Symbiosis and Evolution in Aphids

Below is a hodgepodge of info discussing the symbiotic relationship between pea aphids (Acyrthosiphon pisum) and the bacteria Buchnera aphidicola as it relates to adaptation and evolution. By and large, the discussion centers on recent research by Naruo Nikoh and Atsushi Nakabachi; however several other sources are listed at the bottom of this post – including the Douglas Lab’s website which is a great place to follow-up on the latest aphid-symbiosis research.

Most aphids host mutualistic bacteria, Buchnera aphidicola, which live inside specialized cells called bacteriocytes. Buchnera are vital to the aphids well being as they provide essential amino acids that are scarce in its diet. Now research published in the open access journal BMC Biology suggests that the aphids' ability to host Buchnera depends on genes they acquired from yet another species of bacteria via lateral gene transfer (LGT).

symbiotic bacteria Buchnera are located in specialized cells

called bacteriocytes (shown green in figure) in the aphid body cavity. [Drawing by Tomás Lazo]

Atsushi Nakabachi from Japan's RIKEN institute with his colleagues had previously uncovered two clusters of mRNA sequences from the bacteriocyte of the pea aphid Acyrthosiphon pisum that were encoded in the aphid genome, but similar to bacterial genes. Naruo Nikoh from The Open University of Japan and Nakabachi determined these sequences in full for more detailed analysis, and used real-time quantitative RT-PCR experiments to investigate the genes' expression levels in the aphid bacteriocytes.

Phylogenetic Analysis (Figure 3 - From Article)

The evidence points to LGT from bacteria to aphids. Genetic family trees show that one of the genes came from a bacterium closely related to Wolbachia, a common inherited symbiotic microbe, which infects a high proportion of insects. The aphid strain used for the study is free from Wolbachia and other closely related bacteria, but the transferred gene could be a remnant of an infection in the distant past. The evidence suggests that the aphids use these acquired genes to host Buchnera, which has lost many genes that appear to be essential for bacterial life. The association between aphids and Buchnera is over 100 million years old, and has evolved so that today neither the bacteria nor the host can reproduce without the other.

According to Nakabachi,

"the cases presented here are of special interest in that these transferred bacterial genes not only retain their functionality, but are highly expressed in the bacteriocyte that is differentiated so as to harbour Buchnera, which lack such genes."

LGT (also referred to as horizontal gene transfer) occurs when genetic material from one organism finds its way into another organism other than its offspring. Genetic engineering uses LGT deliberately, but there is increasing evidence that LGT has taken place in many organisms (usually between unicellular organisms) naturally. This has caused a major shift in how biologists view genetic family trees.

Symbiotic relationships have a great deal to teach us regarding evolution.Such interdependent relationships are not unusual in the natural world. What is unusual, report Helen Dunbar, Nancy Moran, and colleagues in a new study published [last year] in the open access journal PLoS Biology, is that a single point mutation in Buchnera's genome can have consequences for its aphid partner that are sometimes detrimental, and sometimes beneficial.

The authors probe Buchnera's and A. pisum's ability to tolerate heat. When exposed to high temperatures, Buchnera is supposed to activate special "heat-shock" genes whose products help to protect proteins from heat-related degradation. By using microarrays to assess activity of A. pisum and Buchnera genes, the researchers discovered that after a four-hour exposure to 35 °C temperature, some of their laboratory strains of Buchnera upregulated the heat-shock genes, but others did not. Further analysis showed the genetic basis for the difference: a single missing nucleotide in an adenine-filled stretch of DNA, called a promoter, that's involved in activating the heat-shock gene. Testing at a range of temperatures from 15 °C to 35 °C showed that activation of the heat-shock gene was consistently lower in the lines with the missing nucleotide than in the normal bacteria.

What does this mean for A. pisum's ability to tolerate tough conditions? To answer that, the researchers asked whether exposing juvenile aphid hosts of Buchnera with either long or short promoters to four hours of high temperatures (35 or 38 °C) affected their ability to reproduce. They found that few of the aphids with bacteria bearing short promoters reproduced after the heat treatment, while those with bacteria bearing the longer promoters had no trouble. In addition, aphids that had been exposed to the high temperatures and had the short-promoter-bearing bacteria weighed less as adults and had far fewer Buchnera inside them than did their counterparts with long-promoter-bearing bacteria.

Given these seemingly huge disadvantages to dropping a single adenine, it's hard to believe the mutation could last long in a Buchnera population. Yet, by sequencing and comparing the Buchnera associated with various A. pisum lines, the researchers discovered that the short-promoter option had arisen and been fixed twice in laboratory stock and was also found at frequencies of 21% and 13%, respectively, in bacteria in field-collected aphids from Wisconsin and New York.

Population genetic theory predicts that when a mutation is maintained in a population at high frequencies, it likely confers some benefit to its bearer. What could be the advantage of carrying a gene that causes one to lose the ability to reproduce at high temperatures?

A clue to the answer comes from the wild populations in which the mutation was not found: those living in Arizona and Utah. Could the bacterial mutation confer a competitive advantage that's only relevant in cooler climates? To find that out, the researchers performed a second test using a range of four-hour exposure temperatures. They discovered that short-promoter bacteria-bearing aphids produced progeny faster than did the normal ones when raised at 15 °C or 20 °C. Thus, though aphids containing bacterial symbionts with the heat-shock-promoter mutation fare worse than normal aphids after exposure to high temperatures, they do better under cool conditions, giving the mutation a selective advantage that causes it to be maintained in the population.

In addition to their explorations of A. pisum and its Buchnera, Moran's team also looked for and found multiple-adenine stretches related to heat-shock genes in Buchnera symbiotic with other aphid species. This offers fertile ground for further study of the intriguing interplay among aphids, bacteria, and temperature.

Sources: Eureka Alert, PhysOrg and Douglas Lab

Nikoh, N., & Nakabachi, A. (2009). Aphids acquired symbiotic genes via lateral gene transfer BMC Biology, 7 (1) DOI: 10.1186/1741-7007-7-12

Dunbar, H., Wilson, A., Ferguson, N., & Moran, N. (2007). Aphid Thermal Tolerance Is Governed by a Point Mutation in Bacterial Symbionts PLoS Biology, 5 (5) DOI: 10.1371/journal.pbio.0050096