Heterochrony: the Evolution of

DevelopmentbyKenneth J. McNamara

Prepared by Ahmet VARIS

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OUTLINE

• Darwinian view• Aim • What is heterochrony?• Types of heterochrony• Relative Frequency of Heterochrony• Heterochrony and Sexual Dimorphism• Conclusion

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Darwin’s view of Evo/Devo

• “Embryology rises greatly in interest, when we look at the embryo as a picture, more or less obscure, of the progenitor, either in its adult or larval state, of all members of the same great class.’’

• ‘’This is one of the most important subjects in the whole round of natural history.”(Darwin ,1878, p.396 and p.386)

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AIM

• Understand how developmental stages evolve from ancestor to next generation.• Observe how species from a common ancestor have

different physiology as a result of their evolution of development.• Examples from species

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Evolution & Development

• Within populations of a single species, individuals do not all grow and develop at the same rate or for the same duration.• The same rule occur between closely related species, the

main morphological differences arise from variations to the rate and duration of growth. • It is now recognized that developmental genes, particularly

those regulating embryonic or larval development, play a major role in evolution

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HETEROCHRONY

• Earlier definition (1800s) ‘The term heterochrony was used to describe cases in which the ontogenetic sequence of events did not recapitulate or repeat the sequence of events in phylogeny’.• Ontogeny is recapitulation of phylogeny dependent on

the physiological functions of heredity (reproduction) and adaptation (nutrition).

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Embryos of different vertebrates share basic primitive features such as gills (red) and tails (blue)

(The Earth Through Time, Harold L. 8th edition, Chapter 6 - page 5)

(http://evolution.berkeley.edu/evolibrary/article/side_o_0/ontogeny_01)

The course of a chick's development, if recapitulation of phylogeny by ontogeny is tottally true

• Today’sdefinition of heterochrony ‘the mechanism that produces a parallel between ontogeny and phylogeny’.• Heterochrony involves a shift in the timing of

developmental processes so an event occurs earlier, later, or at a different rate in a taxon compared to its ancestor.

8(Leonard & James, 1993)

Embryonic development and stripe patterns of the zebras Equus burchelli (top) and grevyi (bottom). Illustration drawn by Carole B.

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HETE

ROCH

RON

Y Paedomorphosis

Progeneis

Neoteny

Postdisplacement

Peramorphosis

Hypermorphosis

Acceleration

Predisplacement

Paedomorphosis

• The evolutionary process in which larval or juvenile features of an ancestral organism are displaced to the adult forms of its descendants.• If the period of growth of the descendant form is

stopped prematurely: progenesis• If onset of growth is delayed: postdisplacement• If the rate of growth is less in the descendant than in

the ancestor: neoteny

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Second stage of the development is the last stage anymore.

The rate of growth is less when it compared to ancestor.

The onset of growth is delayed so the organisms development stop when it become mature and differentiate from ancestor.

(Kenneth J. 2012)

( https://www.geol.umd.eduBSCI392,10-5-07)X axis:time

Y axis: development

Peramorphosis• Phylogenetic change in which individuals of a species

mature past adulthood and take on traits which havent seen yet.• if the period of growth in the descendant is extended:

hypermorphosis• if the onset of growth occurs earlier in the descendant

than in the ancestor: predisplacement• if the growth rate is increased: acceleration

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Maturation occur later than ancestor

The speed of development is faster than ancestor

The structures begin development earlier than ancestor

(Kenneth J. 2012)

( https://www.geol.umd.eduBSCI392,10-5-07)X axis:time

Y axis: development

14Differences in shape of the skull between threedifferent breeds of dogs

Evolution of the domestic dog from the wolf by paedomorphosis

(Kenneth J. 2012)

Relative Frequency of Heterochrony

• Amphibians show a dominantly paedomorphosis, which may be related to their large cell size, causing a reduced rate of cellular division.• Paedomorphosis has occurred many times in frogs, for

example, resulting in the development of many miniature species. The 29 smallest species (with the smallest having an average body length of just 7.7 millimeters) are spread across five families and 11 genera.• Peramorphosis may have been more frequent in

dinosaurs, in particular being a major contributing factor to the evolution of very large body size 15

16Peramorphic evolution of protoceratopsian dinosaurs

(Kenneth J. 2012)

Heterochrony and Sexual Dimorphism

• Sexual dimorphism: The condition where the two sexes of the same species exhibit different characteristics beyond the differences in their sexual organs.• Size and morphological variance arises from either

variations in growth rates or differences in the relative times of onset of sexual maturity; in other words, it arises from heterochrony.

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The adult female of the blue boxfish, Stropiurichthys robustus, is paedomorphic compared with the male, resembling the juvenile in body shape and body patterning.

Photocorynus males live parasitically attached to the female.

(Kenneth J. 2012)

(http://www.fishbase.org/summary/Photocorynus-spiniceps.html)

Conclusion

• The evolution of organisms mostly caused from differentiation of heterechronic genes which influence development of organisms• We are (as homo sapiens) a classic example of

dissociated heterochrony, where some peramorphic features are developmental trade-offs for other, paedomorphic, features: some parts of our anatomy are relatively retarded, compared with our ancestors, but others have developed beyond. • So heterochrony is the interaction between development

and evolution from ontegenetic and phylogenic perspective. 19

Articles about Heterochronic Genes

• Koen Geuten and Heleen Coenen, Heterochronic genes in plant evolution and development.• Carl S. Thummel, Molecular Mechanisms of

Developmental Timing in C. elegans and Drosophila.• Eric G. Moss, Heterochronic Genes and the Nature of

Developmental Time.

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REFERENCES• Kenneth J. McNamara (2012), Heterochrony: the Evolution of

Developmen, DOI 10.1007/s12052-012-0420-3• Kathleen K. Smith (2002), Sequence Heterochrony and the

Evolution of Development, DOI 10.1002/jmor.10014• Timing is Everything: Morphogenesis, Heterochrony and

Evolution, 2016• Homeotic Genes and Body Patterns, http

://learn.genetics.utah.edu/• The Earth Through Time, Harold L. Levin, 8th edition, Chapter

6• Ontogeny and phylogeny, http://

evolution.berkeley.edu/evolibrary/article/side_o_0/ontogeny_01

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THANK YOU

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