## 3 Discussion
The upregulation of _MDPC_ and _IPPI_ genes observed in paired fed males in both times, as well as in JH III stimulated males at 8 h and females at 24 h, suggests that the synthesis of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) might occur in the gut of _D. rhizophagus_. Nevertheless, whereas both IPP and DMAPP are essential for the de novo production of frontalin or ipsdienol, the expression of _GPPS/FPPS_ and _GGPPS_ genes recorded in this bark beetle does not reflect the potential for de novo production of these compounds because they do not show a sex-specific expression pattern, nor do they exhibit sustained upregulation over time. Nevertheless, the basal activity of GPPS/FPPS and GGPPS might still allow the synthesis of terpenoid metabolites involved in basic cellular functions, e.g., protein prenylation and glycosylation (dolichols), and the electron transport chain (ubiquinone) [74; 75; 76]. In those bark beetle species that produce frontalin and ipsdienol, the _GPPS/FPPS_ and _GGPPS_ genes have always been recorded as upregulated in males (Table 4). In _I. pini_ and _I. confusus_, it was demonstrated that the _GPPS_ gene is upregulated from 4 to 32 h in fed males [47], whereas it was downregulated in females [32]. The GPPS enzyme and the cytochromes CYP9T2 and CYP9T3 are involved in the final steps of the production of ipsdienol and ipsenol pheromones from myrcene by males of these species [14; 77; 78]. In _Dendroctous_ species, the final steps of frontalin synthesis have not been described, despite the fact that males or females of several species (e.g., _D. ponderosae_, _D. jeffreyi_, _D. armandi_) produce frontalin either as aggregation, antiaggregation, or sexual pheromone [23; 44; 79; 49]. In particular, it has been reported that the _GGPPS_ gene is upregulated from 8 to 72 h in _D. ponderosae_ males, which produce frontalin after phloem feeding, allowing us to hypothesize that the enzyme of this gene, together with cytochrome P450 enzymes, are involved in frontalin production through an undetermined mechanism [36; 71]. Until now, it has been assumed that the upregulation of MVA pathway genes in the gut of bark beetles is directly involved in the synthesis of frontalin or ipsdienol. However, regardless of the observed upregulation of several genes in _D. rhizophagus_, our GC-MS analysis did not detect the presence of frontalin or ipsdienol pheromones. This result corresponds with previous findings in the same species from pre-emerged males and females forced to feed on host tissue in the laboratory, as well as in individuals collected from naturally attacked hosts at different stages of colonization [40]. It has been suggested that the endogenous production of sexual and aggregation pheromones in bark beetles is not necessarily a fundamental physiological process [18], such is the case of inbreeding species as _Dendroctous micans_[80], _D. punctatus_[81], and presumably _D. murrayanae_[82; 83], where females are fertilized by their siblings prior to emergence. Field observations of _D. rhizophagus_ females ovipositing in galleries without a male partner, as well as the presence of sperm in the oviduct of non-emerged females [84], suggest that pheromone production might not be necessary in this species for host colonization and mating. Further studies are needed to expand our understanding of the _D. rhizophagus_ mating system. However, the lack of frontalin may explain why _D. rhizophagus_ does not interfere with its sibling species _D. valens_ in the areas where they coexist in Mexico [85], as they present individual colonization strategies in which _D. valens_ uses frontalin both as a sexual and an aggregation pheromone [46], as opposed to _D. rhizophagus_. The lack of pheromone production by _D. rhizophagus_ prevents the colonization of adult trees, which are instead colonized by _D. valens_, confining its attacks to pine regeneration. However, our GC-MS assays confirmed the abundance of volatile compounds previously identified in _D.