Abstract
Aphids feed on plant phloem sap, rich in sugars but poor in essential amino acids. However, sugars cause osmotic regulation problems for aphids, which they overcome by hydrolysing the sugars in their gut and polymerising the hydrolysis products into oligosaccharides, excreted with honeydew. Aphids harbour primary bacterial endosymbionts, which supply them with essential amino acids necessary for survival. They also harbour secondary (facultative) endosymbionts (sfS), some of which have a positive impact on life history traits, although it is not yet known whether they also play a role in providing effective tolerance to differing levels of water soluble carbohydrates (WSCs). We investigated the relationship between WSC content of cocksfoot cultivars and performance of clones of the English grain aphid Sitobion avenae F. We evaluated how clone genotype and their sfS modulate performance on these different cultivars. We therefore examined the performance of genetically defined clones of S. avenae, collected from different host plants, harbouring different sfS. The performance was tested on 10 Dactylis glomerata L. cultivars with varying WSC content. D. glomerata is known as a wild host plant for S. avenae and is also commercially planted. We found that high WSCs levels are responsible for the resistance of D. glomerata cultivars to specific S. avenae clones. The minimum level of WSCs conferring resistance to D. glomerata cultivars was 1.7% dw. Cultivars with a WSC content of 2.2% or higher were resistant to S. avenae and did not allow reproduction. Our results further indicate that sfS modulate to some extend host plant cultivar adaptation in S. avenae. This is the first study revealing the importance of WSCs for aphid performance. Cocksfoot cultivars with a high content of WSCs might be therefore considered for aphid control or used for resistance breeding in this and other grass species, including cereals.
Highlights
Several sucking herbivores, such as aphids and whiteflies, have adapted to the highly specialized phloem sieve elements of plants as their feeding sites
The aim of this study was to examine (i) whether the genotype of S. avenae and the host plant the clones have been collected from in the field influence their performance on cocksfoot cultivars, (ii) whether different levels of water soluble carbohydrates (WSCs) content relate to the performance of S. avenae clones on these cocksfoot cultivars, and (iii) whether secondary endosymbionts are functional in explaining the performance of S. avenae on these cocksfoot cultivars varying in WSC content
S. avenae clones were unable to colonize D. glomerata cultivars with a WSC content exceeding 2.2% (Figure 2)
Summary
Several sucking herbivores, such as aphids and whiteflies, have adapted to the highly specialized phloem sieve elements of plants as their feeding sites. Despite this adaptation, the composition of the ingested phloem sap still does not match the nutritional requirements of these insects in terms of quantity and profile of essential amino acids and the concentration of water soluble carbohydrates. Aphids adapt to feeding on large quantities of phloem sap by a two-step conversion of sucrose to products of low osmotic activity. The combined rates of polymerization and reabsorption of sucrose hydrolysis products should substantially exceed the invertase reaction, to prevent an increase in the osmotic pressure of the aphid’s gut contents
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