Sym­biont Ex­change in the Aphid·Buchnera Sym­bio­sis (1|2)

by Christoph

For evo­lu­tion­ary bi­ol­o­gists, the aphid·Buch­n­era sym­bio­sis is a fa­vorite model to study 'or­ga­nel­lo­ge­ne­sis', how a once free-liv­ing gammaproteo­bacterium be­came an oblig­a­tory en­dosym­biont of its aphid host – now its buddy rather – over the course of >200 Ma (mil­lion years) of co-evo­lu­tion. To­days Buch­n­era en­dosym­bionts have reached the stage of mi­to­chon­dria or chloro­plasts – well, al­most – and this shines a bright spot­light on Lynn Mar­gulis' fa­mous 'en­dosym­biont the­ory' from 1967. We at STC have fea­tured this sym­bio­sis ear­lier here, and a fairly re­lated case, the mealy­bug story, here. But note, it also got its very own blog: the aphid room (no longer avail­able in 2023).

Jen­nifer Frazer re­ported re­cently here in STC on 'sym­biont swap­ping' in the dark brown lichen Ce­traria ac­uleata. This lichen's fun­gal part­ner ap­par­ently dis­missed the al­gal, pho­to­syn­the­siz­ing part­ner on its way from the Arc­tic to the Mediter­ranean – or vice versa, that's not known for sure yet – and ac­quired a bet­ter adapted one, be­cause 'cli­mate change'. For the cu­ri­ous evo­lu­tion­ary bi­ol­o­gist, this raises the ques­tion whether 'sym­biont swap­ping' is also pos­si­ble for the highly in­ti­ma­te aphid·Buch­n­era sym­bio­sis. Spoiler alert: the an­swer is Yes.

Sym­biont Ex­change in the Wild

The ob­lig­ate Buch­n­era aphidi­cola en­dosym­biont, found so far in most mem­bers of the Aphi­di­dae fam­ily, has been lost in many species of the mono­phyletic Ce­ra­ta­phi­di­nae sub­fam­ily. For ex­am­ple, the co­conut aphid Ce­ra­ta­phis brasilien­sis, har­bors in­stead a yeast-like fun­gal sym­biont, YLS. C. brasilien­sis (Fig. 1) is an in­ter­est­ing in­sect in its own right – and here is its daunt­ingly com­plete ta­xo­no­mic de­scrip­tion – but I will fo­cus on the fun­gal sym­biont YLS.

Fig­ure 1. Cer­at­aphis brasilien­sis (Hempel) with nymphs, on co­conut palm leave. Length of adults: 1–2 mm. (Don't these aphids look just like tiny smushed co­conuts?). Source

In C. brasilien­sis, the YLS re­sides both in­tra- and ex­tra­cel­lu­lar­ly but not in spe­cial­ized cells – as do the Buch­n­era in the bac­te­ri­o­cytes (Fig. 2) – and is trans­mit­ted ma­ter­nally. Ef­forts to cul­ti­vate YLS out­side its host failed so far, un­der­pin­ning the in­ti­macy of this sym­bio­sis. The yeast-like growth of YLS is a mor­pho­log­i­cal trait of the 'true yeasts' like Sac­cha­romyces cere­visiae (As­co­my­ce­tes) but also fre­quently found among other As­comycetes and Ba­sid­iomycetes. In the lat­ter two phyla, one finds species with di­mor­phic growth – i.e., the ca­pac­ity to grow ei­ther yeast-like by bud­ding-off daugh­ter cells or by form­ing sep­tated hy­phae – es­pe­cially among path­o­genic species.

Fig­ure 2. Sym­bionts (Buch­n­era aphidi­cola) with­in a bac­te­ri­o­cyte of a pea aphid (Acyrthosiphon pisum). The cen­tral ob­ject is the host nu­cleus; Buch­n­era cells are round and packed into the cy­to­plasm. Source

A pre­lim­i­nary phy­lo­ge­netic analy­sis – 'pre­li­mi­na­ry' be­cause a min­i­mal­is­tic set of just three highly con­served pro­teins was used for tree con­struc­tion – sug­gests that YLS be­longs to a branch of the As­comycetes that also in­cludes the en­to­mopath­o­genic fun­gus Cor­dy­ceps mi­li­ta­ris. Sev­eral mem­bers of the 400+ species of the Cor­di­ceps fam­ily are im­por­tant in tra­di­tional Chi­nese and Ti­be­tan med­i­cine, and cordy­cepin, iso­lated from C. mil­i­taris, is a base-ana­log, ac­tu­ally 3'-deoxy-adenosine, with a po­ten­ti­al of an anti-can­cer drug. Also in this branch of the As­comycetes one finds Fusar­ium oxys­po­rum "whose strains rep­re­sent some of the most abun­dant and wide­spread mi­crobes of the global soil mi­croflora", as its Wi­ki­pe­dia en­try claims, and which is one of the most heartily hated fungi in mi­cro­bi­o­lo­gy labs be­cause its spores in­e­vi­tab­ly in­oc­u­late all other cul­ture dishes around. Yes, quar­an­tine wards are also an is­sue in mi­cro­bi­o­lo­gy labs. But ob­vi­ously, the C. brasilien­sis aphids have at one point suc­cess­fully 'do­mes­ti­cated' a fun­gus from a po­ten­tially dan­ger­ous fam­ily.

