Embryos of oviparous squamate reptiles typically obtain calcium from both yolk and eggshell but differ from other oviparous amniotes (turtles, birds and crocodilians) because they are heavily dependent on calcium-rich yolk. Eggs of viviparous squamates lack calcareous eggshells, and embryos receive calcium solely from yolk or from both yolk and placenta. The pattern of calcium mobilization by amniote embryos has been predicted to influence the evolution of viviparity if embryos are dependent on calcium from the eggshell and calcium placentotrophy evolves subsequent to viviparity. We studied the pattern of maternal provision and embryonic utilization of calcium of an oviparous and a viviparous population of the reproductively bimodal lizard Lacerta viviparous to test the hypotheses: (1) oviparous embryos are not dependent on eggshell calcium and (2) calcium content of viviparous hatchlings does not differ from oviparous hatchlings. Our findings do not support either of these hypotheses because oviparous females oviposited eggs with heavily calcified shells and calcium-poor yolk, and embryonic mobilization of shell calcium was greater than for other oviparous squamates. The calcium content of yolk from viviparous females did not differ from oviparous yolk, but viviparous eggs lacked calcareous eggshells. Uterine secretion by viviparous females compensated for the low calcium content of yolk, and placental calcium transfer was among the highest recorded for squamates. The pattern of calcium provision in these two populations suggests that dependence on uterine calcium, either stored temporarily in an eggshell or transferred directly across a placenta, did not constrain the evolution of reproductive mode in this lineage.

The primary pattern of embryonic nutrition for squamate reptiles is lecithotrophy; with few exceptions, all squamate embryos mobilize nutrients from yolk. The evolution of viviparity presents an opportunity for an additional source of embryonic nutrition through delivery of uterine secretions, or placentotrophy. This pattern of embryonic nutrition is thought to evolve through placental supplementation of lecithotrophy, followed by increasing dependence on placentotrophy. This review analyzes the relationship between reproductive mode and pattern of embryonic nutrition in three lecithotrophic viviparous species, and oviparous counterparts, for concordance with a current model for the evolution of viviparity and placentation. The assumptions of the model, that nutrients for oviparous embryos are mobilized from yolk, and that this source is not disrupted in the transition to viviparity, are supported for most nutrients. In contrast, calcium, an essential nutrient for embryonic development, is mobilized from both yolk and eggshell by oviparous embryos and reduction of eggshell calcium is correlated with viviparity. If embryonic fitness is compromised by disruption of a primary source of calcium, selection may not favor evolution of viviparity, yet viviparity has arisen independently in numerous squamate lineages. Studies of fetal nutrition in reproductively bimodal species suggest a resolution to this paradox. If uterine calcium secretion occurs during prolonged intrauterine egg retention, calcium placentotrophy evolves prior to viviparity as a replacement for eggshell calcium and embryonic nutrition will not be compromised. This hypothesis is integrated into the current model for evolution of viviparity and placentation to address the unique attributes of calcium nutrition. The sequence of events requires a shift in timing of uterine calcium secretion and the embryonic mechanism of calcium retrieval to be responsive to calcium availability. Regulation of uterine calcium secretion and the mechanism of embryonic uptake of calcium are important elements to understanding evolution of viviparity and placentation.

The primary pattern of embryonic nutrition for squamate reptiles is lecithotrophy; with few exceptions, all squamate embryos mobilize nutrients from yolk. The evolution of viviparity presents an opportunity for an additional source of embryonic nutrition through delivery of uterine secretions, or placentotrophy. This pattern of embryonic nutrition is thought to evolve through placental supplementation of lecithotrophy, followed by increasing dependence on placentotrophy. This review analyzes the relationship between reproductive mode and pattern of embryonic nutrition in three lecithotrophic viviparous species, and oviparous counterparts, for concordance with a current model for the evolution of viviparity and placentation. The assumptions of the model, that nutrients for oviparous embryos are mobilized from yolk, and that this source is not disrupted in the transition to viviparity, are supported for most nutrients. In contrast, calcium, an essential nutrient for embryonic development, is mobilized from both yolk and eggshell by oviparous embryos and reduction of eggshell calcium is correlated with viviparity. If embryonic fitness is compromised by disruption of a primary source of calcium, selection may not favor evolution of viviparity, yet viviparity has arisen independently in numerous squamate lineages. Studies of fetal nutrition in reproductively bimodal species suggest a resolution to this paradox. If uterine calcium secretion occurs during prolonged intrauterine egg retention, calcium placentotrophy evolves prior to viviparity as a replacement for eggshell calcium and embryonic nutrition will not be compromised. This hypothesis is integrated into the current model for evolution of viviparity and placentation to address the unique attributes of calcium nutrition. The sequence of events requires a shift in timing of uterine calcium secretion and the embryonic mechanism of calcium retrieval to be responsive to calcium availability. Regulation of uterine calcium secretion and the mechanism of embryonic uptake of calcium are important elements to understanding evolution of viviparity and placentation.

