Evolution of mutualistic symbiosis without vertical transmission

Mutualistic symbioses are considered to evolve from parasitic relationships . Vertical transmission, defined as the direct transfer of infection from a parent organism to its progeny, has been suggested as a key factor causing reduction of symbiont virulence and evolution of mutualism. On the other h and, there are several mutualistic associations without vertical transmissi on, such as those between plants and mycorrhizal fungi, legumes and rhizobi a, and some corals and dinoflagellates. It is expected that all mutualisms evolve perfect vertical transmission if the relationship is really mutualis tic, because hosts may fail to acquire symbionts if they do not transmit th e symbionts by vertical transmission. We have developed a mathematical mode l to clarify the conditions under which mutualistic symbiosis without verti cal transmission should evolve. The evolution may occur when and only when (i) vertical transmission involves some costs in the host, (ii) the symbion t suffers direct negative effects if it exploits the host too intensively, (iii) the host establishes the ability to make use of waste products from t he symbiont, and (iv) the mechanism of vertical transmission is controlled by the host. We also clarify the conditions under which mutualistic symbios is with vertical transmission evolves. (C) 1999 Academic Press.