EXTENDED EGG RETENTION AND ITS INFLUENCE ON EMBRYONIC-DEVELOPMENT ANDEGG WATER-BALANCE - IMPLICATIONS FOR THE EVOLUTION OF VIVIPARITY

Viviparity in squamate reptiles presumably evolves via gradual increas es in the time that eggs are retained within the oviducts. We evaluate d the extent and consequences of phenotypic plasticity in egg retentio n for the oviparous lizard Scelopours scalaris. This species not only exhibits facultative egg retention, but has close relatives that are v iviparous. Thus, S. scalaris may possess reproductive features that ar e transitional between oviparity and viviparity. We tested the hypothe sis that oviposition normally occurs when further retention would impa ir embryonic development. To do so, we determined the effects of exten ded egg retention on embryonic development and on egg water balance fo r a population in which females normally retain eggs to Dufaure and Hu bert stages 31.0-33.5. Under substrate conditions that inhibited ovipo sition, females retained eggs to stage 39.5, or 0.5 stage units short of hatching. Extending egg retention did not retard the development of embryos relative to that of embryos in control eggs. Water did not ac cumulate in the extraembryonic compartments of retained eggs as it did in control eggs; all water uptake was associated with the embryo. The pattern of embryonic development within the retained eggs does not su pport the hypotheses (1) that oviposition occurs when gas exchange in utero is no longer sufficient to support the needs of the embryos, or (2) that increases in the duration of egg retention and decreases in e ggshell thickness evolve concurrently. Our observations on water uptak e additionally suggest that viviparity may evolve more easily in taxa that are able to preclude storage of excess water in the extraembryoni c compartment of the egg while allowing retained embryos access to suf ficient water for normal embryonic development.