Different species, different responses to cold

In some aphid species, such as Sitobion avenae and Myzus persicae (in the photo below from Invasive.org), overwintering is not reliant on eggs, so that the interaction between the cold tolerance of active stages and the severity of winter temperatures may have a dominant effect on their subsequent population dynamics in terms of the timing of migrations and seasonal abundance.

According to literature data, Myzus persicae could produce genetically distinct clones, both in the holocyclic and anholocyclic life cycle, leading to the possibility that clones differ in their activity thresholds, and, as a consequence, any pattern in thermal tolerance will depend on the composition of the population.  In particular, if activity thresholds are related to the latitude of aphid collection, it could be possible to hypothesize that aphid clones from colder climates where sub-zero temperatures are more common, e.g. Scandinavia, would display greater cold adaptation. At the same time, Scandinavian clones should display a reduced recovery time from chill coma. In contrast, a clone from a warmer climate where sub-zero temperatures are unlikely, e.g. the Mediterranean, would be hypothesized to display greater heat adaptation, but a reduced cold adaptation… but is it really true?

Alford et al. evaluated the activity thresholds in nine anholocyclic clones of the peach-potato aphid M. persicae collected along a latitudinal cline of its European distribution from Sweden to Spain. Interestingly they showed that high-temperature activity thresholds were less plastic comparing clones than low-temperature thresholds. Furthermore they showed that aphid thermal tolerance could be governed more by clonal type than the latitudinal origin of the populations and that aphids of the genus Myzus have little or no ability to maintain the acclimation process over successive generations.

However, the absence of acclimation seems not be a general feature in aphids… since Powell and Bale showed that low temperature exerts an intergenerational effect resulting in increased cold tolerance in the grain aphid  Sitobion avenae with a preferential investment of increased cold tolerance in first born progeny through successive generations.The reported intergenerational increases in cold tolerance can be seen as part of a more generalised ability of aphids to exhibit between-generation directional changes in response to a stress or other environmental perturbation that persists over longer periods of time, such as changes in temperature, host plant quality or carbon dioxide (CO2) concentration.

The ability of aphids to have inherited acclimation or other faster adaptation mechanism seems to be at present still controversial and it could be interesting to have data also on other aphid species since most of the studies on acclimation and thermal tollerance have been mainly performed on S. avenae and M. persicae leaving almost unexplored other intriguing aphid species.

References

• Alford, L., Blackburn, T.,  Bale, J. (2012) Effects of acclimation and latitude on the activity thresholds of the aphid Myzus persicae in Europe. Journal of Applied Entomology, 136, 332-346 DOI: 10.1111/j.1439-0418.2011.01658.x
• Powell, S., & Bale, J. (2008) Intergenerational acclimation in aphid overwintering Ecological Entomology, 33, 95-100 DOI: 10.1111/j.1365-2311.2007.00947.x

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Do you have endosymbionts useful for thermal tolerance?

One of the most studied symbioses occurs between aphids and the protobacterium Buchnera aphidicola that has been identified as primary endosymbiont of the pea aphid Acyrthosiphon pisum.
In view of its long association with aphids, Buchnera lost several genes, including those for the anaerobic respiration, the synthesis of different biomolecules (including amino sugars and some carbohydrates) and the biosynthesis of cell-surface components, such as lipopolysaccharides and phospholipids indicating that Buchnera is completely symbiotic. Buchnera also lost regulatory factors, so that it continuously overproduces tryptophan and other amino acids that are useful for aphids.

Figure. As reported in Nature, different pathways lost genes in the Buchnera genome. Pathways or steps for which no enzymes were identified are pink.

Almost all aphid species have 60–80 huge cells, called bacteriocytes, where Buchnera bacteria (in green in the figure) live. As reported for other symbionts, Buchnera is maternally transmitted to eggs and embryos through host generations, and the mutualism between the host and the bacteria is so obligate that neither can reproduce independently.
Different roles have been suggested for symbionts other than the synthesis of amino acids only. Buchnera might, for instance, play a key role in aphid thermal tolerance. Thermal tolerance of the primary endosymbiont Buchnera is attributed to genes coding for heat shock proteins, which deter degradation of protein secondary structure. Secondary endosymbionts (other optional bacterial species that can be present within the aphid body), such as Serratia simbiotica, play a similar role in the thermal tolerance of their host strengthening the ability of aphids to evolve further adaptations to overcome the impacts of warming.
Buchnera are at least partly able to survive at high temperatures because of constitutive expression of genes that are normally upregulated in response to heat and aphids could be able to thrive under temperatures as high as 35°C in the laboratory.

Unfortunately, the prolonged permanence of aphids at high temperatures (for instance in hot summer with daily mean temperature of 32.5 °C) results in the elimination of Buchnera reducing not only the thermal tolerance of aphids, but also their fecundity since the lack of endosymbionts results in a lost synthesis of amino acids essential for the hosts.

According to these results, global warming could be difficultly faced by aphids in tropical regions due to Buchnera symbiont depletion.  Interestingly, in the presence of low density of primary symbionts, secondary symbionts (such as Hamiltonella defensa,  Serratia symbiotica, Regiella insecticola) could be more present and improve aphid thermal tolerance to high temperatures clearly suggesting a role for heritable symbionts in the adaptation of aphids to their abiotic environments.

This is way I love evolution… there isn’t a unique solution to problems!

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