| Themes > Science > Botanical Sciences > Major Divisions Of Life > Kingdom Fungi > Lichens and The Evolution of Land Plants |
Lichenization, the process by which fungal hyphae and algal cells literally grow together to form a mutualistic association, may help to explain the remarkable evolution of vascular plants on earth. Most textbooks of general botany suggest that land plants evolved from ancestral green algae. However, some authorities believe that vascular plants are far more than simple extensions of green algae. They are comparatively too complex, diversified too quickly, and contain numerous fungus-like cells. In his fascinating article "Are Vascular Plants Inside-Out Lichens" (Ecology 69 (1): 17-23, 1988), Peter Atsatt of the University of California, Irvine discusses some of the evidence supporting a lichenized ancestor to vascular plants. According to Dr. Atsatt, the ancestral lichenization resulted in a "reverse-phase lichen" with a dominant algal component containing an endophytic, mineral scavenging fungus similar to extant mycorrhizal associations. Nuclear fusions between the fungal and algal cells resulted in hybrid nuclei containing the traits of both parents. In true synergistic fashion, this dual genome gave rise to a plant body composed of a mosaic of alga-like photosynthetic cells interspersed with specialized fungus-like cells. Probably the most difficult concept for skeptical botanists to accept is the fungal ancestry in today's vascular plants. Dr. Atsatt discusses several types of cells and tissues in vascular plants which resemble fungal hyphae, including pollen tubes, vascular (xylem) tissue, laticifers, and haustoria. Pollen tubes not only resemble the growth of fungal hyphae, but in Pinus, cycads, and Ginkgo they are branched and actually absorb nutrients from the "host's" megasporangium. The latex-producing laticifers found in many members of the Euphorbiaceae, Asclepiadaceae and other dicotyledonous families are very similar to fungal hyphae. Nonarticulated laticifers are elongate, multinucleate cellular tubes that grow throughout the plant body of these families. Some endophytic parasitic flowering plants, such as certain dwarf mistletoes and the remarkable Pilostyles thurberi of the Colorado Desert, live completely within the host tissues and only emerge from their host to produce flowers. The vascular tissue of these endoparasites literally permeate the host tissues and truly resemble fungal hyphae. The absorptive haustorial organs of many parasitic flowering plants which penetrate the host tissue are also very reminiscent of fungal hyphae. Although there is ample fossil evidence suggesting that algae and fungi lived over 500 million years ago, there is little fossil evidence of true ancestral lichens from that era. However, a startling new hypothesis from Gregory Retallack of the University of Oregon may shed some light on the existence of Precambrian lichens. Since their discovery in southern Australia in the late 1950s, the fossil remains of the Ediacaran biota have puzzled paleobiologists. These strange, flattened creatures lived about 600 million years ago presumedly at the bottom of shallow coastal seas. They have been classified in several primitive animal phyla, from jellyfish, echinoderms and worm-like animals to large alga-like protists, and may have been ancestral to other animal phyla. A number of paleontologists refer to these organisms as "Vendobionta," and regard them as an extinct early experiment in the evolution of life. By about 530 million years ago they were all replaced by shelled Cambrian animals. But according to Dr. Retallack, these bizarre creatures may have been ancient lichens. In his fascinating article "Were the Ediacaran Fossils Lichens?" (Paleobiology 20 (4): 523-544, 1994), Dr. Retallack eloquently discusses the evidence supporting his lichen hypothesis. Comparing their thickness to that of much younger tree-trunk fossils, he concludes that the fossils resisted compaction after burial almost as well as sturdy logs. Their sturdiness, large size (up to one meter across), lack of any mouth, digestive cavity or musculature, and evidence from their microscopic examination all suggest to Retallack that Ediacaran fossils were lichens. The presumed marine habitats of Ediacaran fossils is not crucial to their interpretation as lichens, because rock lichens live in the sea and on land. If one can hypothesize that at least some of the Ediacarans may be ancestral to certain animal groups, then perhaps lichens gave rise to more than vascular plants! According to Blair Hedges of Pennylvania State University (Science, August 2001), aquatic fungi evolved into a terrestrial form about 1.3 billion years ago. These early fungal forms were actually lichens because they formed a symbiotic relationship with primitive aquatic green algae. The early land surface of the Earth at this time contained numerous colorful rock lichens. The bright pigments served to reduce the harmful effects of ultraviolet radiation in a primitive atmosphere. Evidence from mutation rates in 119 genes common to living fungi and plants, indicates that ancient moss-like land plants appeared about 700 million years ago. Lichens are far more than mere biological curiosities. They are very successful and unique life forms that may hold the secrets to complex evolutionary processes, cell differentiation and gene expression in vascular plants. |
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