Downloaded at 9 pm on 2 28 Evolution and Diversity in Plants I Ecol 182 3 2 2006 Big Questions What have been the important constraints and or principles that have shaped the evolution of plants Diversification Form and function Important particularities on evolution and speciation in plants R A Fisher 1958 Fundamental Theorem of Natural Selection Rate of increase in the mean fitness of a population is proportional to the genetic variance in fitness In order for there to be evolution there must be genetic variation Major ways genetic variation is introduced into populations 1 Mutation UV random error 2 Genetic recombination meiosis including crossing over 3 Immigration into population But plants do two additional tricks that enhance genetic variation 4 Polyploidy an organism that has more than one complete set of the normal chromosome compliment most animals are diploids many plants are polyploids occurs through processes such as chromosome duplication 5 Hybridization crossing of closely related taxa usually between species within a genus Multicellularity and plant evolution Multicellularity evolved more than once for plants prokaryotic unicellular algae multicellular algae embryophytes Multicellularity has several interesting advantages Cells can be specialized division of labor requires communication and transport Organism can increase surface area for environmental exchange access to more resources Organism can increase in size better buffering of environmental extremes live longer access to additional resources When is an organism multicellular When neighboring cells adhere interact and physiologically communicate Contact is achieved in four ways 1 Tight junctions proteins in membranes that bond neighboring cells 2 Desmosomes intracellular filaments that adjoin cells often creating a space for material movement 3 Gap junctions pores surrounded by transmembrane proteins direct material movement between cells 4 Plasmodesmata open channels within the plant cell wall that connect cells directly Multicellular plant Single living protoplast of adjoining cells Cell membranes which line plasmodesmata are continuous from one cell to the next Water and small molecules may pass with relative ease essentially through the whole plant Material flow may be modified by altering number and location of plasmodesmata What is a plant Plants are photosynthetic eukaryotes including algae A more derived group of plants is called the embryophytes produce an embryo that is protected by tissues of the parent plant Plants appear monophyletic forming a single branch of the evolutionary tree so says your book Please please remember these endosymbiotic events and the discussion you have had on a tree like phylogeny versus a web like phylogeny Figure 29 1 What Is a Plant Diversity of Embrophytes Embryophytes fall out into 10 phyla Seven include members possessing well developed vascular systems called the tracheophytes Three phyla liverworts hornworts and mosses derived in that order lack tracheids and are collectively referred to as the nontracheophytes Table 29 1 in your book lists the groups and their defining characteristics good source for important knowledge hint Unique characteristics of plants Alternation of generations is a universal feature of the life cycles of plants Life cycle includes both multicellular diploid and multicellular haploid individuals Gametes are produced by mitosis while meiosis produces spores that develop into multicellular haploid individuals The multicellular diploid plant is called the sporophyte The sporangia on the sporophyte produce haploid unicellular spores by meiosis The multicellular haploid plant formed by mitosis of a spore is called the gametophyte Figure 29 2 in the book The gametophyte produces haploid gametes Sporophyte generation from the zygote through the adult multicellular diploid plant The fusion of two gametes results in the formation of a diploid cell the zygote and the cycle repeats Gametophyte generation from the spore through the adult multicellular haploid plant to the gamete Charophytes a group of green algae appear to be the closest living relative of Embryophytes These organisms now occupy the margins of ponds or marshes meaning that the jump to a terrestrial environment was in close proximity The Conquest of the Land Embryophytes invaded the terrestrial environment approximately 400 500 mya Invading the land is more like invading the air rather than soil Water not as available and quickly lost from plant in the terrestrial environment Gravity becomes very important Dispersal of gametes is much more difficult outside of an aquatic environment Some adaptations to life on land Cuticle a waxy covering that prevents drying Gametangia enclosure for gametes to prevent drying Embryos protected young sporophytes Pigments protection against mutagenic UV radiation Spore wall thickening prevent drying and resist decay Mychorhizzae mutualistic association with a fungus to promotes nutrient uptake from the soil Stomata controllable pore in tissue that regulate water loss and CO2 uptake Aerenchyma invaginations in tissue that create moist internal surface area for controlled gas exchange The Conquest of the Land Evolution of specialized water conducting cells tracheids allowed for advancement in the terrestrial environment We distinguish between embryophytes that have tracheids tracheophytes and those that do not non tracheophytes The first land plants either lacked vascular tissue or like some mosses had very simple conducting tissue that developed from dead cells Figure 29 4 From Green Algae to Plants Water and nutrient acquisition by nontracheophytes recall they do not have a vascular system Many grow in dense masses through which water can move by capillary action They have leaflike structures that catch and hold water that splashes onto them They are small enough that minerals can be distributed evenly by diffusion Nontracheophytes Liverworts Hornworts and Mosses Grow in dense mats in moist habitats typically they are small in size Layers of maternal tissue prevent loss of water from the embryo Have a thin cuticle though it is not highly effective in retarding water loss Are widespread across six continents and exist locally on the coast of Antarctica Nontracheophytes visible green structure is the gametophyte Sporophyte produces unicellular haploid spores through meiosis within sporangium or capsules Spores germinate and give rise to a multicellular