Evidence of simultaneous occurrence of tylosis formation and fungal interaction in a late Cenozoic angiosperm from the eastern Himalaya

Information on whether tyloses in fossil angiospermic plants form specifically in response to infestation with pathogenic fungi has been lacking so far, and thus the evolutionary history of tylosis formation as a physical restraint strategy against the advancing fungi remains unresolved. Our study addresses this knowledge lacuna. Carbonized woods were recovered from the upper part of the Siwalik succession (Kimin Formation: late Pliocene to early Pleistocene) of Arunachal Pradesh, eastern Himalaya. Samples from different portions of the wood remains were prepared by using standard thin sectioning techniques and studied under transmitted light compound and scanning electron microscopes. The Plio-Pleistocene wood remains are attributed to Gmelina arborea Roxb. (Lamiaceae) as Gmelina siwalika Khan, Bera M et Bera S, sp. nov. Numerous well-preserved tyloses were found mainly in the central heartwood region of the fossil wood. Rare fungal remains indicative of early-stage colonization are characterized by septate fungal hyphae extending through the vascular ray system within the secondary xylem, and fungal spores. As tyloses are present in large numbers and formed before the spread of fungal remains, they served as an effective physical restraint to the spread of the fungi. We conclude that tylosis formation contributed to an antagonistic relationship between pathogenic fungi and Gmelina plant in Neogene paleoecosystems of the eastern Himalaya. This report also demonstrates that the development of this histological protrusion has been a significant process in woody plants since Cenozoic. (C) 2018 Elsevier B.V. All rights reserved.