This document discusses early developmental similarities between animal embryos and whether they can be used to determine homology of late larval structures. It notes that while cleavage patterns and early cell lineages may be similar between distantly related animals, the fate of blastomeres can vary between species, making it difficult to infer homology of later tissues based on early development alone. Comparative anatomy, not embryology, is argued to be the primary basis for determining homology. While some larval structures like ciliated bands develop from similar blastomeres in spiralians, variability in developmental processes means early similarities do not guarantee late similarities.

1. What can early developmental similarities tell us about the
homology of late larval structures?
Conklin 1897 Lambert 2010
Siewing 1969
Lyons et al 2015
Cleavage patterns, fate maps and cell lineages…
• Andreas Hejnol
• Bruno Vellutini
• Chema Martin-Duran

2. Cleavage Fate Map Cell lineage

3. Spiralia “What a wonderful parallel is this between animals so unlike in their end stages!
How can such resemblances be explained?”
Edwin G. Conklin

4. Spiral cleavage - a homologous character complex
• Four quadrants
• Spiral arrangement
• fate of blastomeres:
• 4d endomesoderm
• ‘ectomesoderm’ 1st, 2nd or 3rd micromeres
Conklin 1897
Lambert 2010
Siewing 1969
Lyons et al 2015
I. Similarity
II. Test for congruence on a tree

6. Homology of the feeding apparatus of spiralian larva
Patella
(after Patten 1886)
Crepidula
(Werner 1956)
Metatroch: McMurrich 1885
3rd micromemere
Polygordius
(after Woltereck 1904)
Metatroch
Prototroch
Metatroch
Prototroch
Hejnol et al
Prototroc

7. prototroch
prototrochmetatroch
1a (left) / 1d (right)
1b (ventral)/1a (left - dorsal) / 1c (right - dorsal)
2b
2a (left) / 2c (right)
3d (left) / 3c (right)
3a (left) / 3b (right)
1a (left) / 1d (right)
1b (ventral)/1a (left - dorsal) / 1c (right - dorsal)
2b
2a (left) / 2c (right)
3d (left) / 3c (right)
prototroch
metatroch
Crepidula fornicata Ilyanassa obsoleta

8. Is there a right or wrong? - Ectomesoderm
Lyons et al 2015After Hejnol et al 2007 - updated after Smith et al 2011 and Lyons et al 2015

9. Can we use early fate maps to homologize
late larval/adult tissues?

10. Can we use fate-maps to
homologize organ systems?
• The variability of blastomere fates between
species renders a straightforward homologization
of later larval and adult tissues impossible:
• We can not infer homology of tissues because
they arose from the same blastomeres.
• We can not reject homology because they arose
from different blastomeres.
Lyons & Henry 2014

11. Apical organCiliated band
Endoderm Mouth
BryozoanSpiralian

12. 1st Quartet
2nd Quartet
3rd Quartet
Macromeres
animal
vegetal
• The general organization along the primary embryonic axis detemines the potential of early blastomeres
D
V
R
L
16 cells:

13. Ciliated bands

14. Frank Rattay Lillie
Edmund B. Wilson 1894
“we must primarily take anatomy
as the key to embryology, and not the
reverse.
Comparative anatomy, not comparative
embryology, is the primary standard for
the study of homologies, and hence of
genealogical descent.”
Lillie 1895

15. The underlying molecular mechanisms allow the loss of
conserved cleavage patterns
• The complete loss of the spiral cleavage program in several
spiralian lineages
• Mollusca: cephalopods
• Gastrotrichs
• Platyhelminthes: Euneoophora)
• Lophophorates: brachiopods, ectoprocts
• maybe Gnathifera (Rotifera)
Santagata et al 2012Martin-Duran et al 2017
Owenia fusiformis
Terebratalia transversa
Novocrania anomala

16. Conclusions
• The more detailed the studies the more they differ.
• Fate maps can not be used to homologize larval or
adult structures.
• Embryos do not recapitulate. They evolve.
Leaving Hell (Inferno XXXIV), Dante, Divina Commedia,
Urbino and Ferrara 1477-1478
Blastaea
Gastraea
Trochaea

17. @Hejnol_lab

19. Homologous organs formed through different
developmental processes
from different textbooks - coloured by Kaul-Strehlow & Stach 2013

20. Cnidaria make homologous endoderm in different ways…

21. "Comparable parts [...] are homologous if their
correspondences are the consequence of the
same genetic or epigenetic information,
and consequently of the same origin."
definition after Dohle 1989
corroboration
corroboration
1st
step:
Delimitation of homology
a
2nd
step:
Test for congruence
b
CHARACTER
HOMOLOGY
ANALOGY
no corroboration
HOMOPLASY
no corroboration
Character analysis
- relative position
- correspondence in structure (similarity)
- gene expression, etc.
Phylogenetic test
- distribution on a phylogenetic tree
- test of parsimony