The endosymbiotic bacteria of the South African bird cherry-oat aphid, rhopalosiphum padi.

Abstract

Cereal crops play a major role in human diet as staple foods, especially in developing countries. These crops are a part of a few edible crops that are widely cultivated globally. However, the production of these crops is constantly being put under strain by abiotic and biotic stresses in the environment, such as aphids. Aphids are the most important insects of cereal crops, not only causing damage through feeding. Aphids such as the bird cherry-oat aphid also transmit harmful plant viruses. They feed using piercing and sucking mouthparts that they insert into the plant while excreting saliva, which suppresses the plant’s defence mechanism. The plant sap these insects feed on is rich in sugars and deficient in essential nutrients required for their optimal growth and reproduction processes. These insects live in symbiotic associations with endosymbiotic bacteria, which synthesise the deficient nutrients for the aphids. These bacteria also produce a chaperon protein that has been hypothesised to be involved in protecting the viruses they transmit from degradation. In South Africa, there is limited information about the endosymbiotic bacteria of the bird cherry-oat aphid. This aphid is estimated to cause substantial cereal crop yield losses through feeding, mostly through transmitting barley yellow dwarf viruses. Persistence of these pests may lead to a reduced harvest of these crops, which might result in a drastic rise in hunger and poverty and serious economic consequences. This study aimed to investigate the association between the bird cherry-oat aphid and its endosymbiotic bacteria, with the hope that the findings will give further understanding on how to manage this pest. Molecular biology techniques were employed to identify the endosymbiotic bacteria of the bird cherry-oat aphid using 16S rDNA. Once identified, the effects of two antibiotics on the survival and reproduction of bird cherry-oat aphids were compared using a flask method which was found to be superior in rearing aphids compared to an artificial diet. This study also assessed the ability of bird cherry-oat aphids to acquire Hamiltonella defensa secondary endosymbiont from infested rose grain aphid through a shared food source. The results obtained in this study show that the obligate primary endosymbiont, Buchnera aphidicola, was found across all the screened samples, while sporadic occurrence was observed for the secondary endosymbionts. In addition, this study also showed that in the absence of their primary endosymbiotic bacteria, bird cherry-oat aphids could not reproduce and though aphid death was not immediate, most of the aphids had died by the end of the experiment. Lastly, this study showed that secondary endosymbionts can be passed between aphids through a shared food source. The field provides crops with a vast number of microbes, which can be interchangeable between plants and aphids. However, the most essential microbe, B. aphidicola, uses the aphid as a host and controlling this endosymbiont might lead to a potential control measure for the bird cherry-oat aphid.