Pax2 and Pax5 arose by gene duplication at the onset of vertebrate evolutio
n and have since diverged in their developmental expression patterns. They
are expressed in different organs of the mouse embryo except for their coex
pression at the midbrain-hindbrain boundary (MHB), which functions as an or
ganizing center to control midbrain and cerebellum development. During MHB
development, Pax2 expression is initiated prior to Pax5 transcription, and
Pax2(-/-) embryos fail to generate the posterior midbrain and cerebellum, w
hereas Pax5(-/-) mice exhibit only minor patterning defects in the same bra
in regions. To investigate whether these contrasting phenotypes are caused
by differences in the temporal expression or biochemical activity of these
two transcription factors, we have generated a knock-in (ki) mouse, which e
xpresses a Pax5 minigene under the control of the Pax2 locus. Midbrain and
cerebellum development was entirely rescued in Pax2(5ki/5ki) embryos. Pax5
could furthermore completely substitute for the Pax2 function during morpho
genesis of the inner ear and genital tracts, despite the fact that the Pax5
transcript of the Par2(5ki) allele was expressed only at a fivefold lower
level than the wildtype Pax2 mRNA, As a consequence, the Pax2(5ki) allele w
as able to rescue most but not all Pax2 mutant defects in the developing ey
e and kidney, both of which are known to be highly sensitive to Pax2 protei
n dosage, Together these data demonstrate that the transcription factors Pa
x2 and Pax5 have maintained equivalent biochemical functions since their di
vergence early in vertebrate evolution.