The whitefly, Bemisia tabaci, is an economically important agricultural pest causing huge damage to crops worldwide. They inflict damage to plants directly and as a vector of several hundred viruses, most of which belong to the genus Begomovirus (>320 species), and other economically important viruses belonging to the genera Ipomovirus, Carlavirus, Crinivirus, Torradovirus, and Polerovirus (Bedford et al., 1994; Jones, 2003; Mugerwa et al., 2021; Navas-Castillo et al., 2011). There are many species and/or biotypes of whiteflies, each with its own preferences for host plants, virus-transmitting abilities, and insecticide resistance (Barman et al., 2022a; De Barro et al., 2011; Gilbertson et al., 2015; Perring, 2001). A wide host adaptability and virus transmission ability make it one of the 100 most dreadful alien invasive species (Barman et al., 2022b; Hogenhout et al., 2008; Lowe et al., 2000). This polyphagous pest has adapted easily to varied temperature regimes across the world, such as in India, ranging from chilling cold temperatures in the hills to oppressively high temperatures in the deserts (Barro et al., 2011; Singh et al., 2012).

Temperature is one of the important determinants of abundance and geographical distribution of every ectotherm including insects (Huey and Kingsolver, 1993). When it comes to the invasive trait of B. tabaci, its heat-resistance ability is considered to be one of the underlying reasons (Cui et al., 2008; Lü and Wan 2008; Wan et al., 2009). Insects in general respond to elevated temperatures and other stresses with increase in synthesis of heat-shock proteins (Hsps) (Zhao and Jones, 2012). The role of Hsps in heat/cold stress adaptation, metamorphosis, and developmental responses in other insects is well documented (Waters and Rioflorido, 2007; Aevermann and Walters, 2008; Waters et al., 2008; Pan et al., 2017; Qin et al., 2018; Xiong et al., 2018; Yu et al., 2018; Parsell and Lindquist, 1993). Based on their molecular weight and homologous relationship, the Hsps are divided into five families and they are Hsp100, Hsp90, Hsp70, Hsp60, and small heat-shock proteins (sHsps) (Li et al., 2009). Stress proteins, such as the Hsps, are a potential candidate responsible for the wide adaptability of whitefly across different geographical niches (Wolfe et al., 1999; Salvucci et al., 2000; Lin et al., 2007; Cui et al., 2008; Lü and Wan, 2008; Mahadev et al., 2009). The mechanism by which the Hsps acts as molecular chaperones is quite distinctive; they stabilize proteins to enable host survival under heat-stress conditions by safeguarding the integrity of the host cell and their homeostasis (Jakob et al., 1993).

Considering the change in the climatic scenario of the Indian sub-continent due to imperious human activities, it is rational to detect the differentially expressed genes under thermal stress condition for better understanding of the tolerance capacity of B. tabaci to varying temperature conditions, thus delineating the underlying mechanism for niche expansion of this pest across India. In addition, symbiotic bacteria also have an important role to play in improving the fitness of their host, enabling it to sustain under novel climatic conditions (Wernegreen and Moran 2001). In accordance with other plant-sucking insects, B. tabaci harbours a diversity of symbionts, which enrich hosts’ nutrient-poor diet by synthesizing essential amino acids (Douglas 1989; Baumann 2005; Barman et al., 2020). Endosymbionts are generally classified into two major classes: primary symbionts/P-symbionts and secondary symbionts/S-symbionts (Baumann 2005; Feldhaar 2011). Candidatus Portiera aleyrodidarum (hereafter, Portiera), being a primary symbiont, occurs in all individuals. Secondary symbionts like Hamiltonella, Arsenophonus, Cardinium, Wolbachia, and Fritschea participate in functions that may not be necessary for the survival of the host but renders noticeable influence on its biological adaptation and ecological requirements (Ali et al., 2018; Feldhaar 2011; Khan et al., 2020). Portiera is described to be involved in the synthesis of nutrients such as essential amino acids (EAAs) and carotenoids, which are not present in a phloem diet (Cheng et al., 2016; Ferrari and Vavre, 2011; Su et al., 2014). Symbionts like Hamiltonella and Arsenophonus have been reported to be associated with the transmission of plant viruses (Gottlieb et al., 2010; Rana et al., 2012). Rickettsia is also reported to induce genes required for thermo tolerance in whitefly (Brumin et al., 2011). Reference has also been drawn indicating the obligate symbionts as “Achilles’ heel” from the perspective of temperature change (Corbin et al., 2017). Variations in the symbiont titre also have a significant impact on the insect fitness (Ali et al., 2019). For example, in Aphis craccivora, the quantity of Buchnera decreases under both low- and high-temperature conditions, which in turn negatively influences aphid reproduction (Chen et al., 2009).

