Transcript Document

Chordate and Vertebrate Origins
Protochordates
Phylum Hemichordata: acorn worms (Balanoglossus)
Phylum Chordata:
• Subphylum Urochordata:
sea squirts (Ciona/Molgula)
• Subphylum Vertebrata:
vertebrates
Common Protochordates Characteristics
Possess some or all chordate characteristics
All are suspension feeding marine organisms
• cilia transports water, mucus collects food, cilia transports to GI tract
Larvae are planktonic, adults are usually benthic
• all chordate characteristics may not appear
at once
Hemichordate larvae (tornaria) resemble
echinoderm larva
• recently validated by molecular techniques
Subphylum Urochordata: Tunicates
3 groups: Ascidiaceans, Larvaceans, and Thaliaceans
Sea squirts, appendicularia, & salps
• approximately 2000 species; most are sea squirts
• wide distribution: fouling organisms to deep sea forms
Planktonic larvae; sessile adults
Suspension feeder specialists
Tunic: flexible outer layer, thick & can be brightly colored
• diagnostic for the group
Subphylum Cephalochordata: Branchiostoma/Amphioxus (lancelets)
Adults exhibit all chordate characteristics
Cosmopolitan distribution in tropical and
warm temperate seas.
1-way water flow thru pharyngeal slits
Cilia driven filter feeding apparatus
Oral hood w/buccal cirri encloses pharynx
Food & mucus:Hatschek’s & epibranchial
groove • thread to gut • wheel organ
Water flows to atrium & out atriopore
Start to see characters of vertebrates
• chevron-shaped myomeres
• precursors to vertebrate organs
-- endostyle: thyroid
-- midgut cecum: liver & pancreas
• cardiovascular:
-- dorsal & ventral aorta
-- paired cardinal arteries; carotids
-- overall circulation similar to
vertebrates
-- pharyngeal arches; aortic arches
Branchiostoma/Amphioxus
Excretory/Osmoregulatory System:
• located in atria along pharyngeal slits
• parts: glomerulus, pedicel, solenocytes,
nephridial tubule, atriopore
-- single cell w/ pedicels surround
the glomerulus vessels
* bridge from glomerulus to nephridial tubule
Pedicels of the solenocytes differ from inverts
• similar to the processes of podocytes in
vertebrate kidneys
Phylogenetic History of Protochordates
Living protochordates: 500 million years of evolution independent of vertebrates
Actual chordate ancestors are extinct; minimal fossil record
• ancestral features can help determine phylogeny
Early theories:
Arthropods & Annelids to Chordates
• All 3: segmented, similar brain organization, similar
but inverted body plan
Weaknesses:
• annelid segmentation differs from myomere segmentation
• mouth & anus positions differ; no evidence of migration
during development
• overlooks protostome/deuterostome developmental patterns
Phylogenetic History of Protochordates
Cephalochordates from Echinoderms: W. Garstang
Echinoderms are deuterostomes; a more likely chordate ancestor
• phylogenetic key are echinoderm larvae
According to Garstang
“Chordates arose from larval echinoderms”
Chordate characteristics 1st appeared in
echinoderm larva
• tornaria larvae share traits with echinoderm larvae
Bilateral symm.
1-way gut
• hemichordate larvae share traits with chordate larvae
Body elongation creates tail that could undulate; post anal tail formed
• causes CCB to elongate, move dorsal & fuse; forms a proto-dorsal nerve cord
• causes CAB to elongate; forms a proto-endostyle
Only needs pharyngeal slits & notochord to be considered a chordate
Which selection pressures are involved?
Phylogenetic History of Protochordates
Changes in body plan must have been advantageous
As size increases SA covered w/cilia cannot keep up
with increase in volume (Ch 4)
• individuals with alternate locomotion favored
• evolutionary solutions: notochord, segmented muscles,
further elongation
SA L2
V  L3
SA  V2/3
24 cm2
Same geometric principle applies for ciliated feeding
• endostyle & pharyngeal slits increase water flow; swimming increases flow
Garstang suggested adult form abandoned
If larvae > successful then adults then sexual maturity at larval stage
• escapes an echinoderm adult life
• new phylogenetic direction
How was metamorphosis into
an adult echinoderm terminated?
48 cm2
Heterochrony
Relative change in timing of developmental event
• change in ontogenetic onset, offset, or timing of a character appearance
• one process of evolutionary change
Paedomorphosis (“child form”): juvenile or
embryonic features are present in adults
(axlotls:gill retention in adults)
3 Mechanisms:
1. progenesis: body growth ends earlier;
sexual maturity achieved earlier than normal
-- paedomorphosis ≠ neotony
3. postdisplacement: features appear late;
features maintain juvenile characteristics
Heterochrony
Peramorphosis (“beyond form”): appearance
of ancestral features in adults; exaggerated
features or characters
3 Mechanisms
• acceleration: rate of growth increases
• predisplacement: onset of growth is early;
characters appear earlier than normal
• best example is Irish
elk
• example of
hypermorphosis
Phylogenetic History of Protochordates
Garstang viewed protochordate evolution as a series of paedomorphic steps
However!
Urochordate larvae (ascidian) have decreased larval time
• Garstang’s theory requires a reversal of time spent as larvae -- unlikely
Ascidian larval morphology divergent
• gut morphology is only analogous to Amphioxus
• true gut does not develop in larvae
Similarities of larval form can be
explained by convergence alone
• strong selection pressures in an
aquatic environment
Phylogenetic History of Protochordates
Malcolm Jollie’s Dipleuruloid Theory
“Similarities of echinoderm & chordate deuterostome development too strong to ignore”
• proposes a theoretical ancestor: Dipleurula
Dipleurula: small, bilateral, & ciliated
• larval characters found in both echinoderm and hemichordate larvae
Pharyngeal slits arose among Hemichordates
• assists ciliary and mucus feeding system
Notochord, tail, nerve cord, & myomeres
develop to serve adult forms
Chordates Split:
• one group secondarily moves back to
filter feeding niche
-- cephalochordates & urochordates
• one group evolves as active predators
Emphasizes trend toward predation
Instead of filter feeding
Chordate Clade
Vertebrates arose within the deuterostome radiation
• includes echinoderms & hemichordates
Chordate evolved from a common echinoderm/chordate ancestor
• chordates did not evolve from echinoderms (sensu Garstang)
Chordate body plan established early in time, among invertebrates
Basic chordate plan includes:
pharyngeal slits, notochord,
dorsal hollow nerve cord, and a
post anal tail.
Locomotion relied upon a notochord
and serially segmented musculature