Sir

You are showing your zoocentric side with your description on the In This Issue page of the rarity of three-way symbioses (“Ménage a trois”, Nature 411, xi; 2001). This sort of arrangement is not so rare among plants: for example, leguminous plants with mycorrhizal associations — see Lichen Biology (ed. T. Nash III, Cambridge Univ. Press, 1996). An even more intimate relationship exists among a number of tripartite lichens, where a composite organism is formed through the symbiosis of an ascomycete, a chlorophytic alga and a cyanobacterium. This association can be so specific that, from a practical standpoint, at least two of the individual partners may not exist outside the symbiotic relationship.