HYBRIDIZATION EXPERIMENT BETWEEN OVIPAROUS AND VIVIPAROUS STRAINS OF LACERTA VIVIPARA: A NEW INSIGHT INTO THE EVOLUTION OF VIVIPARITY IN REPTILES

The lizard Lacerta vivipara exhibits both oviparous (egg-laying) and viviparous (live-bearing) modes of reproduction. Hybridization cannot occur in nature because oviparous populations are isolated in the extreme southwestern part of the distributional range. Hybridization experiments (oviparous x viviparous, F, F2, and back-crosses) enabled an analysis of the degree of reproductive isolation between viviparous and oviparous populations and also provided insight into the evolution of viviparity. We observed sterile eggs and embryonic mortality in both normal and hybrid crosses. Comparative analysis of breeding success failed to demonstrate clear-cut re- productive isolation between oviparous and viviparous strains. This incomplete reproductive iso- lation agrees with previous genetic studies, which indicated that both reproductive strains are closely related. Hybrid females (F1) laid eggs with an incompletely calcified eggshell. The stages of embryonic development at egg-laying in hybrid females were intermediate (stages 35-36), between those in oviparous populations (stages 31-34) and in viviparous females (final stage = 40). Incubation duration was shorter for eggs of the hybrids than for normal oviparous eggs. Our hybrid strain is thus an interesting, although artificial, illustration of the classical model for evolution of viviparity in reptiles: it occupies an intermediate stage on the oviparous-viviparous continuum, and it supports predictions of eggshell regression, more advanced embryonic development at egg-laying, and shortening of incubation. The intermediate reproductive phenotype of first generation hybrids indicates that transmission of reproductive mode is not dominant-recessive. However, more research (on F2 hybrids) is needed before one can assert whether reproductive mode behaves like a Mendelian or a polygenic character.