Heterochrony in Avian Brain Development
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Heterochrony in Avian Brain Development Georg F. Striedter Department of Neurobiology & Behavior and Center for the Neurobiology of Learning & Memory, University of California, Irvine. Result #2 - The optic tectum is consistently smaller in budgerigars than in quail or chicks. The Question.

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Heterochrony in Avian Brain DevelopmentGeorg F. StriedterDepartment of Neurobiology & Behavior and Center for the Neurobiology of Learning & Memory, University of California, Irvine

Result #2 - The optic tectum is consistently smaller in budgerigars than in quail or chicks

The Question

Result #1 - Chicken time runs fast

How does evolution change development to generate diverse adult brains?

Although the illustrated embryos are all 6 days old, the chicken embryo (B) is considerably larger and more mature than the quail embryo (A) or the budgerigar embryo (C). Scale bar = 1 mm.

Optic Tectum

Medulla

The Approach

Quantify brain region growth in embryos of different species

The Species

This suggests a species difference in how much tissue is initially allocated to become optic tectum.

Developmental Stage

Brain Size

Result #3 - Telencephalic neurogenesis appears selectively delayed in the budgerigar

Budgerigar

Northern Bobwhite Quail

Telencephalon

The budgerigar telencephalon goes through a relative growth spurt between days 6 and 7.

Domestic Chicken (White Leghorn strain)

If you stretch chicken time by 40%, then chickens are pretty much like bobwhite quail, in terms of when they reach a given stage (Hamburger-Hamilton) and a given brain (or body) size.

Relative

Growth Spurt

Time to hatching: 18, 23, and 21 days

Adult Brain Allometry

This growth spurt could be due to budgerigars selectively in-creasing telencephalic cell cycle rate or selectively delaying telencephalic neurogenesis. The latter hypothesis is supported by the following observation:

Cumulative BrdU Labeling

The proliferative cell layer (PCL) is thinner in budgerigars (A-C) than in quail (D-F) at 6 days of incubation, even though the remaining brain’s maturity is similar in both species.

Scale = 500 µm for A&D, 50µm for others.

PCL

Tc ~ 10 hrs

The Conclusion

The brains of embryonic birds vary a) in overall growth rate, b) in how much tissue is initially allocated to the various brain regions, and c) in neurogenesis timing.

We are now using cumulative BrdU labeling to test the hypothesis that brain cell cycle times are 40% faster in chickens than quail.


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