September 2017
Drs Scott Sydenham and Vicki Tolmay, ARC-Small Grain, Bethlehem
Crop pests have plagued producers since the infancy of agriculture. It has recently been predicted through extensive modelling that most agriculture regions of the world will reach pest saturation point by 2050.
Third on the top ten list of diseases and pests expected to reach saturation point on grain crops is the aphid species complex (as a group). This is on the back of the demand and need for higher yielding crops, as more than a 50% increase in food production is required to meet global demand of the ever-growing human population.
Global redistribution and introduction of pest insect species by and in response to human activities has become defining. The arrival of invasive exotic insect pests of various crops is undeniably negative for international food security and national agricultural sustainability.
The Russian wheat aphid (RWA), (Diuraphis noxia), insect species is clonal, and all female offspring are identical to the mother aphid. In South Africa, there is no documented evidence of male aphids of D. noxia, which eliminates the possibility of sexual recombination.
Telling the difference between the biotypes is particularly difficult and the screening process is cumbersome. The biotypes are classified according to the reaction of a set of international differential wheat lines which containdifferent Dn (Dn1-9, Dnx and Dny) resistance genes.
This process takes a month to complete and is very labour intensive. Recently the genome of Russian wheat aphid was published and very limited, if any, variation was found at DNA sequence level between different biotypes around the world.
The local context
In South Africa the Russian wheat aphid has been a significant insect pest on wheat for decades. Russian wheat aphid was an infamous insect pest of winter/facultative wheat under dryland production, causing significant yield losses in the mid-1980s and 1990s in South Africa.
With the steady decline in area planted to wheat since the 2000s – especially in the Free State Province – the presence of damagingly high Russian wheat aphid numbers in producers’ fields has become less common and sporadic.
There is evidence in the last few seasons that certain RWASA biotypes have acquired specific secondary host preferences and have been found in the fields of irrigated spring wheat, as well, suggesting potential biotype adaption. Importantly for now, the number of Russian wheat aphid found in the Western Cape and irrigation production areas remain low.
With climate change the availability of secondary hosts and the increasing trend in global human travel, the development of new virulent Russian wheat aphid biotypes and/or introductions into new wheat production regions appear unavoidable. This was the case in 2016, when Australia had its first ever incursion of an unidentified Russian wheat aphid biotype.
Currently, there are four known biotypes of Russian wheat aphid in South Africa, RWASA1 (identified in 1978), RWASA2 (identified in 2005), RWASA3 (identified in 2009) and RWASA4 (identified in 2011). The current cultivar spectrum has good resistance levels to RWASA1 and RWASA2, with lower resistance levels to RWASA3 and limited, if any to RWASA4.
This high resistance level to RWASA1 and RWASA2 is primarily due to the constant investment in pre-breeding for Russian wheat aphid host plant resistance done at ARC-Small Grain. The limited number of resistant cultivars available to RWASA3 and RWASA4, could be due to the shorter time that has transpired since these biotypes were found, as resistant cultivars take many years to develop. What is concerning, is the RWASA4 biotype is becoming more prevalent in survey studies.
In the last five years, a number of studies across different insect species (honey bees, termites and ants) have been successful in exploiting epigenetic variation to identify potential biomarkers. Epigenetic variation refers to variation in the presence of methyl-groups that are attached to the DNA of a species. In these studies methylation sensitive, molecular techniques were used to identify differences in methylation patterns attached to the DNA.
These different methylation patterns are commonly known as biomarkers. Differences in methyl groups attached to the single base pairs of DNA have shown to influence gene expression epigenetically.
These differences in methylation were identified within different insect species and have shown to influence insect specialisation (for example the different roles/activities that different bees in a hive perform) or the adaptation and virulence of insect species.
Looking towards a brighter research future
Being able to tell the difference between the Russian wheat aphid biotypes quickly and effectively will enable scientists to spend their time studying the ecology and life history of the different biotypes. With this information, predictions could be made as to which biotype(s) would be better adapted to which environmental conditions, allowing breeders to pre-emptively develop resistant cultivars.
In total 512 different primer combinations were tested with just a handful showing differences between the four RWASA biotypes. More primer combinations showed differences in RWASA2 and RWASA4 biotypes, suggesting the presence of more methylation pattern variation.
From years of phenotypic screening experience with RWASA biotypes, this is an interesting result as RWASA2 and RWASA4 appear the more virulent and aggressive biotypes. RWASA3 and RWASA1 were harder to differentiate.
In other insect species, methylation has been linked to virulence changes. A few promising biomarkers that clearly identify each of the four RWASA biotypes have been identified, namely primer pair A (RWASA4), primer pair B (RWASA3), primer pair C (RWASA2) and primer pair D (RWASA1). The red arrows in each photo (Photo 3 to Photo 6) identify the unique biomarker for each RWASA biotype.
This is very encouraging because previously published SSR and RAPD markers had been tested on the four RWASA biotypes without successful differentiation. These international publications reported on DNA based markers that could distinguish the specific Russian wheat aphid biotypes that had been compared in those studies but not necessary all Russian wheat aphid biotypes.
This methylation sensitive methodology might be the key to identifying biomarkers for all Russian wheat aphid biotypes around the world and could possibly be a potential tool to identify or track Russian wheat aphid biotype introductions.
Before publishing this data in a scientific journal, further evaluation of these specific biomarkers will be done for the four RWASA biotypes to validate their repeatability on both blind and mixed biotype samples.
For further information, contact Drs Scott Sydenham or Vicki Tolmay at 058 307 3400 or sydenhams@arc.agric.za.
Publication: September 2017
Section: Focus on
