thin wall cells similar to palisade parenchyma. The fossil arbuscules appear very similar to those of existing AMF. The cells containing arbuscules have thickened walls, which are also observed in extant colonized cells. Mycorrhizas from the Miocene exhibit a vesicular morphology closely resembling that of present Glomerales. This conserved morphology may reflect the ready availability of nutrients provided by the plant hosts in both modern and Miocene mutualisms. However, it can be argued that the efficacy of signaling processes is likely to have evolved since the Miocene, and this can not be detected in the fossil record. A finetuning of the signaling processes would improve coordination and nutrient exchange between symbionts while increasing the fitness of both the fungi and the plant symbionts. The nature of the relationship between plants and the ancestors of arbuscular mycorrhizal fungi is contentious. Two hypotheses are: Mycorrhizal symbiosis evolved from a parasitic interaction that developed into a mutually beneficial relationship. Mycorrhizal fungi developed from saprobic fungi that became endosymbiotic. Both saprotrophs and biotrophs were found in the Rhynie Chert, but there is little evidence to support either hypothesis. There is some fossil evidence that suggests that the parasitic fungi did not kill the host cells immediately upon invasion, although a response to the invasion was observed in the host cells. This response may have evolved into the chemical signaling processes required for symbiosis. In both cases, the symbiotic plant-fungi interaction is thought to have evolved from a relationship in which the fungi was taking nutrients from the plant into a symbiotic relationship where the plant and fungi exchange nutrients. Molecular evidence Increased interest in mycorrhizal symbiosis and the development of sophisticated molecular techniques has led to the rapid development of genetic evidence. Wang et al. (2010) investigated plant genes involved in communication with order Glomales fungal partners (DMI1, DMI3, IPD3). These three genes could be sequenced from all major clades of modern land plants, including liverworts, the most basal group, and phylogeny of the three genes proved to agree with then current land plant phylogenies. This implies that mycorrhizal genes must have been present in the common ancestor of land plants, and that they must have been vertically inherited since plants colonized land. AM fungi and cyanobacteria symbiosis? It was revealed that AM fungi have the bacterial type core enzyme (ribonuclease III) of sRNA processing mechanism possibly related with symbiosis, by the result of