Comparative Embryo Morphometry

Adult brains differ between species in the proportional sizes of their major subdivisions. The telencephalon, for example, occupies at least 70% of the entire brain in parrots, 55% in quail, and less than 50% in adult chickens.
Embryos2
How do these differences in brain region proportions arise developmentally? Do the proportions differ from the beginning (i.e., from the stage where these regions are specified developmentally)? Does the telencephalon grow more quickly in some species than in others? Or does it grow for a longer period of time? To answer these questions, we are constructing growth curves for all major brain regions in several species of birds. The figure on the left shows embryos of (A) a bobwhite quail, (B) a domestic chicken, and (C) a budgerigar (parakeet) at 6 days of incubation. All three embryos are drawn to the same scale (bar =1 mm). The first thing you might notice is that the chicken embryo is considerably larger and more mature. Indeed, the growth curves indicate that chickens grow and mature significantly faster than other birds. Second, you might notice is that budgerigar embryos have surprisingly small eyes. This is interesting because the main brain region getting inputs from the eyes, the optic tectum (a section through this region is shown on the right),
MuESection
is also significantly smaller in budgerigars than in the other species, relative to the remaining brain and at comparable stages of development. This difference in optic tectum size is probably caused by a species difference in how much tissue is initially allocated to become optic tectum (rather than some other brain region). Surprisingly, the telencephalon is not much bigger in budgerigars at early stages of development. Our working hypothesis is that adult budgerigars have unusually large telencephalons because telencephalic neurogenesis is selectively delayed (i.e. proliferation is prolonged) in budgerigars, relative to chickens and quail.