1. Evolution of Life Histories: Echinoderms as Model Systems

2. Types of development in sea stars
The typical sea star larva is the bipinnaria, it drifts
in the water and feeds on algae : planktotrophic
Planktotrophic development is the ancestral type.
Other types of development evolved from planktotrophic
Some species have swimming larvae that
rely on maternal yolk for food: lecithotrophic
Other species protect (brooding) their eggs and young
until a juvenile stage is reached

3. 0.5 mm
1.0 mm
0.2 mm
(in maternal brood chamber)
1 mm
2 mm
0.5 mm
2.5 mm
1.5 mm

4. Evolution of developmental strategies depends
on the balance between many selective forces:
Think about evolution in terms of Trade OFFS
Planktotrophic
Larvae
low investment/egg
high fecundity
wide dispersal
must find food
long development
very high mortality
Lecithotrophic
larvae
investment is higher
lower fecundity
limited dispersal
no food needed
limited duration
some mortality.
Protected
development
very high investment
very low fecundity
no dispersal
no food needed
parent protection
low mortality

5. Studies of Evolution of Development
Stepwise transition from indirect to direct larval development in sea urchins
Studies of Evolution of Development
In Sea Urchins
The first step involves the switch from obligatory to facultative feeding
The second step involves the loss of the ability to feed (but vestigial structures may remain)
3. The third step involves the minimizing of the amount of time required to complete metamorphosis (regulatory genes)
Complete loss of structures associated with feeding in the larvae
Schmoo from Lil Abner cartoon

6. Studies of Evolution of Development In Sea Urchins
Australian Sea urchins of the genus Heliocidaris

7. Australian Sea urchins of the genus Heliocidaris
Gastrula pluteus larva pluteus with rudiment

8. Egg Diam (microns) 90 430 Days to juvenile 42 4 Egg biochem Protein
Heliocidaris tuberculata
Heliocidaris erythrogramma
Two strikingly different
pathways of early development
lead to a similar outcome
H.t H.e
Egg Diam
(microns) 90
430
Days to
juvenile 42 4
Egg biochem
Protein
rich
Lipid
Larva
Feeding, complete
relic
tiny enormous

9. Fate Map Neurons Gut Skeletal Coelom Ectoderm Stipple: non-ectoderm
N : Neurons
C : Coelom
E : Ectoderm
G: Gut
P : Pigment
S : Skeletogenic
Heterotopies and Heterochronies
Big egg
Little egg
Neurons
Gut
Skeletal
Coelom
Ectoderm
Divisions 1 2 3
“Modifications are not limited to deletions in the larval program of development but profoundly affect the embryonic origin of adult tissues and structures as well”
Wray & Raff, 1991

10. Heterochronies in Wnt genes and juvenile rudiment development
- Wnt genes important regulatory transcription factors for signaling pathways in animal development.
In Echinoderms, Wnt genes are expressed in tissues that form the early adult rudiment and the adult radial nervous system.
Wnt expression peaks at 28 days in Ht feeding larvae and hrs in He nonfeeding larvae
Wnt expression domains do not change (no heterotopies) but are accelerated in rudiment both in absolute time as well as relative to the formation of other larval structures; this is part of overall earlier onset of development of adult structures.
They parallel the accelerated rate of development seen in embryonic and larval tissues.

11. Studies of Heliocidaris Hybrids
by Raff et al. 1999
Reciprocal crosses
H.e. eggs x Ht sperm
H.t. eggs x He sperm

12. Studies of Heliocidaris Hybrids by Raff et al. 1999
H. “tiny” eggs x H. “enormous” sperm
results in a lethal hybrid cross
due to loss of larval polarity (at gastrulation) … Explain?
Gastrulation is first stage where bilateral symmetry is established
In Ht and other species with feeding larva only the A-V axis is established maternally; other axes are determined zygotically. So the sperm provides information that is essential to development of other (bilateral) axes.
In He, all of the axes must be determined maternally; sperm no longer provide information relevant to larval polarity.

13. Studies of Heliocidaris Hybrids
by Raff et al. 1999
H. t eggs x H. e. sperm
results in a lethal hybrid cross
due to loss of larval polarity (at gastrulation)
H. ‘tiny’ sperm x H. “enormous” eggs
indirect developing paternal influence on direct developing egg results in a novel ontogenetic species

14. Explain…. H. tuberculata sperm x H. erythrogrammica eggs
Formation of a complete gut with surface connections
Spicules
Development of the adult rudiment within a vestibule on the left side of the mouth
Pluteus arms (feeding structures) and a complete ciliary band that lies between the mouth and anus
Ectodermal territories are restored
Hybrid Larva
Explain….

15. Explain?
Indicates a strong paternal role in determination of ectodermal territory and the feeding structures they form

16. Summary of Hybridization Studies
Ht (m) x He (f) hybrids produced a novel synthetic morphology
 Indicates that some key aspects of development, particularly the feeding structures are of paternal origin (I.e. the sperm).
 The genes for oral ectoderm, digestive system etc. are no longer expressed in He sperm
Ht x He (m) fertilization (when successful) resulted in a developmental arrest at gastrulation. The developing embryos were unable to establish proper embryonic axes
 Changes in how polarity is established took place in the transition from feeding to non feeding; In He the establishment of embryonic polarity (dorsal-ventral axis) is no longer sperm mediated; it is due to maternal patterning.

17. In Heliocidaris erythrogramma (big eggs)
A mosaic of conserved and radically changed developmental features
A. Conserved
General rules of primary A-V axial formation and cleavage (except micromeres)
Embryonic cell division rates and time of gastrulation
Wnt8 patterning (of the entomesoderm, the A/V axis of the larva
and in the hydrocoel-vestibule complex; Wnt genes code for proteins that are integrally involved in animal development)
Ferkowicz and Raff 2001
Kaufman and Raff 2003

18. A mosaic of conserved and radically changed developmental features
B. Lost or Modified
Cell lines and cell fates (heterotopies)
Expression of genes that control the appearance of pluteus features including arms, oral ectoderm and gut (no longer provided by sperm)
Wnt expression is accelerated in rudiment both in absolute time as well as relative to the formation of other larval structures.
These “heterochronies” have a highly conserved developmental outcome and are part of overall early onset of development of adult structures
Ferkowicz and Raff 2001
Kaufman and Raff 2003