Differential Mitochondrial Genome Expression of Four Skink Species Under High-Temperature Stress and Selection Pressure Analyses in Scincidae

As ectotherms highly sensitive to environmental temperature fluctuations, skinks (a small lizard) are increasingly vulnerable to population instability due to global heatwaves. A clade model analysis of four Chinese skink species (<i>Plestiodon capito</i>, <i>Plestiodon chinensis</i>, <i>Sphenomorphus indicus</i>, and <i>Scincella modesta</i>) revealed positive selection acting on the ND6 gene in <i>Sp. indicus</i>. This species exhibits codon alterations in ND6, shifts its expression pathway and potentially decouples ND6 from high-temperature stress response mechanisms. To validate these findings, transcriptomic profiling was conducted to assess mitochondrial protein-coding gene (PCG) expression patterns under thermal stress. Using <i>RT</i>-qPCR, liver mitochondrial PCG transcript levels were compared between high-temperature (34 °C) and control (25 °C) groups in skink populations from distinct latitudes. Low-latitude species (<i>P. chinensis</i> and <i>Sc. modesta</i>) exhibited metabolic downregulation, characterized by a significant suppression of mitochondrial gene expression. Specifically, <i>P. chinensis</i> showed the downregulation of six mitochondrial genes (COII, COIII, ATP6, ND2, ND4, ND6) while upregulating one (ND1). By contrast, <i>Sc. modesta</i> showed the downregulation of nine genes (COI, COII, COIII, ATP8, ND1, ND3, ND4, ND4L, CYTB) and upregulated two (ND5, ND6). By contrast, high-latitude species exhibited divergent patterns: <i>P. capito</i> downregulated four genes (COI, COII, COIII, ND4L) and upregulated four others (ND1, ND2, ND3, ND4), whereas <i>Sp. indicus</i> downregulated six genes (COI, COII, ND2, ND3, ND4, ND4L) and upregulated one (ND5). These regulatory disparities suggest that low-latitude skinks have a greater capacity for metabolic depression to cope with chronic stress, whereas their high-latitude counterparts exhibit different adaptations. The findings provide valuable insights into assessing the adaptive potential of species in warming environments, particularly for ectotherms with limited thermoregulatory capacities.