Good Guys, Bad Guys

by Merry

Be­cause it prefers to dine on some of our val­ued crop plants, the pea aphid (Acyrthosiphon pisum) is consider­ed a ma­jor pest – thus a Bad Guy from our per­spec­tive. Pea aphids are not with­out their en­e­mies. En­emy num­ber one is a par­a­sitoid wasp, Aphid­ius ervi. As par­a­sitoid wasps are used to do, fe­males pro­vide for their off­spring by de­posit­ing their eggs in the haemo­coel of an­other in­sect, in this case a pea aphid. The wasp larva feeds and de­vel­ops there for about a week, even­tu­ally killing the host. When the host vis­cera have been con­sumed, the larva causes the aphid's cu­ti­cle to harden and dry. Soon, an adult wasp emerges from the aphid "mummy." Agri­culturally speak­ing, par­a­sitoid wasps are Good Guys, im­portant part­ners in our in­te­grated pest man­age­ment strate­gies. You can buy wasp eggs for your fields by the hun­dreds or thou­sands on­line.

The Good Guy. Aphid­ius wasps are com­mon­ly used for for bio­con­trol of aphids. Here, a fe­male wasp hits her mark. Source

Early on it was no­ticed that aphid (A. pisum) clones vary greatly in their re­sis­tance to the wasps. In some, the de­vel­op­ment of the wasp larva is ar­rested and the host sur­vives. Be­cause the pea aphid was known to har­bor Buch­n­era aphidi­cola as a pri­mary en­dosym­biont and also as many as five other bac­te­ria as sec­ondary en­dosym­bionts, it was sus­pected that this vari­a­tion in re­sis­tance might be due to which en­dosym­bionts are on board. (What makes a sym­biont a pri­mary rather than a sec­ondary sym­biont? That de­pends on who you ask. Some dis­tin­guish based on the evolu­tionary age of the sym­bio­sis, oth­ers men­tion the ob­lig­ate na­ture of the re­la­tion­ship for both part­ners.)

The Bad Guy. SEM of an aphid, sure to have Buch­n­era on board. Source

The nu­tri­tional role of the Buch­n­era en­dosym­bionts is well known. Aphids feed on phloem – a high sugar, low amino acid plant sap. The Buch­n­era genome en­codes the genes re­quired to make all the es­sen­tial amino acids needed by the aphids. The Buch­n­era live within specia­lized aphid cells (called bac­te­ri­o­cytes) along with a vari­able num­ber of sec­ondary sym­bionts. The sec­ondary sym­bionts char­ac­ter­ized so far in­clude three γ‑proteo­bac­teria, a Rick­ettsia, and a Spiro­plasma. In or­der for these ma­ter­nally-in­her­ited sym­bionts to per­sist or spread in aphid pop­u­la­tions, they must con­fer some ben­e­fit, at least un­der some con­di­tions. Of the three γ‑proteobacteria, the pres­ence of one (Regiella insec­ti­cola) im­proves the sur­vival rate of pea aphids in­fected by a ma­jor aphid fun­gal pathogen (the En­to­moph­tho­rales fun­gus Ery­nia neoaphidis). For the other two (Hamil­tonella de­fensa and Ser­ra­tia sym­bi­ot­ica), the pres­ence of ei­ther one con­fers some re­sis­tance to the par­a­sitoid wasp – thus mak­ing them Bad Guys on the side of the aphid.