The effect of heat stress in B. tabaci has been explored in the viewpoint of survival and reproduction (Byrne and Bellows 1991; Cui et al., 2008; Wolfe et al., 1999). Keeping these points in mind, the present research experiment was envisioned to assess the survivability of whitefly under sub-optimal and supra-optimal temperature conditions. In particular, we discussed certain key research priorities to shed light on the complex interaction between insect functioning, their microbial communities, and the Hsps gene. Primarily, the following questions were addressed: 1) what are the changes in the expression pattern of three Hsps (Hsp40, Hsp70, and Hsp90) under temperature stress conditions? 2) What are the relative changes in the symbiont titre harboured in B. tabaci after exposure to a temperature shock? And 3) is any sort of relation between symbiont titres and the Hsp gene expression? Many studies indicate such complex interactions in insects worldwide; however, this study represents an important step in emphasizing possible mechanisms for developing thermal resistance in B. tabaci, which is responsible for its sudden outbreak and wide spread in the country, and suggesting new management strategies.

Temperature is one of the important determinants of an organism’s distribution and abundance (Colinet et al., 2010). The ability to tolerate thermal stress is a vital parameter enabling the survival of B. tabaci under varying temperature conditions, thus playing a pivotal role in its wide distribution pattern in the Indian sub-continent (Samanta et al., 2021). In the current study, we determined the mortality rate of whitefly adults on the exposure to thermal stress and highlighted the molecular aspects underlying the heat responsive mechanism in B. tabaci. The varying mortality percentage of whitefly on subjection to different temperature stress conditions was observed with a higher mortality rate at 12°C than the mortality rate noted at 44°C. Cui et al. (2008) also reported a low mortality rate of whitefly at higher temperatures (40°C). A possible reason for this variation may be the differential expression pattern of Hsp genes that are reported to play a vital role in protecting organisms under heat stress conditions (Salvucci et al., 2000; Hoffmann et al., 2003; Wang et al., 2019).

Hsp genes are a central protagonist in helping organisms to cope with different environmental challenges, such as pathogen infection, xenobiotic substances, and thermal stress conditions (Chakraborty et al., 2021; Derecka et al., 2013; Somero, 1995; Sun and MacRae 2005; Tissiêrs et al., 1974). The present study puts forward the differential expression of three Hsp genes (Hsp40, Hsp70, and Hsp90) in B. tabaci under a heat-shock condition. The temperature treatment of whitefly resulted in an induced expression of Hsp70 under all the three conditions (12°C, 18°C, 44°C, and 26°C acting as control) with maximum expression observed at 44°C. The elevation of Hsp70 at extremes of temperature suggests its involvement in both heat and cold adaptation (Xiao et al., 2019). Reports suggest that the optimal expression level of Hsp70 is critical to the maintenance of cell function and homeostasis (Wheeler et al., 1999; Kristensen et al., 2002) and for chaperons to bind peptide chains (Fink, 1999). The transcript level of Hsp40 was also significantly upregulated upon the exposure of whitefly to extreme cold (12°C) and hot (44°C) conditions with a much higher expression level at 44°C, indicating the possible involvement of Hsp40 in heat adaptation of whitefly. Previous studies mentioned the association of Hsp genes in insects with important functions such as the regulation of growth and reproduction that, henceforth, increased their ability to fit under adverse environmental conditions (Lu et al., 2015; Quan et al., 2022). Work pertaining to the role of Hsp40 in whitefly seems limited; however, there are unequivocal indications regarding the role of Hsp40 in Hymenoptera, wherein, upon protein denaturation, Hsp40 delivers unfolded protein to Hsp70, and they together facilitate refolding by ATP binding and hydrolysis (Nguyen et al., 2016; Rinehart et al., 2007). Upregulation of the two Hsps (Hsp40 and Hsp70) under thermal stress conditions are indicative of their role in preventing cell damage under such stress conditions.

Contrary to the upregulation of most Hsp genes subjected to heat stress, a low basal expression of Hsp90 was observed, when exposed to extremes of temperature. Hence, it would not be wrong to say that the expression of Hsp90 is less dependent upon temperature stress than other Hsps. However, in the case of Hsp90, the transcript level attained a peak at 18°C. This clearly suggests that Hsps might have evolved from different expression patterns under different temperature conditions. Reports indicate the participation of Hsp90 in the negative regulation of proteins (Lindquist and Craig, 1988). The susceptibility of B. tabaci to temperature has been reported to vary according to geography and genetic groups (Pusag et al., 2012). This highlights the importance of thermal tolerance for insects including whitefly to thrive under such a diverse climatic condition as that of India.

