## 3 Discussion
rhizophagus_ in both conditions (solitary and paired) and both times. Females only exhibited upregulation in _AACT_ at 24 h. The _HMGR_ gene, whose corresponding enzyme synthesizes mevalonate, was the only gene that exhibited upregulation in both fed males and females at both times and conditions (solitary or paired). HMGR is a highly regulated enzyme [69], and its gene is one of the most studied in bark beetles, together with _HMGS_. These last genes achieved similar expression levels in _D. armandi_ males both in phloem-fed insects and JH III stimulated insects [48], but in _I. confusus_, both genes reached higher expression levels in phloem-fed insects than in stimulated insects. In contrast to what was observed in fed insects, neither the enzymatic activity of HMGS, HMGR, and GPPS nor pheromone production was recorded in the JH III stimulated insects of _I. confusus_. It has been hypothesized that the deficiency of pheromone \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Gene** & **Species** & **Male** & **Female** & **Technique** & **Treatment** & **Reference** \\ \hline \multirow{2}{*}{_AACT_} & _I. pini_ & U & **U*** & RT-qPCR & Feeding & [32] \\  & _I. pini_ & C & **U*** & RT-qPCR & JH III & [70] \\ \hline \multirow{6}{*}{_HMGS_} & _I. pini_ & U & **U*** & RT-qPCR & Feeding & [32] \\  & _I. pini_ & U & U & RT-qPCR & JH III & [70] \\  & _I. pini_ & U* & U & RT-qPCR & JH III & [67] \\  & _D. jeffreyi_ & U* & C & Northern blot & JH III & [27] \\  & _D. armandi_ & U* & U & RT-qPCR & Feeding & [48] \\  & _D. armandi_ & U* & C & RT-qPCR & JH III & [48] \\  & _I. confusus_ & U & NR & RT-qPCR & Feeding & [47] \\  & _I. confusus_ & U & NR & RT-qPCR & JH III & [47] \\ \hline \multirow{6}{*}{_HMGR_} & _I. paraconfusus_ & U* & U & Northern blot & JH III & [68] \\  & _I. pini_ & U* & U & Northern blot & JH III & [25] \\  & _I. pini_ & U & **U*** & RT-qPCR & Feeding & [32] \\  & _I. pini_ & U & **U*** & RT-qPCR & JH III & [70] \\  & _I. confusus_ & U & NR & RT-qPCR & Feeding & [47] \\  & _I. confusus_ & U & NR & RT-qPCR & JH III & [47] \\  & _D. jeffreyi_ & U* & C & Northern blot & JH III & [28] \\  & _D. ponderosae_ & U* & U & RT-qPCR & Feeding & [71] \\  & _D. armandi_ & U* & U & RT-qPCR & Feeding & [48] \\  & _D. armandi_ & U* & C & RT-qPCR & JH III & [48] \\ \hline \multirow{2}{*}{_MK_} & _I. pini_ & U* & C & RT-qPCR & JH III & [70] \\ \hline \multirow{2}{*}{_MDPC_} & _I. pini_ & U* & U & RT-qPCR & Feeding & [32] \\  & _I. pini_ & U & **U*** & RT-qPCR & JH III & [70] \\ \hline \multirow{2}{*}{_IPPI_} & _I. pini_ & U & **U*** & RT-qPCR & Feeding & [32] \\  & _I. pini_ & U & **U*** & RT-qPCR & JH III & [70] \\ \hline \multirow{6}{*}{_GPPS/FPPS_} & _GPPS I. pini_ & U* & D & RT-qPCR & Feeding & [32] \\  & _GPPS I. pini_ & U & **U*** & RT-qPCR & JH III & [70] \\  & _FPPS I. pini_ & U* & D & RT-qPCR & Feeding & [32] \\  & _FPPS I. pini_ & U* & D & RT-qPCR & JH III & [70] \\  & _D. ponderosae_ & U* & C & RT-qPCR & Feeding & [36] \\  & _GPPS I. confusus_ & U & NR & RT-qPCR & Feeding & [47] \\ \hline \multirow{2}{*}{_GGPS_} & _D. ponderosae_ & U* & C & RT-qPCR & Feeding & [71] \\ \hline \end{tabular}
\end{table} 
Table 4: Previous reports of relative expression analysis of bark beetle MVA pathway genes after feeding on phloem or JH III treatments. production in JH III stimulated insects might be due to the absence of a brain hormone that is only induced by feeding [47]. However, this might also be caused by the presence of a regulatory mechanism mediated by an AMP-activated protein kinase (AMPK) analog to that reported in mammals, in which HMGR enzymatic activity is regulated according to the available energy. A high AMP:ATP ratio induces HMGR phosphorylation by AMPK, which inhibits the enzyme [72]. An analog strategy has been reported in yeasts, where the enzyme is inactivated when the available energy is low [73]. These strategies were presumably developed by convergent evolution and might also be present in insects, whose regulation mechanisms are not yet known [72].