We now know quite a bit more about H. de­fensa, thanks to work car­ried out in Nancy Moran's lab. Some in­sights came from their re­cent se­quenc­ing of its com­plete genome. At 2.1 Mb, H. de­fensa has un­der­gone sig­nif­i­cant genome re­duc­tion com­pared to its free-liv­ing close rel­a­tives such as Yersinia and Ser­ra­tia with their 4.6 – 5.4 Mb genomes. On the other hand, it has suf­fered far less gene loss than Buch­n­era, whose genome is a mere 640 Kb. How­ever, H. de­fensa lacks the genes needed to syn­the­size eight es­sen­tial amino acids. For those, it re­lies – like its aphid host – on syn­thesis by Buch­n­era. What H. de­fensa does con­tribute is de­fense against the wasp. It earns its keep when par­a­sitoids are present but oth­er­wise car­ries a fit­ness cost. As the re­sult, H. de­fensa oc­curs only spo­rad­i­cally in these aphids, its in­fec­tion fre­quency fluc­tu­at­ing in con­cert with par­a­sitoid pres­sure.

Ac­com­plices. TEM of aphid bac­te­ri­o­cyte show­ing the larger Buch­n­era pri­mary sym­bionts (ar­row­heads) and the smaller H. de­fensa sec­ondary sym­bionts (ar­rows). Bar = 10 μm. Source

The story be­comes even more com­plex be­cause not all strains of H. de­fensa are de­fend­ers. Those ef­fec­tive in block­ing de­vel­op­ment of the wasp lar­vae carry a λ‑like phage known as APSE. (The source of this acronym? A. pisum secondary endosym­biont.) Even among the phage-car­ry­ing H. de­fensa, some are bet­ter de­fend­ers than oth­ers, likely due to dif­fer­ences be­tween their phage. All of the ASPE phage stud­ied so far have been found to have 95% or more se­quence iden­tity and to en­code some nasty tox­ins. Some of their toxin genes are ho­mol­o­gous to those found in well-known mam­malian pathogens, e.g., genes for Shiga and Campy­lobac­ter je­juni tox­ins. The par­tic­u­lar toxin-en­cod­ing genes present dif­fer from one phage strain to an­other, likely ac­count­ing for the vari­able ef­fec­tive­ness of their H. de­fensa hosts.

That H. de­fensa's de­fense of its aphid host re­quires the ASPE phage does not mean that H. de­fensa doesn't play a part – only that it can't do it alone. H. de­fensa comes from a line of pathogens and has re­tained abun­dant weaponry, in­clud­ing type-III se­cre­tion sys­tems, pu­ta­tive toxin genes, and re­lated reg­u­la­tory fac­tors. Nev­er­the­less, it seems clear that the phage is an es­sen­tial part­ner. To quote the in­ves­ti­ga­tors:"...the evo­lu­tion­ary in­ter­ests of phages, bac­te­r­ial sym­bionts, and aphids are all aligned against the par­a­sitoid wasp that threat­ens them all."

But the wasp, too, is an es­sen­tial part­ner, bring­ing the count of par­tic­i­pants so far up to five: wasp, aphid, Buch­n­era, H. de­fensa, and ASPE phage. With­out the wasp, H. de­fensa would dis­ap­pear from the aphid pop­u­la­tion like so much ex­cess bag­gage. With­out H. de­fensa and its phage, the aphids would be more vul­ner­a­ble to the wasps. With­out Buch­n­era, the aphids would starve and the wasp would have to look for some other host to nour­ish its young. In this pen­tan­gle, there are nei­ther Good Guys or Bad Guys, only an in­ter­wo­ven mé­nage à cinq.

Ref­er­ences

Oliver KM, Deg­nan PH, Hunter MS, & Moran NA (2009). Bac­te­rio­phages en­code fac­tors re­quired for pro­tec­tion in a sym­bi­otic mu­tu­al­ism. Sci­ence (New York, N.Y.), 325 (5943), 992–4 PMID 19696350

Moran NA, Deg­nan PH, San­tos SR, Dun­bar HE, & Ochman H (2005). The play­ers in a mu­tu­al­is­tic sym­bio­sis: in­sects, bac­te­ria, viruses, and vir­u­lence genes. Pro­ceed­ings of the Na­tional Acad­emy of Sci­ences of the United States of Amer­ica, 102 (47), 16919–26 PMID 16195380