Growing up gator: a proteomic perspective on cardiac maturation in an oviparous reptile, the American alligator (Alligator mississippiensis)
We recently described lasting changes in the cardiac proteome of American alligators (Alligator mississippiensis) reared under hypoxic conditions, that resemble what embryos encounter in natural nests. While these changes were consistent with functional differences in cardiac performance induced by developmental hypoxia, the magnitude of this response was dwarfed by a much greater effect of development alone (76% of the total differentially abundant proteins). This means that substantial differences in relative steady-state protein expression occur in the hearts of alligators as they mature from egg-bound embryos to 2-year-old juveniles, and this developmental program is largely resistant to variation in nest conditions. We therefore performed functional enrichment analysis of the 412 DA proteins that were altered by development but not hypoxia, to gain insight into the mechanisms of cardiac maturation in this ectotherm. We found that the cardiac proteome of alligators at 90% of embryonic development retained a considerable capacity for transcription and translation, suggesting the heart was still primarily invested in growth even as the animal approached hatching. By contrast, the cardiac proteome of 2-year-old juveniles was weighted towards structural and energetic processes typical of a working heart. We discuss our results in the context of differences in cardiac development between ectothermic and endothermic oviparous vertebrates, and argue that the robust developmental program of the alligator heart reflects a slow-paced ontogeny, unburdened by the requirement to support the elevated peripheral oxygen demand typical of endothermic animals from a young age.