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Telome Theory

The document discusses the Telome Theory, proposed by Walt Zimmermann in 1930, which outlines the evolutionary processes of aquatic plants transitioning to terrestrial plants, focusing on structures such as telomes and their arrangement. It describes key evolutionary processes including overtopping, planation, syngenesis, reduction, and curvation that contribute to plant development and the origin of sporophylls in various lineages. The theory has merits in explaining plant evolution but faces criticisms for its limitations in addressing specific plant structures and arrangements.

Science◦
Submitted By: Deepak Das Roll No M.Sc. Life Sciences (Specialization in Plant Sciences)
Contents • Introduction • Meaning of Telome Theory • Elementary Processes • Concept of Telome Theory in Evolutionary processes • Merits • Demerits
Introduction Aquatic plants → Land plants Early vascular land plants Evolved from Rhynia Rhynia
• Telome theory was first proposed by – Walt Zimmermann in 1930 • Based upon : Fossil record • Evolutionary modification :  True Leaf  True Stem  True Root  Complex Vascular System  Protected Sporangia
Meaning of Telome Theory • Telome - “Single nerved ultimate terminal portion of a dichotomising axis” • 2 types of Telome: Fertile Telome – Terminated by Sporangium Sterile Telome – Without Sporangium • The connecting axex between two telomes - Mesomes • Several telomes grouped together to form Telome truss/Syntelome • Syntelome: (i) Phylloid Truss – Only Sterile telomes (ii) Fertile Truss – Only fertile telomes
A figure of Rhyniaceae showing different telomes Kenrick, P. (2002). The telome theory. Developmental genetics and plant evolution,
Process of Telome Theory • Rhynia group of plants underwent certain evolutionary processes – Overtopping – Planation – Syngenesis – Reduction – Curvation
Overtopping • One of the two dichotomising branches outgrows the other and become larger and mechanically stronger • Larger branch forms – Axis • Overtopped branch forms – lateral branches Overtopping process
Planation Planation Process • Rearrangement of telomes and mesomes from a three dimensional pattern to a single plane
Syngenesis Syngenesis Process • Telomes and mesomes fuse tangentially • Parenchymatous tissue is developed in between • Known as webbing process • In stem of Selaginella polystelic condition due to - simple parenchymous webbing
Reduction Reduction Process • Activity of terminal meristem of each telome supressed resulting into much shorter branches • Responsible for – Microphyllous leaves of lycopsida and Sphenopsida Needle like leaves of conifers
Curvation Recurvation process • The fertile Telomes become curved or bend downwards • Two sub processes  Incurvation - downward shifting of the sporangia from terminal or ventral surface of the leaf in Pteropsida  Recurvation - downward bending of the sporangia occurs along with the sporangial stalks in Sphenopsida
Concept of Telome Theory Origin of Sporophylls in Lycopsida • Planation of the fertile and sterile Telomes • Reduction of the mesomes and in number of spornagia • Protolepidodendron and Psilotum – like axillary sporangia and single veined leaves are evolved
Origin of Sporophylls in Sphenopsida • Recurvation and Syngenesis processes are involved • The intermediate developmental stages are seen in some fossil genera like Calamophyton, Hyenia, Protocalamostachys and Asterocalamites
Origin of Sporophylls in Pteropsida • Overtopping, reduction, syngenesis and incurvation are the processes involved • Pinnately-veined sporophylls with marginal sori developed by the lateral fusion of mesomes • In many ferns, the sori are shifted downwards due to the incurvation process
Merits of Telome Theory • Describes the origin and evolution of sporophytes of land plants • Structure of the sporophytes of the most primitive known plants is defined • The exact relationship between the root, stem and leaves • Connects the living and fossil plants.
Demerits of Telome Theory • Does not explain how a telome-like characteristic body has been developed [Bower (1935)] • Telome theory does not explain the whorled or spiral arrangement of sporangia [Thomas (1950)] • For origin of Lycopsida, it is somewhat hypothetical [Andrews (1960)] • Does not provide an acceptable origin of all leafy structures • It does not explain the derivation of the dictyostelic condition [Stewart (1964)]
Reference • Beerling, D. J., & Fleming, A. J. (2007). Zimmermann's telome theory of megaphyll leaf evolution: a molecular and cellular critique. Current opinion in plant biology, 10(1), 4-12. • Kenrick, P. (2002). The telome theory. Developmental genetics and plant evolution, 365-387. • O.P.Sharma (2016), Series on “Diversity of Microbes and Cryptogams – Pteridophyta” (pp 28.1-28.5) New Delhi, India, McGraw Hill Education Pvt. Ltd. • Vashishta, Sinha, Kumar (2006), Pteridophyta (pp 32- 36) New Delhi, India, S. Chand & Co Pvt. Ltd.
Telome Theory