Typical Sea star life cycle represented by Crown-of thorns Protostomes and Deuterostomes Embryological Evidence for Distinction of the Chordate-Echinoderm Line and the Mollusc-Annelid line (remembering they are all triploblastic) General processes of embryogenesis, from zygote to multicellular triplobastic larva 1. Cell division 2. Cell differentiation 3. Formation of primary cell layers and tissues 4. Formation of the principal body cavity (coelom) Similarities in early development may be taken as indication of common ancestry!! (indeed, Ernst Haeckle went as far as saying “Ontogeny Recapitulates Phylogeny”) Why might early development be such a reliable indicator of phylogeny? 5 Organogenesis 6. Growth Phylogeny of the “big nine”: Protostomes and Deuterostomes Cellular level Multicellular Patterns of embryonic cell division Equal vs. unequal : ? no bearing on phylogeny Complete vs. incomplete: no bearing on phylogeny (holoblastic vs. meroblastic) ? Highly Derived Spiral vs. radial divisions: important…!! The orientation of the cleavage plane is inherent in the genetic program and this appears to be a conservative character. Protista 1 Planes of Division Animal pole Radial Cleavage in echinoderms, acorn worms, chordates…. Longitudinal plane A-V Axis Equatorial plane Vegetal pole morula Oblique plane Note how blastomeres end up stacked neatly one on top of another or directly to the side Radial Cleavage in echinoderms, acorn worms, chordates…. morula blastula blastula Note how blastomeres end up stacked neatly one on top of another or directly to the side Spiral Cleavage in molluscs, annelids Note oblique plane of mitotic spindle at 3rd division and the spiral arrangement of the blastomeres from the third division onward (better space use than radial) Goulding MQ. 2009. Cell Lineage of the Ilyanassa Embryo: Evolutionary Acceleration of Regional Differentiation during Early :10.1371/ journal.pone.0005506 Development. PLoS ONE 4(5): e5506. doi The Third Cleavage Patterns of the Dextral and Sinistral Limnea stagnalis Embryos and Their Adult Snails. (A) The 4-cell to the 8-cell stage of the sinistral and dextral L. stagnalis embryos observed by light microscopy. Shibazaki Y, Shimizu M, Kuroda R (2004) Body handedness is directed by genetically determined cytoskeletal dynamic in the early embryo. Current Biology 14:1462-1467.P.Z. Myers:// scienceblogs.com/pharyngula/2006/04/spiral_cleavage.php 2 Sponges cleavage and morphogenesis, leading to the larvae. 1–4 Cleavage patterns in sponges: incurvational (1), polyaxial (2), radial (3), and chaotic (4). Three main form of sponges blastula: stomoblastula (5), coeloblastula (6), and stereoblastula (7). Different larval types of sponges: amphiblastula of Calcaronea (Calcarea) (8);; calciblastula of Calcinea (Calcarea) (9);; coeloblastula (10), parenchymella (11), and disphaerula (12) of Halisarca (Halisarcida);; parenchymella of Vaceletia crypta (Verticillitida) (13);; pseudoblastula of Chondrosia reniformis (Chondrosida) (14);; trichimella of Oopsacas minuta (Hexactinellida) (15);; juvenile of Tetilla under direct development (16);; parenchymella of Tethya aurantium (Pallas, 1766;; Hadromerida) (17);; coeloblastula of Polymastia robusta Bowerbank, 1866 (Hadromerida) (18);; parenchymella of Dictyoceratida (19);; parenchymella of Incurvational polyaxial freshwater Haplosclerida (20);; parenchymella of Poecilosclerida (21);; cinctoblastula of Homoscleromorpha (22). What factors could potentially be important in determining the fate of individual cells in a developing embryo? radial chaotic Cnidaria: radial, sometimes chaotic (different in sibs!) Placozoans: radial Ctenophores: biradial Radial cleavage is the ancestral condition Timing of cell fate-determination: evidence of phylogenetic differences Blastomere Separation Expts Abnormal larvae Small but Normal larvae Echinoderms ,chordates, etc. Regulative determination Or indeterminate cleavage Echinoderms ,chordates, etc. Regulative determination or indeterminate cleavage Monozygotic twins How do cells know where to go and what to become? Chemical signals in the cytoplasm Other cells Other physical factors Timing of cell fate-determination In echinoderms, acorn worms & vertebrates the fate of embryonic cells is not established until later in cleavage (16 cell or later) In molluscs and other “protostomes” the fate of embryonic cells is predetermined by cytoplasmic factors very early in cleavage Molluscs, Annelids etc. Mosaic determination or determinate cleavage 1 in 15 thousand human pregnancies result in monozygotic Quadruplets There is no Two sets of healthy Octuplets No healthy nonuplets Polyembryonic 9-banded Armadillos (only six species of genus Dasypus) can have Purple sea urchins But not in Earthworms Or Snails consistent phylogenetic pattern in the type of blastula formed Coeloblastula Discoblastula Stereoblastula Periblastula 3 46 Fig. 7: The trend in sponge embryonic development publications between 1870 and 2006. Formation of larva in sponges There is no consistent phylogenetic pattern in how gastrulation is achieved invagination ingression delamination Fig. 8: Different types of morphogenesis in sponges resulting in larva formation: A. Cell delamination (Hexactinellida – Oopsacas minuta);; B. Morula delamination (Demospongiae: Dendroceratida, Dictyoceratida, Halichondrida, Haplosclerida);; C. Invagination (Halisarca dujardini, Demospongiae);; D. Multipolar ingression (H. dujardini, Demospongiae);; E. Multipolar egression (Homoscleromorpha, Demospongiae);; F. Polarized delamination (Demospongiae: Poecilosclerida and Halichondrida);; G. Excurvation (Calcaronea, Calcarea);; H. Formation of blastula (pseudoblastula) by means of ingression of maternal cells into the embryo (Chondrosia reniformis, Demospongiae: Chondrosida);; I. Unipolar proliferation (Demospongiae: Verticillitida – Vaceletia crypta). (From: Ereskovsky and Dondua 2006). epiboly Ereskovsky 2007 Phylogenetic implications of early development: • Patterns of cell division Important differences exist in: the fate of the blastopore Snail gastrula sea urchin gastrula • Timing of cell fate-determination • Mechanism of blastula and gastrula formation • Fate of the blastopore • Origin of the mesoderm • Method of coelom formation Becomes the mouth Becomes the anus Important differences exist in: the origin of the mesoderm Snail gastrula sea urchin gastrula cleavage of 4D cell outpocketing of archenteron 4 Important differences exist in: the mechanism of coelom formation Enterocoelous coelom formation Important differences exist in: the mechanism of coelom formation Snail gastrula schizocoelous coelom formation Finally: Protostomes Deuterostomes Molluscs, Annelids, etc. Echino., chordates etc. Spiral cleavage Radial cleavage Mosaic cell fate Blastopore is mouth Regulative cell fate Blastopore is anus Mesoderm from 4D cell Meso. from archenteron Coelom by splitting of solid mesodermal mass Coelom by pinching off of enterocoels 3 conditions with respect to the coelom: Acoelomate Pseudocoelomate Coeloms, Pseudocoeloms and Phylogeny Review of Principal Body Plans Eucoelomate Acoelomate Condition No coelom “False” coelom “True” coelom Traditional View is that these are homologous primitive and derived states Eucoelomate condition Pseudocoelomate condition 5 Phylogeny of the “big nine”: Protostomes and Deuterostomes Cellular level Peritoneal cavity Pleural Cavity Pericardial Cavity Multicellular Are mesodermal lining that surround organs in vertebrates Protista 6