Vo­gel and Moran se­quenced the genome of YLS to learn more about its meta­bolic prop­er­ties and to com­pare genome evo­lu­tion of prokary­otic and eu­kary­otic en­dosym­bionts un­der con­di­tions of small pop­u­la­tion sizes and re­strained pos­si­bil­i­ties for ge­netic ex­change (ex­cept with their host). YLS ap­pears to have a com­plete set of func­tional re­com­bi­na­tion pro­teins, in­clud­ing the genes ne­cessary for mei­otic di­vi­sion (al­though meio­sis and the for­ma­tion of sex­ual spores hasn't been ob­served yet). This is in stark con­trast to bac­te­r­ial en­dosym­bionts whose genomes usu­ally lack re­com­bi­na­tion func­tions al­to­gether. Bioin­for­matic analy­sis of the YLS genome re­vealed the pre­sen­ce of com­plete meta­bolic path­ways for the use of glu­cose and sev­eral other sug­ars as car­bon source, aer­o­bi­cally and fer­men­ta­tive. The path­ways for the syn­the­sis of all amino acids, es­sen­tial and non-es­sen­tial, are present in the YLS genome, which would al­low YLS to sup­ply its host as ef­fi­ciently as Buch­n­era. In ad­di­tion, the YLS genome en­codes a plethora of path­ways for the syn­the­sis of sec­ondary metabo­lites, many of them po­ten­tial an­timi­cro­bials, which could con­tribute to the de­fense of  C. brasilien­sis against preda­tors of any size.

As com­pared to its free-liv­ing clos­est rel­a­tives, a num­ber of or­thol­o­gous genes show a higher de­gree of ge­netic drift – ac­cu­mu­la­tion of codon sub­sti­tu­tions – in YLS, which is sim­i­larly found in genes from bac­te­r­ial en­dosym­bionts, in­clud­ing Buch­n­era, when com­pared to those of free-liv­ing bac­te­r­ial rel­a­tives. How­ever, there are no signs of genome re­duc­tion but rather genome ex­pan­sion: in the cases com­pared, e.g., in­trons in or­thol­o­gous genes tend to be larger in YLS than in its rel­a­tives. For now, this type of genome analy­sis is at its very be­gin­ning, yet it's al­ready ap­par­ent that genome dy­nam­ics in prokary­otic and eu­kary­otic en­dosym­bionts are quite di­verse.

Fig­ure 3. Bac­te­ri­o­cytes in their typ­i­cal bi-lobed loca­lization in the aphid's ab­domen, con­tain­ing Buch­n­era cells (stained green). Size bar: not leg­i­ble. (Credit: An­gela Dou­glas). Source

When did the sym­biont ex­change in the Cer­at­aphid­i­nae hap­pen? Fos­sils of their pri­mary host plant, Styrax sp., date back to the late Cre­ta­ceous (99 – 65 Ma) and their con­tin­ued into the Eocene (55 – 35 Ma). There­fore, the es­tab­lish­ment of YLS in lieu of Buch­n­era as ob­lig­ate sym­biont in an an­ces­tor of C. brasilien­sis may be con­si­de­rab­ly more re­cent than the aphid·Buch­n­era sym­bio­sis with its as­sumed age of >200 Ma. By the way, 'more re­cent' seems some­what in­ap­pro­pri­ate when talk­ing of pe­ri­ods of tenth of mil­lions of years but these are evo­lu­tion­ary time scales. Yet there are not only the fos­sil record or ex­trap­o­la­tions from phy­lo­ge­netic stud­ies! Braen­dle and cowork­ers found by an­ti­body stain­ing of tis­sue sam­ples that in Tu­beraphis styraci, an­other mem­ber of the Cer­at­aphid­i­nae that car­ries YSL as sym­biont in place of Buch­n­era, the dif­fer­en­ti­a­tion of two types of bac­te­ri­o­cyte pre­cur­sor cells and their lo­cal­iza­tion in em­bryos (Fig. 3) fol­low the same timely or­der as in A. pisum, al­beit at a largely re­duced num­ber of bac­te­ri­o­cyte pre­cur­sor cells. These au­thors could also show that bac­te­ri­o­cyte dif­fer­en­ti­a­tion and lo­cal­iza­tion pro­ceeds largely undis­turbed in Buch­n­era-de­pleted A. pisum aphids – de­ple­tion was achieved by an­tibi­otic treat­ment – which demon­strates that bac­te­ri­o­cyte dif­fer­en­ti­a­tion is not trig­gered by the en­dosym­bionts. There­fore, the still largely func­tional path­way of bac­te­ri­o­cyte for­ma­tion in Tu­be­ra­phis styraci makes it likely that adap­ta­tion to its (now lost) Buch­n­era en­dosym­biont had pre­ce­ded the ac­qui­si­tion of the YSL. At present we can only spec­u­late on why the Cer­at­aphid­i­nae branch of the aphids skipped their al­ready es­tab­lished sym­bio­sis with Buch­n­era in fa­vor of a sym­bio­sis with a fun­gus. But it ap­par­ently hap­pened and it was not just a brief es­capade.

To come full cir­cle, next week's post will ad­dress sym­biont ex­change in the lab...