & enamel

-surface denticles (small “teeth”)

-develop dermis (project thru epidermis)

chondrichthyes:

reproduction (4)

-internal fertilization

-males: clasper on pelvic fin

-oviparous: tough, leathery egg cases
-viviparous: nutrition via yolk, uterine milk, ‘placenta’

who are the osteichthyes?

-bony fish

Subclass Actinopterygii and Sarcopterygii

who are part of the Actinopterygii subclass?

-ray finned fishes

(trout, etc)

(eels)

who are part of the subclass Sarcopterygii?

-lobe finned fishes

(lungfishes)

Actinopoterygii

summary (3)

~225 mya (late Triassic)

-fins supporte by numerous endoskeletal rays

-muscles that control the fins are within the body

Neopterygii (grp within Axctinopterygii)

-teleosts

-20000 living species (salmon, trout, catfish, eeils, perch, etc)

sturgeons (4)

-largest species of fresh water fishes (8m, 1400 kg)

-some migrate between fresh water and marine habitat (distances over 2500 km)

-toothless

-live up to 100 yrs (reach sex maturity ~20 y)

palaeonisciformes

-extinct grp of Actinopterygii

who are part of the subclass Sarcopterygii?

lungfishes

coelacanth

Sarcopterygii

summary (lungfishes) (3)

-fleshy fins composed of soft muscles

-gave rise to terrestrial verts

-three extant genera of lungfishes

what did the tetrapod limb evolve from?

Sarcopterygian fin

Lepidosiren

summary (3)

-Sarcopterygii

-can breath air when oxygen levels in water drop or when water dries out

-prominent notochord

Coelacanth

summary (7)

-thought to have become extinct 75 mya

-specimen found (1938) off S. Africa

-new species discovered off Indonesia

-tiny vertebrae

-especially well-developed notochord

-possess a swim bladder (filled with fat)

– Lobe fins used to support and position fish in currents