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Summary: 145
Low Oxygen: A Constraint on the Evolution of Viviparity in Reptiles
Robin M. Andrews*
Department of Biology, Virginia Polytechnic Institute and
State University, Blacksburg, Virginia 24061
Accepted 12/6/01
ABSTRACT
The evolution of reptilian viviparity (live bearing) from ovi-
parity (egg laying) is thought to require transitional stages of
increasingly longer periods of embryonic development in utero,
that is, longer periods of egg retention by the gravid female.
Studies on sceloporine lizards demonstrate that embryonic re-
sponses to egg retention that is extended beyond the time of
normal oviposition range from developmental arrest to normal
development. The present study was designed to test the hy-
pothesis that O2 availability is the proximate factor that deter-
mines the rate and degree of development that reptilian em-
bryos undergo in utero. Eggs of Sceloporus undulatus were
incubated under conditions of low (LOX), normal (NOX), and
high (HOX) oxygen both early and late in development. The
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