Evolution and Diversity in Plants I - Ecol 182 – 4-7-2005

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Evolution and Diversity in Plants I - Ecol 182
4-7-2005
Re-downloaded at 710am on 4-7
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Big Questions

• What have been the important constraints and / or
  principles that have shaped the evolution of
  plants.
• Diversification
• Form and function

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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)
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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)
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Multicellularity and plant evolution
Multicellularity evolved more than once! -for
plants, prokaryotic unicellular algae ?
multicellular algae ? embryophytes Multicellular
ity 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
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• 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

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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
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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

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Figure 29.1 What Is a Plant?
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Diversity of Embrophytes

• Embryophytes fall out into 10 phyla
• Seven include members possessing well-developed
  vascular systems are 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)

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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.

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• 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.
• The gametophyte produces haploid gametes.
• The fusion of two gametes results in the
  formation of a diploid cell, the zygote, and the
  cycle repeats.

Figure 29.2 in the book Sporophyte generation
from the zygote through the adult, multicellular,
diploid plant. Gametophyte generation - from the
spore through the adult, multicellular, haploid
plant to the gamete.
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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)
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The Conquest of the Land

• Embryophytes invaded the terrestrial environment
  approximately 400500 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

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• 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
• Stomatacontrollable pore in tissue that
  regulate water loss and CO2 uptake
• Aerenchymainvaginations in tissue that create
  moist internal surface area for gas exchange

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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
  (tracheophytes) and do not (non-tracheophytes)
  have tracheids
• The first plants either lacked vascular tissue
  or, like some mosses, had very simple conducting
  tissue that developed from dead cells.

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Figure 29.4 From Green Algae to Plants
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• Water and nutrient acquisition by
  non-tracheophytes (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.

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NontracheophytesLiverworts, 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.

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• 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, haploid gametophyte whose cells
  contain chloroplasts.

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• Gametangia are where gametes are formed.
• The archegonium is a multicellular female sex
  organ with a long neck and a base that contains a
  single egg (a above)
• The antheridium produces sperm (b above)
• The sporophyte produces a sporangium, or capsule,
  within which meiotic divisions produce spores and
  thus the next gametophyte generation.

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• Liverworts - most ancient surviving plant clade.
• Rhizoids absorb water with filaments found on the
  lower surfaces gametophytes.
• Several genera have both sexual and asexual
  reproduction
• Asexual reproduction - by simple fragmentation of
  the gametophyte.

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• The hornworts, phylum Anthocerophyta, mosses and
  tracheophytes, all have unique adaptations to
  life on land
• These groups all possess stomata that allow the
  uptake of CO2 and the release of O2, but they can
  be closed to prevent excessive water loss (in
  some groups).

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• Two characteristics distinguish hornworts from
  liverworts and mosses
• Cells of hornworts contain a single large,
  platelike chloroplast, whereas liverworts and
  mosses contain numerous small, lens-shaped
  chloroplasts.
• Cyanobacteria often populate internal,
  mucilage-filled cavities within hornworts.
• These cyanobacteria are able to fix atmospheric
  nitrogen gas into a form that can be used by the
  hornwort.

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• The phylum Bryophyta (mosses) are probably sister
  to the tracheophytes.
• Hydroid cells, in many mosses, are a likely
  progenitor of the water-conducting cells of the
  tracheophytes.
• When hydroid cells die, they leave a tiny channel
  through which water can flow.

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The Tracheophytes

• The sporophyte generation of a now-extinct
  organism produced a new cell type, called the
  tracheid.
• Allowed for the radiation of a novel life form
• The tracheid is the principal water-conducting
  element in the xylem in all tracheophytes except
  the angiosperms.

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• The tracheophytes have well-developed
  vasculature, consisting of
• Phloem conducts photosynthetic products from
  production sites to sites where they are used or
  stored (think source-sink).
• Xylem conducts water and minerals from the soil
  to the aerial parts of the plants, or from one
  place in the soil to another.
• Xylem can provide support as it is stiffened by
  lignin.

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• The evolution of tracheids had two important
  consequences
• provided a pathway for long-distance transport.
• provided rigid structural support.
• Tracheids set the stage for invasion of land by
  plants.
• Tracheophytes also feature a branching,
  independent sporophyte.
• We break tracheophytes down into at least seven
  different groups (see fig. 29.10) with the
  biggest distinction of those that produce seeds,
  and those that do not produce seeds

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Figure 29.10 The Evolution of Todays Plants
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The Tracheophytes

• Recall plants invaded land about 400-500 million
  years ago.
• During the Devonian period club mosses
  (lycopods), horsetails, and ferns made the
  environment more hospitable to animals.
• Trees dominated during the Carboniferous period,
  resulting in forest that eventually become coal
  deposits.
• At the end of the Permian period, the
  200-million-year reign of the lycopodfern
  forests came to an end as they were replaced by
  forests of seed plants.

