Download Chapter 109 Eukaryotic Cells and Multicellular Organisms

Chapter 10
Plants and Fungi as Branches of
the Tree of Life
Overview
• Plants, fungi and animals are the three kingdoms of
multicellular organisms.
• Land plants are known to have evolved from organisms
similar to some living green algae.
• Plant life cycles consist of an alternation of generations.
Overview
• Plants transition from a dominant gametophyte generation in
lower bryophytes to a dominant sporophyte generation in ferns,
gymnosperms, and angiosperms.
• Many of the adaptations of land plants reflect the transition from
water to land.
• Fungi are not simple plants but a sister group of animals.
This material was covered extensively in General Biology Lab—you
need to read this chapter to remind yourself of the key concepts on
this topic. You will be responsible for this content.
An Abominable Mystery
• Charles Darwin : “The rapid development as far as we can judge of
all the higher plants within recent geological times is an
abominable mystery.”
• Origination from green algae, a long-standing hypothesis, has
received support recently.
Dr. Strohmeyer admonitions…
– Another largely circumstantial case analogous to endosymbiotic theory (if
you were a juror… would you be convinced beyond a reasonable doubt?).
– Indeed, as you read through this chapter, you are going to repeatedly
encounter language that betrays the highly controversial (?) hypotheses and
uncertainty of plant evolutionary history/phylogeny. Knowing the bias of this
text, this level of transparency here should be a clue to you just how strong
the controversy is.
Key things to remember!
• Note carefully the plant tree of life on the next slide—it conveys
key concepts and plant adaptations you should recall from your
gen bio labs and perhaps other courses.
– Green Algal-like ancestor gave rise to plant tree of life 
Bryophytes (liverworts, hornworts, mosses)
Lycophytes (club mosses)
Pteridophytes (Ferns, whisk ferns, horsetails)
Gymnosperms (Conifers, cycads, gneta, ginkgo)
Angiosperms (Monocot and Dicot flowering plants)
• Recall that seedless plants include the Bryophytes, Lycophytes, and
Pteridophytes and that seeded plants include gymnosperms
(naked seed in cones) and angiosperms (enclosed seed in fruits)
• Recall that non-vascular plants include Bryophytes and vascular
plants include all the rest.
Figure 01: Phylogenetic tree of 24 families of plants
An Algal Ancestry
• Molecular evidence allows us to construct the following
likely sequence of the origin of land plants.
1. Unicellular green algae that had incorporated new genes
through horizontal gene transfer and gained organelles by
endosymbiosis, transformed into multicellular photosynthetic
organisms by remaining attached rather than separating after
each cell division
(read BOX 10.1 for theories of how multicellularity arose).
An Algal Ancestry
• Molecular evidence allows us to construct the following
likely sequence of the origin of land plants.
2. These first small multicellular “ plants” captured additional
genes by horizontal gene transfer, increased in size and in
organelle and genome complexity, enabling their component
cells to compartmentalize and specialize.
An Algal Ancestry
• Molecular evidence allows us to construct the following
likely sequence of the origin of land plants.
3. Organisms with mostly somatic reproduction (formed body
cells) gave way to germ cell reproduction (only a few were
reproductive, forming gametes).
• Based on ultrastructural features, the ancestor of land
plants is now thought to have been a single-celled
flagellated green alga that lost flagella and became
sessile.
• Using 18S rRNA, rbcL, and nuclear and
mitochondrial genomes’ sequences,
demonstrates that numerous lineages of aquatic
algae invaded the land.
– In other words, the molecular data seem to indicate
that land plants arose multiple times. So the plant
tree of life is not really an accurate depiction.
• Transformation of one of these lineages gave rise
to two major lineages:
– chlorophyte green algae;
– charophyte algae and the land plants.
• Green algae and land plants share various
similarities reflecting their common origins. This
perceived evolutionary origin is built on
circumstantial evidence of certain morphological
and molecular similarities.
– Green algae store their carbohydrate reserves as
starch (fungi and animals store glycogen).
– Many species of algae have rigid, cellulose-reinforced
cell walls, as do all land plants (fungi have chitin).
– Green algae and vascular plants use similar types of
pigments in metabolic pathways, both green
chlorophyll (a and b) and yellow- orange carotenoids
( a and ß) (fungi are heterotrophs).
• Other similarities appear at first sight to be
the result of parallelism (a term that is not
the same as convergent evolution but often
confused with it—more on these later).
– Parallelism: apparently similar traits arose in related
lineages, that do not share a recent common
ancestor, due to similar environmental selective
pressures.
– Both sea lettuce (Ulva sp.) and razor algae (Caulerpa
sp.) have membranous forms that simulate the
morphology of some vascular plants but show their
separate evolutionary ancestries by passing through
an alga-like, filamentous stage.
– The transformation of marine algae to terrestrial
forms may have occurred as early as the Ordovician
460 Mya, possibly as the result of falling ocean
levels, higher oxygen content in atmosphere, ozone
formation, etc
In the next several sections you will read and review 1) the major
plant phyla characteristics, 2) their various key adaptations to
occupy their various niches on land, 3) be reminded of the
importance of the alternation of generations (read box 10.2) and
what this life cycle looks like in the various phyla and
4) note the apparent need to resort to multiple evolutionary
origin/lineage hypotheses to account for the diversity of features in
the various groupings of plants (see below).
