Abstract Mutualistic associations between different or- ganisms are theoretically expected when the interests of independently reproducing units are aligned to form a single reproductive unit. This alignment does not come about easily, because models show that hosts and symbi- onts can be in conflict over the transmission of symbi- onts. Selection will favour hosts that are able to limit ge- netic variation of symbionts, for example by enforcing uniparental vertical transmission, while symbionts will be selected to disperse independently of the host. A cru- cial factor determining the evolution and elaboration of symbiotic relationships is therefore who controls the transmission of symbionts. In the fungus-growing ter- mites (Macrotermintinae) horizontal transmission seems to be the rule as the termites normally acquire their culti- vated fungus (Termitomyces) from the environment. In spite of this general pattern, uniparental, vertical trans- mission has evolved in two unrelated Macrotermitinae genera, where only one sex of the two primary reproduc- tives carries asexual spores from the fungal comb of its parent colony to inoculate the new fungus comb. Re- markably, symbiont transmission is exclusively paternal in Macrotermes bellicosus, whereas symbionts are ma- ternally inherited in all Microtermes species studied so far. Thus, in Macrotermitinae horizontal transmission is the ancestral state with two independent origins to unipa- rental, vertical transmission. This is in contrast to fun- gus-growing ants where uniparental, vertical transmis- sion is the rule. Causes and consequences of this differ- ence are further discussed. Despite this fundamental dif- ference both groups evolved a similar symbiosis that is probably the key for their ecological success: the fungus- growing ants in the neotropics and the fungus-growing termites in the paleotropics. Keywords Evolutionary conflict · Host symbiont conflict · Mutualism · Symbiosis · Termitomyces Introduction Mutualisms are best viewed as reciprocal exploitations that nonetheless provide net benefits to each partner (Herre et al. 1999). This view implies that there is no qualitative distinction between “truly parasitic” and “tru- ly mutualistic” interactions, but that the degree of mutu- alism in symbiotic relationships can be considered as a continuous variable. Among the factors that have been thought to promote more mutualistic interactions is ge- netic homogeneity of symbionts (e.g. Herre et al. 1999). Symbiont fitness has two components: one arises from the overall success of the group of symbionts within a host, and the other originates from the competitive suc- cess and transmission relative to other symbionts within the same host. As relatedness declines within hosts, a genotype’s success depends more on its ability to out- compete its neighbours and less on the overall success of the group (Hamilton 1972). Thus, declining relatedness of symbionts should favour them to compete more in- tensely. This intense competition is likely to decrease the overall success of the group of symbionts and to have virulent side-effects on the host. Therefore, it is in the in- terest of hosts to keep symbionts genetically homogene- ous. One way to achieve this is to reduce mixing of sym- bionts by clonal and vertical uniparental transmission (Frank 1996). The synthetic model by Frank (1996) arose from a precursor that addressed the evolution of uniparental inheritance of cytoplasmic elements like mi- tochondria and chloroplasts (Cosmides and Tooby 1981; for a recent review see Hurst et al. 1996). Recently, the theory has also been applied more generally to mutualis- tic relationships (Frank 1996; Herre et al. 1999). Before Communicated by A. Cockburn J. Korb ( ✉ ) Biologie I, Universität Regensburg, 93040 Regensburg, Germany e-mail: judith.korb@biologie.uni-regensburg.de Tel.: +49-941-9432461, Fax: +49-941-9433304 D.K. Aanen Department of Population Ecology, Zoological Institute, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen, Denmark Behav Ecol Sociobiol (2003) 53:65–71 DOI 10.1007/s00265-002-0559-y REVIEW Judith Korb · Duur K. Aanen The evolution of uniparental transmission of fungal symbionts in fungus-growing termites (Macrotermitinae) Received: 27 September 2002 / Revised: 23 October 2002 / Accepted: 26 October 2002 / Published online: 4 December 2002 © Springer-Verlag 2002