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Introducing the Tracheophytes

• The first tracheophytes were in the now-extinct
  phylum Rhyniophyta.
• They had the structural features found in all
  other tracheophyte phyla
• Club mosses (Lycophyta), appeared in the Silurian
  period.
• Ferns, horsetails, and whisk ferns (Pteridophyta)
  appeared in the Devonian.
• These groups (Lycophyta and Pteridophyta) had
  true roots, true leaves, and a differentiation
  between two types of spores.

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The Tracheophytes

• Roots had their origins as branches, either as
  rhizomes or aboveground portion of stems.
• Early roots were simple structures that
  penetrated soil, branching and anchoring the
  plant (absorbing water and minerals?)
• Belowground and aboveground environments are
  quite different.

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The Tracheophytes

• A leaf is a flattened photosynthetic structure
  emerging laterally from a main axis or stem and
  possessing true vascular tissue.
• There are two leaf types microphylls and
  megaphylls.
• The microphyll has a single vascular strand that
  has departed from the stem without disturbing the
  stems vascular structure. The club mosses have
  microphylls.
• Microphylls may have evolved from sterile
  sporangia.

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Figure 29.13a The Evolution of Leaves
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The Tracheophytes

• The megaphyll is larger, and more complex found
  in ferns and seed plants.
• May have arose from flattening of stems and
  development of overtopping (one branch
  differentiates from and extends beyond rest).

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Introducing the Tracheophytes

• Plants that bear a single type of spore are said
  to be homosporous.
• The most ancient tracheophytes were all
  homosporous.
• Both the gametophyte and the sporophyte are
  independent and usually photosynthetic.
• A single type of gametophyte bears both female
  and male reproductive organs.

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Introducing the Tracheophytes

• Plants with two distinct types of spores evolved
  later, and are said to be heterosporous.
• In heterosporous plants, the megaspore develops
  into a larger, specifically female gametophyte
  (megagametophyte).
• The microspore develops into the smaller, male
  gametophyte (microgametophyte).
• Heterospory evolved independently and repeatedly,
  suggesting that it affords selective advantages.

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Figure 29.14a b Homospory and Heterospory
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The Surviving Nonseed Tracheophytes

• The club mosses (phylum Lycophyta) have
  microphylls, exhibit apical growth, and have
  roots that branch dichotomously.
• Sporangia in many club mosses are contained
  within conelike structures called strobili,
  clusters of spore-bearing leaves inserted between
  a specialized leaf and the stem.
• There are both homosporous and heterosporous
  species.
• The Lycophyta and the Pteridophyta were the
  dominant phyla during the Carboniferous period.

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Figure 29.15 Club Mosses
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The Surviving Nonseed Tracheophytes

• The horsetails, whisk ferns, and ferns form a
  clade, the phylum Pteridophyta.
• The horsetails (all are genus Equisetum) have
  true roots that branch irregularly, and sporangia
  on short stalks called sporangiophores.
• The leaves are reduced megaphylls and grow in
  whorls.
• Stem growth is from the base of the stem segments.

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Figure 29.16 Horsetails
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The Surviving Nonseed Tracheophytes

• The whisk ferns are two genera of rootless,
  spore-bearing plants, Psilotum and Tmesipteris.
• Psilotum has only minute scales instead of true
  leaves.
• Although whisk ferns resemble the most ancient
  tracheophytes, they are now considered to be
  highly specialized plants that evolved fairly
  recently.

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Figure 29.17 A Whisk Fern
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The Surviving Nonseed Tracheophytes

• The sporophytes of the ferns typically have true
  roots, stems, and leaves.
• The ferns first appeared during the Devonian.
• About 97 of fern species belong to one clade,
  the leptosporangiate ferns. These ferns have
  sporangia with walls only one cell thick, borne
  on a stalk.
• Ferns are characterized by fronds, large leaves
  with complex vasculature.
• Sporangia are found on the undersurfaces of the
  fronds, clustered in groups called sori.

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Figure 29.19 Fern Sori Are Clusters of Sporangia
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Figure 29.18 Fern Fronds Take Many Forms
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The Surviving Nonseed Tracheophytes

• The sporophyte generation dominates the fern life
  cycle.
• Spores germinates and form a gametophyte, bearing
  antheridia or archegonia (or both).
• The antheridia release sperm that swim to a
  nearby archegonium and fertilize an egg.
• The sperm are guided by chemical attractants
  released from the archegonia.
• The resulting diploid embryo forms roots and
  fronds, and grows into the familiar sporophyte
  life stage.

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Figure 29.20 The Life Cycle of a Fern
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Figure 29.10 The Evolution of Todays Plants
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