In the last day I read with great interest the intriguing review entitled “A symbiotic view of life: we have never been individuals” written by Scott F. Gilbert, Jan Sapp and Alfred I. Tauber and published in The Quarterly Review of Biology.
Due to their parthenogenetic reproduction aphids are generally considered a sort of clone so that each individual is identical to the others in the population. According to this suggestion, several Authors refereed to aphids as a single genome species. Actually, as well stated by Gilbert et al, the one-genome/one-organism doctrine of classical genetics has been eclipsed by recent studies on symbiosis. In particular, aphid microbial symbionts form a second type of genome and genetic inheritance (Moran 2007; Gilbert 2011). As frequently suggested, insects may acquire their symbionts vertically though the maternal germline as well as horizontally from the environment (such as during feeding). In particular, in aphids, symbiotic bacteria provide selectable allelic variation (thermotolerance, color, parasitoid resistance) that enables some hosts to persist better under different environmental conditions (Dunbar et al. 2007; Tsuchida et al. 2010).
A well-studied example is the pea aphid, Acyrthosiphon pisum since variants of its symbiont Buchnera provide the aphid with thermotolerance, even if at the expense of fecundity at normal temperatures; Dunbar et al. 2007). The second bacterial symbiont Rickettsiella is responsible for color change, turning genetically red aphids into green through the synthesis of quinones (Tsuchida et al. 2010). Furthermore, variants of Hamiltonella symbionts provide immunity against parasitoid wasp infection (Oliver et al. 2009). Interestingly, in the last case, the protective role of Hamiltonella is due to the incorporation of a specific lysogenic bacteriophage within the bacterial genome. The aphids are therefore infected by Hamiltonella that must be infected by phage APSE-3. As Oliver et al. (2009) wrote, “In our system, the evolutionary interests of phages, bacterial symbionts, and aphids are all aligned against the parasitoid wasp that threatens them all. The phage is implicated in conferring protection to the aphid and thus contributes to the spread and maintenance of H. defensa in natural A. pisum populations” .
However, symbioses are frequently not for free for the hosts and even if aphids have some advantages due to symbionts in the presence of parasitoids having their beneficial protection, in the absence of parasitoid wasps aphids carrying the bacteria with lysogenic phage are not as fecund as those lacking them. Similarly, a trade-off occurs in aphids that carry the thermotolerant genetic variants of Buchnera, meaning that more heat-resistant aphids have less fecundity at milder temperatures than their sisters whose bacteria lack the functional allele for the heat-shock protein.
As Gilber et al reminded at the ned of their review, not all scientists involved in evolution agree about the important role of symbiosis so that, for instance, in the 2009 “Homage to Darwinism” debate held at Oxford University, Richard Dawkins questioned the bringing of symbiosis into evolutionary theory: “Take the standard story for ordinary animals, [where] you’ve got a distribution of animals [and] you’ve got a promontory, or an island or something and so you end up with two [geographical] distributions. And then on either side you get different selection pressures, and so one [group] starts to evolve this way, and [the other] one starts to evolve that way, and what’s wrong with that? It’s highly plausible, it’s economical, it’s parsimonious. Why on Earth would you want to drag in symbiogenesis when it’s so unparsimonious and uneconomical?”. As Lynn Margulis replied at that time… simply because symbiosis exists and it is common in living organisms.
References
- Dunbar HE, Wilson AC, Ferguson NR, & Moran NA (2007). Aphid thermal tolerance is governed by a point mutation in bacterial symbionts. PLoS biology, 5 (5) PMID: 17425405
- Gilbert S. F. 2011. Symbionts as genetic sources of hereditable variation. pp. 283–293. In Transformations of Lamarckism: from sbtle fluids to molecular biology, edited by S. B. Gissis and E. Jablonka. Cambridge (Massachusetts): MIT Press.
- Moran NA (2007). Symbiosis as an adaptive process and source of phenotypic complexity. Proceedings of the National Academy of Sciences of the United States of America, 104 Suppl 1, 8627-33 PMID: 17494762
- Oliver, K., Degnan, P., Hunter, M., & Moran, N. (2009). Bacteriophages Encode Factors Required for Protection in a Symbiotic Mutualism Science, 325 (5943), 992-994 DOI: 10.1126/science.1174463
- Tsuchida T., Koga R., Horikawa M., Tsunoda T., Maoka T., Matsumoto S., Simon J.-C., Fukatsu T. 2010. Symbiotic bacterium modifies aphid body color. Science 330:1102–1104.