Yolk reserves supply the majority of embryonic nutrition in squamate reptiles, including calcium. Embryos of oviparous squamates exploit the eggshell for supplemental calcium, while embryos of viviparous species may receive additional calcium via the placenta. Developmental uptake of calcium in oviparous snakes increases during the interval of greatest embryonic growth (stage 35 to parturition). However, the pattern of embryonic calcium acquisition is unknown for viviparous snakes. Furthermore, while the uterus of oviparous species transports calcium early in embryonic development during mineralization of the eggshell, the timing of uterine calcium secretion in viviparous snakes is unknown. We studied a viviparous snake, Virginia striatula, to determine the ontogenetic pattern of yolk and embryonic calcium content. The pattern of embryonic calcium uptake of V. striatula is similar to that of oviparous snakes but the sources of calcium differ. In contrast to oviparous species, embryos of V. striatula acquire half of total neonatal calcium via placental provision, of which 71% is mobilized between stage 35 and parturition. Furthermore, we report for the first time in a viviparous squamate an increase in yolk calcium content during early stages of embryonic development, indicating that uterine secretion of calcium occurs in V. striatula coincident with shelling in oviparous squamates. Thus, uterine calcium secretion in this viviparous species may either occur continuously or in two phases, coincident with the timing of shelling in oviparous species and again during the last stages of development. Whereas, the pattern of embryonic calcium acquisition in V. striatula is plesiomorphic for squamates, the pattern of uterine calcium secretion includes both retention of a plesiomorphic trait and the evolution of a novel trait.

Embryos of oviparous lizards have two sources of calcium for embryonic development: 1) calcium that accumulates in yolk during vitellogenesis, and 2) calcium carbonate deposited in the eggshell from oviductal secretions. Eggs of viviparous lizards lack a calcified eggshell and calcium secreted by the uterus is delivered to the embryo across a placenta. Whereas oviparous lizard embryos recover calcium from the eggshell during late developmental growth stages, viviparous embryos have a lengthy intimate association with the uterus and the potential for an extended interval of placental calcium transfer. We compared the pattern of calcium mobilization of embryos of the viviparous, placentotrophic scincid lizard, Pseudemoia pagenstecheri, to that of a closely related oviparous species, Saproscincus mustelinus, to determine if the timing of uterine calcium secretion was influenced by reproductive mode. Embryos of both species receive a substantial amount of calcium from either the eggshell or placenta (54% and 85% respectively). The ontogeny of calcium uptake by embryos of P. pagenstecheri reveals that the onset of embryonic acquisition of calcium occurs earlier relative to embryonic stage but the timing of peak uterine secretion of calcium is delayed, compared to S. mustelinus.

Embryos of oviparous squamate reptiles obtain all organic and most inorganic nutrients from yolk; yolk provides 19-86% of hatchling calcium content. The remaining calcium is extracted from the eggshell. Yolk calcium provision to viviparous embryos also is variable and includes three patterns. The contribution of yolk to embryonic development for most viviparous squamates is similar to oviparous species, but the attenuated eggshell of viviparous species is a poor source of calcium because it lacks an outer layer of calcium carbonate, and embryos supplement yolk calcium via placental transfer. In a second pattern, yolk provides all organic nutrients and calcium. The final pattern occurs in viviparous species that are substantially placentotrophic and placental transfer accounts for most organic and inorganic nutrients, including calcium. The many independent evolutionary transitions to viviparity among squamates have inspired interest in a possible link to patterns of embryonic calcium nutrition. A prominent model predicts that the pattern of maternal provision and embryonic uptake of calcium unique to squamates facilitates the evolution of viviparity. A primary assumption of the model is that the evolution of viviparity precedes the evolution of calcium placentotrophy. An alternative model predicts that viviparity and placentotrophy evolve concurrently because mechanisms for nutrient provision and mobilization are not dependent on reproductive mode. These hypotheses have not been tested directly but review of the literature indicates that neither fully explains the diversity of squamate embryonic calcium nutrition. Viviparous species differ from oviparous species primarily in the timing of uterine calcium secretion and structure of eggshell calcium. Future studies should focus on the mechanisms that promote these differences.