One Sym­biont Is Good, Two Are Bet­ter

The For­ever Fas­ci­nat­ing Story of the Leaf-Cut­ting Ants and Their Bac­te­ria

by Elio

Now here's a ques­tion you've been ask­ing all along about the in­ter­ac­tion be­tween the leaf cut­ting (At­tine) ants, the fungi they cul­ti­vate, and the bac­te­ria that make an­ti­fun­gals against un­wanted fun­gal species. Have these bac­te­ria evolved along with the ants to pro­tect their gar­dens from un­wanted "weeds," or do the ants pick up such bac­te­ria from their en­vi­ron­ment? New data sug­gest both things hap­pen.

Leaf-cut­ting ants on their way home. Source

To re­mind you, leaf-cut­ting ants prac­tice fung­i­cul­ture, and have been do­ing this for about 50 mil­lion years. The bits of leaves and flow­ers that they bring to the nest get chewed up, fer­til­ized, placed in suit­able "gar­dens" within the nest, and seeded with fun­gal ma­te­r­ial from pre­vi­ous gar­dens. Ob­vi­ously, fun­gal gar­dens can be over­run by un­wanted species, with dis­as­trous re­sults for the colony. To keep this from hap­pen­ing, the ants de­pend on se­lec­tive an­ti­fun­gals made by actin­o­mycetes. We have vis­ited this topic in the past (click here and here).

Look­ing at three colonies of leaf-cut­ting ants col­lected in Trinidad, the au­thors of a re­cent pa­per ex­am­ined two kinds of actin­o­mycetes present, a Pseudono­car­dia and a Strep­to­myces. The Pseudono­car­dia are ver­ti­cally trans­mit­ted through the queen ants and may well have evolved with the ants. In the words of Cameron Cur­rie:

...when a queen leaves for her mat­ing flight, she car­ries a pel­let of fun­gus, col­lected from her na­tal nest, in her in­frabuc­cal pocket, and trans­ports the mu­tu­al­is­tic bac­te­ria on the cu­ti­cle... In­deed, both mi­cro­bial sym­bionts show some de­gree of broad-scale phy­lo­ge­netic con­gru­ence with the ant host. Strict ant-sym­biont phy­lo­ge­netic con­gru­ence is, how­ever, dis­rupted by sym­biont switches (hor­i­zon­tal trans­mis­sion) be­tween fun­gus-grow­ing ant colonies, species, and even gen­era.

The di­verse colony mor­phol­ogy of Actin­o­mycete species iso­lated from Acromyrmex­oc­tospinosus worker ants. Strep­to­myces strains S1-S9 and Pseudono­car­dia strains P1-P2. Source

The view that ants and Pseudono­car­dia co­e­volved has been chal­lenged on the grounds that there is plenty of free-liv­ing Pseudono­car­dia in the en­vi­ron­ment (plus some other ar­gu­ments). The Strep­to­myces, on the other hand, are agreed upon to be new ar­rivals be­cause they are com­mon in the en­vi­ron­ment and, I'm guess­ing here, not known to be car­ried by the queens to the new nests. The Pseudono­car­dia make an un­usual an­ti­fun­gal called dentigerumycin, plus a rel­a­tive of nys­tatin, whereas the Strep­to­myces make the well-known an­ti­fun­gal can­di­cidin. Genome scan­ning re­vealed that these Pseudono­car­dia have the genes needed to pro­duce the nys­tatin-like an­ti­fun­gal.

When you think of it, the ants bet­ter be clever in their se­lec­tion of antifungal–producing bac­te­ria. Too broad the ac­tiv­ity spec­trum of an an­ti­fun­gal and the cul­ti­vated fungi may be af­fected, lead­ing to a calami­tous loss of food. So, one would guess that once a suc­cess­ful sym­bio­sis has been es­tab­lished, it would keep go­ing. But here and there, some ad­di­tional help from bac­te­ria in the en­vi­ron­ment may add an­ti­fun­gal fine-tun­ing to the sit­u­a­tion. What emerges is that the greater the num­ber of part­ners in a sym­bio­sis, the more wooly it gets.

Ref­er­ences

Barke J, Seipke RF, Grüschow S, Heav­ens D, Drou N, Bibb MJ, Goss RJ, Yu DW, Hutch­ings MI. (2010). A mixed com­mu­nity of actin­o­mycetes pro­duce mul­ti­ple an­tibi­otics for the fun­gus farm­ing ant Acromyrmex oc­tospinosus. BMC bi­ol­ogy, 8. PMID 20796277

Poulsen M, Cur­rie CR. (2010). Sym­biont in­ter­ac­tions in a tri­par­tite mu­tu­al­ism: ex­plor­ing the pres­ence and im­pact of an­tag­o­nism be­tween two fun­gus-grow­ing ant mu­tu­al­ists. PloS one, 5 (1). PMID 20090958