Ascidians: sessile filter feeders bag like body covered by a tunic Phylum Chordata (49,400 species) (tunicin is a type of cellulose) solitary SubPhylum Vertebrata (48 K species) SubPhylum Urochordata SubPhylum Cephalochordata out Siphons Colonial in Compound • Chordates are 1 of 4 deuterostome groups • 1400 species of invertebrate chordates Ascidians: (sea squirts) Endostyle (thyroid gland homologue produces iodine rich mucus) Pharyngeal slits sessile filter feeders bag-like body covered by a tunic made of cellulose in some spp. siphon Atrial cavity Water enters incurrent siphon passes through porous mucus and exits into atrial cavity through pharyngeal slits. Particles trapped in mucus are swallowed. Megalodicopia hians (Megabyte sea squirt) a b Many Ascidians are solitary, but most are colonial (b) or compound (c,d), the latter living within a single tunic sheath an often have a shared atrial siphon. c d Tasmanian devil-squirt catches prey using a “fly-trap” system Deep sea tunicate with a pharynx that is lacking ciliated stigmata 1 Chordate-like characters of Ascidians are more prominent in the tadpole larva pharynx Post anal Tail Nerve cord Adhesive paillae Radical metamorphosis: -- Involves loss of notochord, tail, dorsal hollow nerve cord, swimming musculature -- Rapid growth of region around buccal siphon causes the body to rotate bout 90 deg Atrial siphon --Atrium expands endostyle --# gill slits increases Notochord Stomach and intestine Buccal siphon Sub Phylum Urochordata Class Larvacea: Small (5-6 mm) planktonic, tadpolelike; lives in midwater and secretes a gelatinous house. One Hypothesis states that Larvaceans evolved from ascidian tadpoles by a process known as Neoteny but there is new evidence on this relationship (later) Larvacean Tadpole http://vimeo.com/32278043 SubPhylum Cephalochordata (lancelets; 2 dozen spp) -- resemble urochordates in feeding biology -- but have much in common with vertebrates: distinct notochord that persists through life dorsal hollow nerve chord, post anal tail…. Dorsal hollow nerve cord Notochord Muscles Gill slit Fig 24.9 Post anal tail Endostyle Reproductive organ Transverse muscle anus Gill slits Notochord dorsal nerve cord 2 SubPhylum Cephalochordata -- feeding biology resembles urochordates -- but have much in common with vertebrates: distinct notochord that persists through life dorsal hollow nerve chord Clearly not a Vertebrate, which have: -- a nerve chord elaborated into a brain -- a protective cranium -- a notochord replaced by a vertebral column But generally accepted as the closest living relative of vertebrates and much studied as a model of vertebrate ancestors Deuterostome Phylogeny • 4 phyla, different from all others but also considerably different from each other Echinoderms, Hemichordates, Xenoturbella, Chordates • key characters are embryological, morphological, and more recently genetic • A variety of scenarios; as early as 1890 • Key questions: • echinoderm-hemichordates-xenoturbellids already discussed • chordate relationships (Urochordates?)?? • ancestral chordate??? Summary : According to Garstang how chordates and vertebrates evolved How did vertebrate innovations evolve? 1. Chordates formed as a deuterostome larva in an ascidian when its larva was modified to a tadpole-like larva with a notocord, DNC, Endostyle (origin) 2. Tadpole like larva becomes adult Larvacean-like chordate through neoteny (a type of paedamorphosis) 3. Selection favors traits that continue to improve locomotion and activity: • complex sensory organs, brain • complex musculature, respiration • addition of bone ....Vertebrate condition (this also happenned in Urochordate line later) Garstang speculated that at some point larvae developed gonads and reproduced in the larval stage and a new group of free swimming animals evolved, perhaps two separate times Figure 23_05 Cladogram consistent with Garstang’s view • Garstang called this process neoteny (somatic development is slowed), a type of paedomorphosis (adult assumes traits previously seen in juveniles) in which juvenile or larval traits appear in the adult body Selection for increased swimming etc. Neoteny (paedomorphosis) Ascidacea (with tadpole that has evolved chordate characters) 3 Dorsoventral Inversion in Adult: Current Hypothesis that is contradictory to Garstang’s hypotheses. A dorsoventral inversion of body plan axis in an adult worm like animal was a key event in the evolution of deuterostomes. Dpp (suppresses neural fate) and its homologue BMP-4 are expressed in a pattern that supports inversion. SOG and its homologues (Chordin etc.) suppress dpp action Are Hemichordates like acorn worms representative of the common ancestor? http://www.mun.ca/biology/desmid/brian/BIOL3530/DB_Ch15/fig15_11.jpg Cladogram Showing New “Molecular” View Figure 23_05 Cladogram consistent with Garstang’s view Cladogram consistent With Cameron’s view Cameron, C.B., Garey, J.R. and Swalla, B. J. (2000) Evolution of the chordate body plan: New insights from phylogenetic analyses of deuterostome phyla. Proc. Natl. Acad. Sci.97: 4469-4474. See also Fig. 22-10 Two major clades , Echinoderm-Hermichordates and Chordates Common ancestors of chordate/ hemi-echinoderms had pharyngeal slits, dorsal hollow nerve cord. Larvacean like animal is ancestor of all chordates? Key New Contributions of Molecular Studies 1. Echinoderm-Hemichordates are sister groups therefore stem deuterostome: • indirect development with auricularia • adult with pharyngeal slits and dorsal nerve cord 2. Non-Larvacean Urochordates are united in a single derived clade. Larvacean clade is the stem sister group. Larvaceans have no larval stage. So larva played no prominent role in evolution of chordate characters. 3. New phylogeny contradicts Garstang’s hypothesis in several ways but is neutral with respect to D-V inversion 4
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