Abstract
One hypothesis to explain the high incidence of independent evolutionary transitions from oviparity to viviparity among squamate (snake and lizard) reptile lineages proposed that embryonic development is independent of eggshell calcium. Recent research on embryonic calcium nutrition does not support this hypothesis as at least 25% of the calcium in hatchling oviparous squamates is extracted from the shell. An alternative hypothesis is that shell calcium supplements calcium from yolk in oviparous squamates, but is not obligatory for embryonic development. We tested this hypothesis by physically peeling the outer layers of the shell, and thus removing the calcium, early in development of Pantherophis guttatus (corn snake) eggs. We found support for the hypothesis because survivorship to hatching did not differ for peeled eggs compared to intact eggs, yet hatchlings from peeled eggs were shorter (273.6 ± 3.4 vs. 261.0 ± 3.7 mm, p=0.0028, n=16), lighter (6.36 ±0.22 vs. 5.75 ± 0.23 g, p=0.0158, n=16), and had reduced calcium (40.8 ± 1.7 vs. 30.5 ± 1.8 mg, p<0.001, n=16), which could impact hatchling fitness in the wild. We also tested the hypothesis that embryos detect and respond to calcium availability by assaying the two tissues involved in embryonic calcium acquisition for changes in calcium transport protein expression (by immunoblotting) following shell calcium removal. The yolk sac, involved in yolk calcium transport, showed no detectable change in the developmental expression of calbindin‐D28K, a marker for calcium transport activity. However, the chorioallantois, involved in shell calcium transport, showed reduced calbindin‐D28K expression relative to samples from intact eggs, suggesting eggshell calcium regulates chorioallantoic calcium transport. Our findings suggest that evolution of viviparity is enhanced by a mechanism for detection and mobilization of calcium in oviparous embryos that also functions in the uterine environment. (Supported by an APS‐IOSP fellowship to HF)
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