How Does Mitochondrial Protein-Coding Gene Expression in <i>Fejervarya kawamurai</i> (Anura: Dicroglossidae) Respond to Extreme Temperatures?

Unusual climates can lead to extreme temperatures. <i>Fejervarya kawamurai</i>, one of the most prevalent anurans in the paddy fields of tropical and subtropical regions in Asia, is sensitive to climate change. The present study focuses primarily on a single question: how do the 13 mitochondrial protein-coding genes (PCGs) respond to extreme temperature change compared with 25 °C controls? Thirty-eight genes including an extra tRNA-Met gene were identified and sequenced from the mitochondrial genome of <i>F. kawamurai</i>. Evolutionary relationships were assessed within the Dicroglossidae and showed that Dicroglossinae is monophyletic and <i>F. kawamurai</i> is a sister group to the clade of (<i>F. multistriata</i> + <i>F. limnocharis</i>). Transcript levels of mitochondrial genes in liver were also evaluated to assess responses to 24 h exposure to low (2 °C and 4 °C) or high (40 °C) temperatures. Under 2 °C, seven genes showed significant changes in liver transcript levels, among which transcript levels of <i>ATP8</i>, <i>ND1</i>, <i>ND2</i>, <i>ND3</i>, <i>ND4</i>, and <i>Cytb</i> increased, respectively, and <i>ND5</i> decreased. However, exposure to 4 °C for 24 h was very different in that the expressions of ten mitochondrial protein-coding genes, except <i>ND1</i>, <i>ND3,</i> and <i>Cytb</i>, were significantly downregulated. Among them, the transcript level of <i>ND5</i> was most significantly downregulated, decreasing by 0.28-fold. Exposure to a hot environment at 40 °C for 24 h resulted in a marked difference in transcript responses with strong upregulation of eight genes, ranging from a 1.52-fold increase in <i>ND4L</i> to a 2.18-fold rise in <i>Cytb</i> transcript levels, although <i>COI</i> and <i>ND5</i> were reduced to 0.56 and 0.67, respectively, compared with the controls. Overall, these results suggest that at 4 °C, <i>F. kawamurai</i> appears to have entered a hypometabolic state of hibernation, whereas its mitochondrial oxidative phosphorylation was affected at both 2 °C and 40 °C. The majority of mitochondrial PCGs exhibited substantial changes at all three temperatures, indicating that frogs such as <i>F. kawamurai</i> that inhabit tropical or subtropical regions are susceptible to ambient temperature changes and can quickly employ compensating adjustments to proteins involved in the mitochondrial electron transport chain.