Dynamics of size and shape variation during embryonic development in the Balkan crested newt

Despite its relevance for developmental and evolutionary studies, intraspecific variation, particularly among-individual variation in size and shape during embryogenesis, remains poorly understood. Here, we apply geometric morphometrics to quantify external morphology and to examine variation in embryo size and shape in Triturus across the tailbud and prehatching phases. Triturus newts, an emerging model system in developmental and evolutionary biology, possess a balanced lethal system that delays development, alters morphology, and induces embryonic arrest in approximately half of the embryos. We hypothesize that this arrest increases morphological variance during the tailbud phase, a period that should be highly constrained due to organogenesis. In contrast, we expect more uniform variance during the prehatching phase, which follows arrest, when only viable embryos remain and approach hatching, a major developmental milestone. Our results reveal gradual changes in size and shape across both developmental phases. Morphological variance increased at tailbud stage 23, driven in part by allometric effects, and coincided with the onset of developmental arrest. During the prehatching phase, variance was uniform but higher than that reported in a previous study of the posthatching period. Overall, our findings provide new insights into morphological variability during early development and establish a foundation for comparative studies aimed at testing developmental hypotheses, including the effects of the balanced lethal system.