Progressive differentiation of the W sex‐chromosome between oviparous and viviparous populations ofZootoca vivipara(Reptilia, Lacertidae)

Populations of Zootoca vivipara appear karyologically well differentiated by heterochromatin distribution both in autosomes and sex‐chromosomes; allopatric oviparous populations possess few elements with centromeric heterochromatin and W acrocen‐tric sex‐chromosomes with heterochromatin only on the centromere; Alpine, as well as central‐western European viviparous populations possess conspicuous centromeric bands of heterochromatin on all of the chromosome pairs and a metacentric W chromosome with paracentromeric and telomeric heterochromatin bands; viviparous populations, living east and possibly north of the Carpathian Mountains, have many elements with interstitial and telomeric heterochromatin and an acrocentric W chromosome with telomeric and subtelomeric heterochroma. These data suggest an origin and evolution of sex‐chromosomes in this species, that, interestingly, parallels the recent hypothesis (Heulin et al., 1993) on the rise and evolution of viviparity and speciation in Zootoca.