Interactions between hosts and their pathogens are numerous and come in so many forms that we continue to categorize them to this day. A unique example often seen in insects is the capability known as polyphenism, where two or more different phenotypes, or physical traits, can emerge from the same genome. This is commonly seen in animals via them donning a winter coat once snow arrives on the scene, the color change trait being derived from the same genes, but alternatively activated due to environmental stimuli.
For insects, this usually involves them changing to suit the amount of food available and the state of the host plant they feed on. It also plays into reproductive cycles and how conditions can cause them to abandon such a cycle entirely if there isn’t suitable nutrients in the region for their young to survive on. When it comes to flying insects, wing polyphenism is a tradeoff that occurs due to the high amount of energy cost that is needed to grow larger wings, meaning that it is only done under extremely special circumstances.
A key ecological factor in wing growth is the quality of the host plant, as this plays directly into the amount of food that the insects have to survive on. If the host plants are malnourished, then likely few of them will be growing in that area with poor soil or whatever factor is affecting them. It is in such a case that the short-winged brown planthopper decides what type of wings to depend on for its lifespan.
This feeder on rice plants couples together its short winged form with its reproductive process. In times of hardship, it grows significantly longer wings and instinctively leaves to find a better home. There are a variety of factors for the rice plant that influence these changes, including growth stage and temperature, along with insect factors like density of nymphs of the species in the surroundings and the hormone production levels for metamorphosis. Research into this phenomenon over the years have isolated several signal transduction pathways that control the type of wing growth that occurs.
But even with this, directly how the life stage of the plants is understood and transmitted by the planthoppers into wingspan remained unknown. To finally figure out this mystery, a collaborative study was done between China Jiliang University and Washington State University. They decided to start with the molecule glucose, as it is a critical development and signaling system in plants and animals, though for differing purposes. It was suspected that glucose levels in the rice plants may play a role in planthopper wing formation.
Therefore, the first step was to measure and model glucose levels in rice plants during each stage of growth, proving that glucose levels increase after the seedling stage up to the grain-filling stage, but then drop off after the plant is matured. Next was to test those glucose levels on live planthoppers and see how their wing growth differed. This was done with special supplemented rice plants with elevated glucose levels in order to see if that offset the ratios and timings. An additional experimental group was set up based on the density of nymphs in the same chamber, ranging from 1 to 20.
The scientists found that, indeed, both nymph density and glucose concentration affected planthopper wings, but only in the female specimens. When densities of nymphs were higher, more long-winged females would happen and this would be independent of plant glucose levels, meaning that nymph density is a stronger controlling trigger for long wing development. As for the glucose supplemented plants, these caused a decrease in long-winged females, showing that food quality or close to harvest quality was a necessity for the insects and without it, they would grow long wings to look for food elsewhere.
Interestingly, for the males, they only saw an increase in long-wingedness at the highest densities and the highest glucose supplementation, both. As noted, for the latter trait, this is the opposite of how the wing development functions for females, perhaps as a means to cause males in areas with lots of insects and plentiful food to leave to seek out females in less well-off regions. A sort of general dispersal and spread mechanism to allow the population to not congregate in only the best food areas, where they could possibly be wiped out all at once from a catastrophe.
Insulin and RNAi Manipulation
Direct injection of glucose into the female nymphs also improved long wing development, as expected. Following these basic confirmations of mechanisms, the researchers looked into seeing if they could mess with or alter wing development by using insulin-related materials. They chose metformin, a drug used to treat diabetes and lower blood glucose levels. Injection of the drug into the nymphs suppressed the effect of glucose supplementation on the development of short wings in females, but also prevented long wing development at high nymph densities, indicating that glucose metabolism is still connected to the response at those high densities.
The researchers were unable to measure glucose levels in the nymphs during the metformin treatment, however, so how exactly it changed the metabolism is unknown. What they were able to detect is that certain gene transcript levels are elevated during consumption of glucose supplemented rice plants. The insulin receptors NlInR1 and NlInR2 showed increased transcription after 24 hours of exposure and this kept up for 48 hours afterwards.
The last part of their experiment involved these two receptor genes. They had actually been involved in prior research studies in relation to wing length, via the use of RNAi to downregulate the expression of each. Knocking out R1 seems to force a shift to short wings, meaning that it is needed for long wing development. R2, meanwhile, did the the reverse. The scientists wanted to test whether this was true under glucose supplementation as well. What they discovered that that R1 downregulation did still result in short wings, but the ratio was amplified at the middle 5 and 10 nymph densities, reaching 100% short wings unlike with the controls.
The scientists hypothesize that nymph density actually runs through a separate non-insulin pathway, but it is the conflicting signals between the glucose vs density pathways that causes the ratios of short wings to long wings, which is why it reaches 100% when knocking out one such pathway. A unique distinction is that R1 knockdown meant that interplays of glucose and density still played a role and were only altered slightly from the gene loss. But when R2 was knocked out, glucose concentration and density no longer mattered, all of the insects became long winged, including regardless of sex. The only exception to the 100% long wings result was at a 1 nymph density on regular non-supplemented plants, where there were at least some short wings that appeared.
Getting Rid of Pests
This study resulted in a much greater and expanded detail on how brown planthoppers relate to the rice plants they feed on and how those plants and the insect numbers play into their wing development. Since long wing production inherently forces the insects to leave their feeding site, it would be preferable to farmers if this could be induced somehow. With the knowledge obtained from this study, it appears possible to use RNAi to do just that, especially with the R2 insulin receptor gene.
But that will be work for future scientists to accomplish.
Photo CCs: Planthopper (Fulgoroidea) (8686208002) from Wikimedia Commons