How Many Genomes Does It Take to Change a Light Bulb?

by Elio

Let me rephrase that. How many genomes does it take for an aphid to make tryp­to­phan? If you thought it was one, you'd be wrong: the an­swer is two, nei­ther be­ing the aphid's. Re­mem­ber that aphids live off plant sap that is poor in some amino acids? And that they get some of the miss­ing amino acids from the me­tab­o­lism of an endo­sym­biont, Buch­n­era aphidi­cola ? That's true enough, but it turns out that in the cedar aphid, Cinara cedri, the re­sident Buch­n­era has lost its abil­ity to sup­ply the host with tryp­to­phan. Ac­cord­ing to in­ves­ti­ga­tors from the Univer­sity of Va­len­cia, Spain, this Buch­n­era car­ries the tr­pEG genes on a plas­mid, but lacks tr­pD­CBA, the re­main­ing genes of the path­way.

Fig­ure 1. The cedar aphid and a TEM of a cell con­tain­ing two en­dosym­bi­otic bacte­ria. Cour­tesy of A. La­torre and J. M. Go­salbes

This genic four­some is lo­cated on the chro­mo­some of an­other sym­biont, Can­di­da­tus Ser­ra­tia sym­bi­ot­ica. (To re­mind you, Can­di­da­tus means that the bac­terium has not yet been cul­ti­vated. In Ro­man times, can­di­da­tus re­ferred to a sol­dier await­ing pro­mo­tion.) Both sym­bionts are present in about the same num­ber within spe­cial­ized cells, the bac­te­ri­o­cytes. Why this di­vi­sion of la­bor? This is grist for the evolutionist's mill. We in­vite your views.

Fig­ure 2. The pre­dicted biosyn­the­sis path­way and tryp­to­phan flux. Source

Such a com­ple­men­ta­tion of meta­bolic abil­i­ties is far from unique to this aphid. An even more per­va­sive ex­am­ple of di­vi­sion of la­bor is seen in a sap-feed­ing in­sect, the leaf­hopper known as the glassy-winged sharp­shooter (Homa­lodisca co­ag­u­lata). Ac­cord­ing to McCut­cheon and Moran, bac­te­r­ial en­dosym­bionts act as sep­a­rate bio­chem­i­cal sup­ply houses. (We dis­cussed an ear­lier 2006 re­port from Moran and oth­ers here.)

Fig­ure 3. The meta­bolic con­tri­bu­tions of Sul­cia and Bau­man­nia to the glassy-winged sharp­shooter. The ma­jor con­stituents of the sap are shown in green, above. The large col­ored ar­rows show com­pounds pro­duced by the sym­bionts and needed by the host; the small col­ored ar­rows in­di­cate hy­poth­e­sized shar­ing be­tween sym­bionts. Source (Click to en­large)

As de­ter­mined by their cod­ing ca­pac­i­ties, one of the sym­bionts, Sul­cia muel­leri, a mem­ber of the Bac­teroidetes, pro­vides nine amino acids plus a gene for fatty acid syn­thesis. This is pretty sporty of this or­gan­ism, be­cause its highly re­duced genome (a mere 245,530 bp) en­codes a pal­try 228 pro­tein genes. The other sym­biont, the γ‑pro­teobacterium Bau­man­nia ci­cadellini­cola, sup­plies two amino acids and a gemisch of fatty acids, coen­zymes, purines, pyrim­idines, plus a col­lec­tion of other meta­bolites.

This story mat­ters to wine mak­ers and wine drinkers be­cause the sharp­shooter car­ries the bac­terium Xylella fas­tidiosa, the agent of the grape vine killing Pierce's dis­ease. Click here for more on sym­bioses be­tween sap-feed­ing in­sects and bac­te­ria.

Ex­am­ples of mul­ti­ple-part­ner sym­bioses range across phyla. A ma­rine worm, the oligo­chete Olav­ius al­gar­ven­sis, lacks a mouth, gut, and nephridia (kid­ney-like or­gans). To make a liv­ing, this worm needs the work of no less than four sym­bi­otic bac­teria. These sym­bionts ox­i­dize sul­fur, re­duce sul­fate, and fix car­bon, thus pro­vid­ing the host with var­ied sources of nu­tri­ents.

Fig­ure 4. Pierce's dis­ease of grapevines, caused by the bac­terium Xylella fasti­di­osa, which is trans­mit­ted by a sym­biont-car­ry­ing glassy-winged sharpshoot­er. Source

The sym­bionts also par­tic­i­pate in re­cy­cling of the worm's waste prod­ucts, which may be how the worms make do with­out an ex­cre­tory sys­tem – a unique skill among this group of an­i­mals. (Fig. 5).

Fig­ure 5. Olav­ius al­gar­ven­sis, a gut­less worm that shut­tles its sym­bi­otic bac­te­ria to op­ti­mal en­ergy sources in the up­per oxy­gen-rich and the lower oxy­gen-de­­pleted coastal sed­i­ments. In ex­change, the mi­crobes pro­vide their worm host with fixed car­bon and all the amino acids and vi­t­a­mins it needs Source

Lynn Mar­gulis once called ours the Sym­bi­otic Planet and wrote a book by that ti­tle. Hardly an over­state­ment! This would be a good time to read the re­view on sym­bio­sis as an adap­tive process by Nancy Moran.