Meeting Abstract
30.3 Wednesday, Jan. 5 Origin of the in-series circulation in amniotes OWERKOWICZ, T.*; SPIKINGS, T.; ELSEY, R.M.; HICKS, J.W.; UC Irvine; UC Irvine; Rockefeller Wildlife Refuge, Grand Chenier, LA; UC Irvine towerkow@uci.edu
Among tetrapods, mammals and birds possess a derived in-series circulatory design. The capacity to shunt blood between systemic and pulmonary circuits, as found in amphibians and non-avian reptiles, represents the ancestral condition. We hypothesised that changes in atmospheric oxygen (O2) levels may have exerted selection pressures on the vertebrate cardiovascular function and led to the evolution of the in-series circulation. We measured shunt fraction (forward systolic flow in the left aorta, LAo) of juvenile American alligators breathing air of varied (8-36%) O2 content. Shunt flow increased with hyperoxia and decreased with hypoxia, suggesting active control of arterial haemoglobin saturation. We also studied heart outflow tracts from alligator hatchlings raised for three months under hypoxia (12% O2), normoxia (21% O2) or hyperoxia (30% O2). Using histologic sections, we measured the perimeter of the LAo relative to the right aorta (RAo), and calculated their maximum (circular) cross-sectional area. The LAo:RAo area ratio was lowest in hypoxic and highest in hyperoxic animals. Altogether, this indicates that LAo morphology responds to shunt magnitude. We propose that a pulmonary bypass shunt was strongly maintained in tetrapods living in the hyperoxic atmosphere of the Late Carboniferous-Early Permian, but the shunt was drastically reduced by global hypoxia of the Late Triassic-Early Jurassic. The latter period coincides with the appearance of mammals and theropod dinosaurs (bird ancestors) in the fossil record. Once the shunt was lost, the mammalian and theropod cardiovascular systems would have been committed to obligatory in-series circulation. Funded by NSF grant IOS-922756.
