Transgenerationally inherited incisor aberration mediated by stress in two isolated populations of <i>Microtus hartingi</i> vole

Abstract Environmental and social stress can alter phenotypic expression at both individual and population levels. We examined the effects of ancestral social stress, imposed through forced polygyny, on reproductive success and maxillary incisor overgrowth in two isolated populations of Microtus hartingi vole: from Central Anatolia (CAP) and Eastern Rhodope (RP). In addition to ancestral social stress, CAP voles were exposed to environmental stress during prenatal, lactation, and early-life periods. Under control conditions, the prevalence of incisor overgrowth was at a stable rate of ∼16.7% in RP and occurred very rarely (0.5%) in CAP. Stress exposure markedly increased prevalence in CAP but not in RP, independent of female kinship, ancestor sex, or generation. We propose that upper incisor overgrowth arises from an interaction between genetic background and stress-sensitive regulatory mechanisms, with stress releasing cryptic genetic variation. The RP population, shaped by long-term habitat fragmentation, lack of dispersal, polygyny, and possibly inbreeding, appears to exhibit compensatory stabilization of this aberration, potentially through increased stress tolerance and earlier reproductive onset. CAP, originating from more stable environments, can be considered as a model of the early stage of RP adaptation to habitat fragmentation, as their heightened stress responsiveness leads to pronounced fluctuations in incisor overgrowth levels. Together, these findings suggest that stress-induced regulatory effects can persist across generations and may contribute to population divergence.