The tail of the two: a study of twin embryos in Darevskia lizards

Abstract Monozygotic twinning, which produces genetically identical organisms, exemplifies the widespread ability of embryonic regulation. Although the incidence of twinning varies considerably, it is found in many taxons. Only a few cases of monozygotic twinning have been reported in lizards. Previously, we reported a single observation of an incompletely separated twin in a parthenogenetic Caucasian rock lizard, Darevskia armeniaca . To clarify whether twinning is particularly common in this lizard species, we studied eggs of this and two other species (one parthenogenetic and one bisexual) of this genus. Among 125 examined, we identified four pairs of twin embryos. Separated twins (three cases) or embryos with axial bifurcation (one case) were recorded exclusively in Darevskia armeniaca . These twin or bifurcated embryos were found in the clutches of three out of 15 studied gravid females of this species, and in four out of the 69 studied embryos. No anomalies were observed in 39 eggs of parthenogenetic D. dahli and 17 eggs of bisexual D. valentini . Statistical analysis confirmed significantly higher rates of anomalies in D. armeniaca . Twin embryos were smaller than singleton embryos but showed advanced limb and scale development despite delayed progress in certain traits compared to bisexual D. valentini . Twin embryos of D. armeniaca were larger and more developed than singleton embryos of the parthenogenetic D. dahli , highlighting species-specific developmental differences. We discuss that an excessive proportion of twinning in D. armeniaca is due to the complex genetic origin of this parthenogen and its adaptation to a less stable environment, forcing females to invest more energy in a single reproduction than two other species. Simultaneously, twinning develops in the largest eggs of D. armeniaca , which may be deposited by aged females and whose size exceeds the threshold that may be common for all studied closely related species.