The process of hyphal fusion requires (i) precontact, (ii) contact, adhesion, and cell wall breakdown, and (iii) pore formation and cytoplasmic flow. In germling

fusion, germinating signaling pathway conidia can be fused by germ tube fusion or by the formation of small hyphal bridges (conidial anastomosis tubes), which are significantly narrower than germ tubes (Roca et al., 2003; Pandey et al., 2004). The germling fusions are density- and nutrient-dependent; the fusion is suppressed on nutrient-rich media (Fleißner et al., 2008). The frequency of hyphal fusion within a vegetative colony varies from the periphery to the interior of a colony (Hickey et al., 2002). At the periphery, hyphae grow straight out from a colony and exhibit avoidance behavior. In the inner portion of a colony, hyphae show a different behavior. Certain hyphae or hyphal branches show autotropism,

directed growth and hyphal fusion. Similar to germling fusion, the frequency of hyphal fusion depends on the availability of nutrients. It has been hypothesized that the attraction of hyphae involved in vegetative fusion is mediated by diffusible substances, which results in re-directed polarized hyphal tip growth. These unidentified diffusible signals possibly regulate the behavior of the Spitzenkörper that is found in growing hyphal tips or at the sites of branch initiation (Glass et al., 2004). Localization of the Spitzenkörper in a eltoprazine hyphal apex has been associated with directionality of growth. After making contact, hyphae involved in fusion switch from polar to isotropic VX-809 molecular weight growth, resulting in swelling of hyphae at the fusion point. After the fusion of plasma membranes occurs with the help of pore-formation enzymes, the cytoplasm of the two participating hyphae are mixed. Spitzenkörper disappears at the end of the process. Chemotrophic interactions observed during hyphal and germling fusion suggest that receptors and signal transduction pathways are involved. The mitogen-activated protein kinase (MAPK)

pathway is either involved in early communication between the fusion partners or required for rendering conidia and hyphae competent to undergo fusion. No attempts have been made to enhance the conidial thermotolerance of entomopathogenic fungi using hyphal fusion in artificial media, although a similar phenomenon was recently observed in Beauveria bassiana (Bal.) Vuil. (Ascomycota: Hypocreales) isolates when applied to target insects (Castillo et al., 2004; Güerri-Agulló et al., 2010). Low frequency of heterokaryosis was observed on the cadavers of Colorado potato beetles when they were infected with nitrate non-utilizing mutants from vegetative compatibility groups in B. bassiana (Castillo et al., 2004). On the elytra of red palm weevil, frequent episodes of hyphal and conidial fusion were found (Güerri-Agulló et al., 2010).