In this work we have found that a single genotype of the aphid S. avenae proliferates differentially in two hosts depending on the presence of a facultative bacterial endosymbiont. The most striking result of our study was that among aphids sharing the same genetic background, those developing on wheat and harbouring the facultative endosymbiont R. insecticola exhibit larger colony development, while the opposite was true for the aphids developing in barley. A similar positive effect of this endosymbiont on reproduction has been found on the Vicia-specialized pea aphid populations (Tsuchida et al. 2004). However, that result was not reproduced in other pea aphid or other aphid species, with studies showing negative or neutral effects of R. insecticola on aphid reproduction (Ferrari et al. 2004, 2007; Wang et al. 2016; Luo et al. 2017; Liu et al. 2019). Nevertheless, our results are consistent with field-based monitoring of S. avenae in wheat plantations showing R. insecticola increased the prevalence throughout time when compared with uninfected individuals (Zepeda-Paulo and Lavandero 2021). It should be noted that G1 genotype dominates early in the season, but a recent study showed that its predominance significantly decreased at mid-season (Zepeda-Paulo and Lavandero 2021). However, although G1 decreases its prevalence during the season studied, it persists over the years as describe by Figueroa et al. )2005).
Ramírez-Caceres et al. (2019), studying another S. avenae genotype (G2) also described a higher population growth of E + aphids on wheat than on barley, while E- negatively affect the growth of aphids on barley. All this support that R. insecticola entail within-genotype difference on S. avenae performance across two common food plants, wheat and barley. This suggest that host plant x facultative endosymbiont interaction may drive host specialization even within a genotype. Thus, clones composed by E + aphid on wheat and E- aphids on barley, are expected to be positive selected and negatively selected, respectively, leading to ecological divergent populations dominated by asexual clones. However, given the temporal instability of the cereal plantations in Chile, these populations might not reach such a divergence (González U et al. 2013). The lack of host-based differentiation of S. avenae populations in Chile confirm this hypothesis (Figueroa et al. 2005). Nevertheless, it is surprising that the same facultative endosymbiont can generate such dissimilar effects on its host aphid, which raises several questions about the underlying mechanisms.
Our result show that E + aphids on both host plants showed higher number of winged individuals, although in a very low number, this contrast with previous studies. For example, E + individuals of asexual lineages of S. avenae from China under crowed conditions, produced less winged offspring than E- aphids (Liu et al. 2019). This result was found at 25°C, which is slightly higher than our conditions (22 ± 1 ºC). They also found that winged morph production did not showed differences among E + and E- at higher temperatures, which suggested that winged morph production was dependent on environmental temperature and aphid density. On the other hand, R. insecticola negatively affect the production of winged offspring in the pea aphid (Leonardo and Mondor 2006). In our case, because the production of winged individuals in E + aphids was irrespective of the host plant, this could be an idiosyncratic capacity of this asexual lineage (G1) to respond to the infection with R. insecticola. Remarkably, this asexual lineage is one the most persistent and predominant in the wheat field of Chile (Figueroa et al. 2005), and frequently found harbouring R. insecticola (Zepeda-Paulo et al. 2017) and thus the evolution of this lineage with R. insecticola might have reached a fixed pattern of response to this endosymbiont.
Surprisingly, the body weight of E + and E- aphids did not fallowed the same trend as reproductive performance. Instead, regardless host plant, aphid showed greater weight when they harboured R. insecticola. This indicates that the production of offspring is uncoupled with the weight of each individual. This contrast with the results found in Rhopalosiphum maidis (Fitch) feeding on barley, where R. insecticola-infected aphids performed poorly in weight (Liu et al. 2023). Regarding the distinct effect of R. insecticola on weight and reproductive performance in aphids of S. avenae sharing the same genetical background, again open questions about the undelaying mechanisms of these responses. Thus, we subsequently develop a proteomic study with the infected and non-infected aphids after their development in wheat or barley, to obtain some light on what the underlying mechanisms are.
Our findings suggest that the presence of R. insecticola generates different changes in the proteomic profile of S. avenae depending in a host-dependent manner, that could account for the difference in reproductive performance. Interestingly, the fact that protein regulation of aphids developing on wheat was comparatively milder and steadier than on barley, suggest that E + reared on wheat inflict lower impact of their physiology. In this regard, due to expansion of gene families associated with resistance to insecticides and plant chemical defenses described in the genome of S. avenae (Villarroel et al. 2022), if R. insecticola would have a beneficial effect on confronting those compounds, a larger regulation of those proteins would be expected when comparing E + and E- aphids. However, no differential regulation of those proteins was found in our study. Interestingly, this is consistent with the lack of association between the presence of R. insecticola and sensitivity to pyrethroids in S. avenae population from Germany (Leybourne et al. 2023). Since populations of S. avenae in Chile are most predominant in wheat than on barley (Apablaza and Fernández 1982; Figueroa et al. 2005) and that the genotype G1 is also predominant (Zepeda-Paulo et al. 2017; Zepeda-Paulo and Lavandero 2021), the comparative lower protein regulation on wheat suggests that much steady physiological response as compared with that on barley, is probably due to a recent adaptation of S. avenae to wheat after introduction. Indeed, it has been described that S. avenae superclones exhibit a broad host range, flat energetic costs for non-induced detoxification enzymes, and low variation in their reproductive performance on different host plants (Castañeda et al. 2010b; Barrios-SanMartin et al. 2016b). The facultative aphid endosymbiont Serratia symbiotica manipulates the expression of specific proteins in the pea aphid impairing plant defence response and improve feeding (Wang et al. 2020), a mechanism that may be occurring in E + in S. avenae aphids.
The lower reproductive performance of E + aphids on barley could be link to the higher number of proteins that were upregulated in these aphids, which could be an indicative of a reaction to a bacterium infection. The upregulation of the 1-acyl-sn-glycerol-3-phosphate acyltransferase enzyme (plsC), which participates several lipid biosynthetic pathways (Chen et al. 2011), and the elongation factor 1-alpha, may both be related with active enzymes delivery as a reaction to R. insecticola. However, the functional role of these upregulated protein on E + aphids remain to be deciphered. On the other hand, downregulation of proteins in E + on barley such as such as murein hydrolase activator EnvC, a protein produced by R. insecticola and normally associated with bacteria proliferation within aphids (Cook et al. 2020), could be aphid defensive response against R. insecticola induced by feeding on barley. It remains to be deciphered what are the host-dependent cues triggering such a striking difference in the aphid physiology.
Regarding the effects of E + and E- aphids on the plants, the higher root/shoot ratio exhibited by E + aphids compared to E- aphids on wheat plants but not on barley, suggests that these plants experienced a greater response stress. Since plants usually show a higher root/shoot ratio under water deficiency, it seems that E + aphids on wheat were more damaging than on E-. It is likely that aphids harbouring R. insecticola obtain and specific benefit on wheat which increase their efficiency in the use of this specific plant. This result contrasts with what was found in Medicago truncatula, where plants treated with pea aphids free or infected with R. insecticola showed no difference on the dry weight of plant shoots (Pandharikar et al. 2020).