Meeting Abstract

P2.46  Jan. 5  Heterochrony in avian brain development STRIEDTER, G.F.; Univ. of California, Irvine gstriedt@uci.edu

The brains of adult birds differ widely in size and proportions. Parrots, in particular, are known for having large brains, relative to body size, and disproportionately large forebrains, whereas domestic chickens have surprisingly small forebrains and optic lobes. The developmental bases of these adult species differences is almost completely unknown. In order to fill this gap, I examined embryonic brain and body development in white leghorn chickens, Northern bobwhite quail, and domestic budgerigars. Embryos of various ages were fixed and sectioned serially. Body, brain, and brain region volumes were estimated from those sections, using standard methods. Shrinkage due to tissue processing was estimated by comparing fresh body weight to body volume as estimated from tissue sections. One major finding is that chicken bodies and brains grow about 40% faster than their quail counterparts until about stage 28 (Hamburger-Hamilton). This finding contradicts a previous claim (based on mammalian data) that the rate of brain growth is highly conserved across species. A second major finding is that, throughout embryogenesis, the optic tectum is much smaller in budgerigars than in quail, even though most other brain regions are similar in size. This implies a species difference in the amount of tissue that is specified to become optic tectum during early brain regionalization. Finally, the telencephalon is initially smaller in budgerigars than in quail, but between days 6 and 7 of incubation, the budgerigar telencephalon goes through a relative growth spurt. Histological examination suggests that this growth spurt is due to a selective delay of telencephalic neurogenesis in budgerigars. Collectively, these findings demonstrate that several different process heterochronies combine in birds to generate diverse adult morphologies.