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Pentose metabolism through the phosphoketolase pathway has been well characterized in bacteria. In this paper, we report the identification of a phosphoketolase homologue Mpk1 in the insect-pathogenic fungus Metarhizium anisopliae. Phylogenetic analysis showed that fungal phosphoketolases are of bacterial origin and diverged into two superfamilies. Frequent gene loss or lack of acquisition is evident in specific fungal lineages or species. The mpk1 gene is highly expressed when grown in trehalose-rich insect haemolymph but poorly induced by insect cuticle or carbohydrate-rich plant root exudate. In addition, mpk1 gene expression and enzyme activity could be upregulated by different sugars including xylose, trehalose, glucose or sucrose. mpk1 null mutants generated by homologous recombination grew similar to the wild type of M. anisopliae on medium amended with xylose as a sole carbon source. However, insect (tobacco hornworm, Manduca sexta) bioassays showed significantly reduced virulence in Δmpk1. The results of this study suggest that the horizontally transferred Mpk1 in M. anisopliae plays an important niche adaptation role for fungal propagation in insect haemocoel. Following the carbohydrate flux from plants to plant-feeding insects and insect pathogenic fungi, a tritrophic relationship is discussed in association with the requirement of fungal phosphoketolase pathway.