The biochemical basis of life history adaptation: Molecular and enzymological causes of NADP ⁺-isocitrate dehydrogenase activity differences between morphs of gryllus firmus that differ in lipid biosynthesis and life history

Although whole-organism aspects of life-history physiology are well studied and molecular information (e.g., transcript abundance) on life-history variation is accumulating rapidly, much less information is available on the biochemical (enzymological) basis of life-history adaptation. The present study investigated the biochemical and molecular causes of specific activity differences of the lipogenic enzyme, NADP ⁺-isocitrate dehydrogenase, between genetic lines of the wing-polymorphic cricket, Gryllus firmus, which differ in lipid biosynthesis and life history. With one exception, variation among 21 Nadp ⁺-Idh genomic sequences, which spanned the entire coding sequence of the gene, was restricted to a few synonymous substitutions within and among replicate flight-capable or flightless lines. No NADP ⁺-IDH electromorph variation was observed among individuals within or among lines as determined by polyacrylamide gel electrophoresis. Nor did any NADP ⁺-IDH kinetic or stability parameter, such as KM for substrate or cofactor, kcat, or thermal denaturation, differ between flight-capable and flightless lines. By contrast, line differences in NADP ⁺-IDH-specific activity strongly covaried with transcript abundance and enzyme protein concentration. These results demonstrate that NADP ⁺-IDH-specific activity differences between artificially selected lines of G. firmus are due primarily, if not exclusively, to genetic variation in regulators of NADP ⁺-IDH gene expression, with no observed contribution from altered catalytic efficiency of the enzyme due to changes in amino acid sequence or posttranslational modification. Kinetic analyses indicate that in vitro differences in enzyme-specific activity between flight-capable and flightless lines likely occur in vivo. This study constitutes the most comprehensive analysis to date of the biochemical and molecular causes of naturally occurring genetic variation in enzyme activity that covaries strongly with life history.