Class I	—increasing: g along temperature following either a linear curve or a quadratic curve.
Class II	—decreasing: Genes with the expression level decreasing along temperature following either a linear curve or a quadratic curve.
Class III	—U-shaped: Genes with the expression level following a quadratic curve with the parabola opening upward. To distinguish between increasing/decreasing quadratic curves, we further required the expression difference between 13 and 29 °C to be less than 80% of that between minimum and maximum expression levels among four temperatures.
Class IV	—bell-shaped: The same criterion was applied as for genes with U-shaped reaction norms but with the parabola opening downward.