Bacterial symbiont as game changers for Aphis craccivora Koch's fitness and survival across distinct climate types.

Symbiotic bacteria play a crucial role in the survival, development, and adaptation of aphids to environmental conditions. Buchnera aphidicola (Enterobacterales: Erwiniaceae), the obligate endosymbiont of aphids, is essential for their fitness, while facultative symbionts may provide additional ecological advantages under specific conditions. A comprehensive understanding of how these symbiotic relationships respond to different climatic environments is essential for assessing aphid adaptability and potential implications for biological control. The present study investigates the vital interactions between the obligate bacterial endosymbiont, Buchnera aphidicola, and four facultative bacterial endosymbionts (Arsenophonus sp., Hamiltonella defensa, Serratia symbiotica, and Regiella insecticola), in black cowpea aphid (BCA), in the context of different climate conditions. The BCA specimens were obtained from the leaves of the host plant, alfalfa, cultivated in three distinct climates: cold semi-arid, hot desert, and humid subtropical climates. The findings, as anticipated, indicated a pervasive prevalence of B. aphidicola in BCAs infesting alfalfa crops across all three climate types. In contrast, the BCAs of each climate type exhibited a distinct array of facultative symbionts. The highest number of facultative endosymbionts was exhibited by BCAs from the humid subtropical climate, followed by BCAs from the cold semi-arid climate, whereas none of them were detected in BCAs from the hot desert climate. Rigiella insecticola was not detected molecularly in any of the BCAs from the three climates. Following the eradication of the obligate symbiont Buchnera aphidicola by the antibiotic rifampicin in BCAs, the effects on three categories of parameters were assessed, including life cycle stages, reproductive traits, and external morphological characteristics of adults. The most significant adverse effects were observed in BCAs inhabiting hot desert followed by those inhabiting cold semi-arid climate; detrimental effects in BCAs of the humid subtropical climate were considerably less pronounced. The observed discrepancies in the parameters of BCAs from the humid subtropical climate can be attributed to the presence of a greater number of facultative symbionts, especially the presence of Serratia symbiotica (Enterobacterales: Yersiniaceae). Following the eradication of B. aphidicola, this facultative symbiont continues to complement the functions of B. aphidicola in the host's survival. Conversely, the low presence of facultative symbionts in cold semi-arid climate or even their absence in hot desert climate exacerbates the negative effects of obligate symbiont eradication. These findings highlight the crucial role of symbionts in aphid biology across a spectrum of climatic conditions, and suggest that shifts in symbiotic relationships may modulate aphid fitness, which could have implications for biological control programs.