In addition to determining the role of Hsp genes in adaptation and survivability of whitefly, an important dynamic which remains unattended is the host–microbe association or to say symbiont-mediated modulation of host traits such as thermal tolerance. Thermal variation has a strong influence on host metabolism, and any deviation from optimum environmental conditions could have a deleterious influence on the hosts’ survival and fecundity (Macmillan, 2019). To shed light on this aspect, we evaluated the change in the relative amount of four endosymbionts in whitefly when subjected to thermal stress. Although we are aware that many host–microbe interactions protect their host partners from pathogens or predators (Flórez et al., 2015), less is acknowledged regarding the influence of these symbionts on insects’ thermal tolerance. However, reports suggest that temperature has a significant influence in the abundance of endosymbionts harboured in insects and their interaction (Bensadia et al., 2006; Burke et al., 2010.; Dunbar et al., 2007.; Montllor et al., 2002). Microbes can either expand or restrict the abiotic niche of their host partners, thus influencing their ability to adapt to the fluctuating environmental condition (Lemoine et al., 2020; Zaynab et al., 2019).

Our findings revealed multiple patterns in the relative abundance of primary and secondary symbionts upon subjection to thermal stress. The primary endosymbiont, Portiera, was relatively unaffected by temperature treatment. Localization of Portiera inside the bacteriosome might be held accountable for their limited response to temperature stress (Su et al., 2014; Li et al., 2017; Caspi-Fluger et al., 2011). On the contrary, depletion in the quantity of the three secondary symbionts (Arsenophonus, Wolbachia, and Rickettsia) was measured at 44°C. This indicates a possible involvement of the facultative symbionts in enhanced thermo-tolerance of B. tabaci at extremely high temperatures and hence the low mortality rates at this temperature. Experiments carried out in the pea aphid also demonstrated the involvement of facultative symbionts in protecting hosts from detrimental effects of thermal stress (Oliver et al., 2010). Brumin et al. (2011) reported the spatial location of Rickettsia outside the bacteriosome, resulting in a significant decrease of the endosymbiont when subjected to thermal stress. Shan et al. (2014) also observed a reduction in Rickettsia population in heat-treated whitefly, although the reduction was non-significant. Disturbances in gut symbionts when exposed to high temperature were noted in many insects. A possible explanation to this could be that an escalation in temperature upsets the stability of some protein that is involved in the transportation of metabolite, thereby restricting supplementation of the metabolites between the partners. It was also seen that titres of Wolbachia were depleted in Aedes albopictus when exposed to elevated temperature conditions (Mouton et al., 2007; Wiwatanaratanabutr and Kittayapong, 2009). Such depletion does not essentially indicate host extinction. Hence, the primary symbionts are reported to be directly involved in vital metabolic functions of insect host, and on the flip side, the secondary symbionts are reported to mitigate the effect of heat stress on the primary symbionts, thus aiding the hosts’ survival (Lemoine et al., 2020). Thus, it would not be wrong to say that host–microbial association has a key role in temperature acclimation; however, the exact dynamics vis-à-vis direct effect of an abiotic factor on any sort of symbiotic interactions still remains unknown and will require further exploration.

Deleterious effects of thermal stress on symbiont quantity may be allayed via other mechanism such as gene expression (Brumin et al., 2011) or modulation of the host behaviour (Truitt et al., 2019). In the current study, the expression of the Hsp genes and relative symbionts titre were highly variable under different temperature conditions. Based on the correlation studies, we assumed that endosymbionts infection modulate host gene expression by altering the expression of specialized Hsp genes or by affecting the activities of transcription factors (Moon et al., 2021; Brennan et al., 2008). In addition, further investigation is required in this direction.

In the light of current disturbances in the ecological and environmental balance caused by human activities, the present investigation can be considered an important step in predicting the potential factors responsible for the adaptation of whitefly in such a diverse temperature regime. However, few vital questions that arise on our way stem from the basic evolution of symbiont-mediated thermal resistance/heat tolerance and the precise molecular interactions occurring between symbionts and host genes. Particularly, through proper clarification of these vital issues, a symbiont-targeted pest management will prove to be an effective control component for the future agricultural community. However, detailed work needs to be carried out before the large-scale application of symbiont-targeted pest management strategies.