Vertebrate Development Maternal to Zygotic Control (Advances in Experimental Medicine and Biology)

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theory, but those accusations were convincingly refuted (Richardson et al. 1997 ;
Richards 2009 ). The existence of the phylotypic stage in both vertebrates and
invertebrates was recently confirmed by three independent studies using bioinfor-
matics analysis of the genes expressed at different stages (Domazet-Loso and
Tautz 2010 ; Kalinka et al. 2010 ; Prud'homme and Gompel 2010 ). These studies
showed that the genes expressed at the phylotypic stage originated further back in
evolutionary history than genes expressed at other stages, and evolved at slower
rates. This indicates that the similarity in morphologies is indicative of a highly
conserved developmental program.

Blastoderm

Adult

YSL

G. gallus

T. scripta

M. musculus

M. domestica

Phylotype

Teleost

D. rerio

pa

som

h

Amphibian

X. laevis

bc

pa h pasom

h som

pa

som

h

pa

lb

som

h

pa

som

h

pa

VE

Mammalian

TE PB

bczp

ICM

Avian/Reptilian

KS

ap ao

hb bc sgs

MZ

ao

a

b

c

Fig. 7.3 Hourglass model of development. Diagrams showing morphologies of different classes
of vertebrates at three different stages of development. (a) Comparison of a blastoderm stage
chicken embryo (EGK stage XI), a mouse E3.5 blastocyst and E 5.5 stage embryo, a unilamellar
blastocyst at day 5.5 (McCrady Stage 8–9) of a possum (M. domestica), an X. laevis blastula (7 h),
and a zebrafish blastoderm stage embryo (3.5 h). (b) A comparison of a 2-day-old chick embryo,
a 4-day-old turtle (T. scripta) embryo, an E8.75 mouse embryo, a day 10 (McCrady Stage 27) pos-
sum embryo, a 32 h (Stage 28) Xenopus tadpole, and a 28 h (prim-6) zebrafish embryo. To facili-
tate comparison, the orientation of the chick embryo has been flipped to match the other embryos.
(c) A juvenile chicken, an adult turtle (T. scripta), possum (M. domestica), frog (X. laevis) and
zebrafish (D. rerio). ao area opaca, ap area pellucida, bc blastocoel, h heart, hb hypoblast, ICM
inner cell mass, KS Koller’s sickle, lb limb bud, MZ marginal zone, pa pharyngeal arch, PB peri-
blast, sgs subgerminal space, som somites, TE trophectoderm, VE visceral endoderm, YSL yolk
syncytial layer, zp zona pellucida (Mate et al. 1994 ; Selwood et al. 1997 ; Keyte and Smith 2010 ;
Kimmel et al. 1995 ; May 2013 ). Note: Drawings are not to scale

W. Tseng et al.

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