• The spore-forming and gamete- forming tissues in algae and the similarity
between the filamentous growth pattern of some green algae and the branching
filamentous stage of many mosses are consistent with an algal origin of
bryophytes, although different groups of bryophytes may have arisen
independently from different algal ancestors.
• Also, given that the sporophyte generation of bryophytes depends for nutrients and
support on the gametophyte while the sporophyte of vascular plants is
independent, bryophytes and vascular plants may have arisen from different algal
lineages.
Bryophytes
• The “simplest” land plants, limited to moist
environments, are bryophytes.
• Paraphyletic group liverworts, mosses, club
mosses, and hornworts.
Figure 03B: Moss
Figure 03A: Liverwort
© Todd Boland/ShutterStock, Inc.
© tom67/ShutterStock, Inc.
• Bryophytes were
important in two
major transitions:
– water to land;
– haploid
gametophytedominated life
cycle to a diploid
sporophytedominated life
cycle.
Figure 04: Life cycle of a liverwort in the genus Marchantia
Early Vascular Plants
• “Vascular” refers to the presence of
conductive tissue:
– xylem that enables water to reach the erect parts
of the plant;
– phloem that enables nutrients to be distributed
from the leaves and stems.
• Silurian earliest fossil vascular plants
– Cooksonia
Figure 05A: Cooksonia caledonica
Figure 05B: Psilophyton princeps
• Devonian
Figure 06C:
Lepidodendron
species (about
46-m tall)
Figure 06A:
Zosterophyllum
myretonianum
(about 10.5 cm)
Figure 06B:
Asteroxylon mackiei
(about 0.6 m tall)
A and B- Adapted from Foster, A. S., and E. M. Gifford, Jr. Comparative
Morphology of Vascular Plants, Second edition. Freeman, 1974.
Adapted from Ste.wart, W.N., and G. Rothwell.
Paleobotany and the Evolution of Plants, Second
edition. Cambridge University Press, 1993
• Evolution of cambiums  wood and
increased size/girth
• Evolution of microspores and megaspores
(heterospory)
Figure 07: Evolution of megaspores
Adapted from Stewart, W. N., and G. Rothwell. Paleobotany and the
Evolution of Plants, Second Edition. Cambridge University Press, 1993.
Ferns
• Origin in early
carboniferous
(350 mya)
Figure 10: Carboniferous tree fern
Adapted from Foster, A. S., and E. M. Gifford, Jr., Comparative
Morphology of Vascular Plants, Second edition. Freeman, 1974.
Figure 09: Polypodium vulgare
Origins of Leaves
• The two major hypotheses:
– Telome hypothesis, the first leaf-like structures, arose from
webs between thin flattened branches (telomes).
– Enation (extension) hypothesis, the earliest leaves, arose from
small flaps or extensions of tissue along the stem.
• There are at least six independent evolutionary origins of leaves.
Although the evolutionary origin(s) of leaf types remain
unresolved, the hormonal and genetic basis for leaf
shape is being revealed. Transcription factors bind to
DNA to activate or repress genes. The transcription
factor, Knox (the knotted-like homeobox family of genes)
establishes a gradient of the plant hormone auxin, which
regulates leaf shape. Knox family members are active in
apical meristems where they regulate leaf and stem
development.
Multiple origin of leaves yet the same answer genetically
for their basis—what an interesting observation!
Figure 11: Representations of two
hypotheses for the origin of leaves.
Moving onto Land
• Three evolutionary changes:
– reduction in the size of the gametophyte;
– evolution of easily dispersible pollen;
– and encasement of spores in seeds.
• Allowed plants to avoid desiccation and so
move away from water.
• Early vascular plants evolved into the two
major lineages of pollen-producing, seedbearing land plants.
– Gymnosperms
– Angiosperms
• Examples of convergent evolution in cacti
American Cacti vs African Euphorbs
Figure 13A:
Figure 13B:
Figure 13C: African
American Cactaceae American Cactaceae Euphorbiaceae
© Alex Neauville/ShutterStock, Inc.
© Photodisc
© Micw/Dreamstime.com
Figure 13D: African
Euphorbiaceae
© Steve Allen Travel
Photography/Alamy Images
• Convergent evolution vs. Parallel evolution
– Parallel evolution is the development of a similar trait in related, but distinct,
species descending from the same ancestor, but from different clades (don’t
share a recent common ancestor).
• Parallel evolution occurs when two independent species evolve together
at the same time in the same ecospace and acquire similar characteristics
– Convergent evolution describes the acquisition of the same biological
trait/phenotype in unrelated lineages due to similar selective pressures.
• The wing is a classic example of convergent evolution in action. Although
their last common ancestor did not have wings, birds and bats do, and are
capable of powered flight.
• Traits arising through convergent evolution are termed analogous
structures, in contrast to homologous structures, which have a common
origin. Bat and pterodactyl wings are an example of analogous structures,
while the bat wing is homologous to human and other mammal forearms,
sharing an ancestral state despite serving different functions.
– Homoplasy is similarity in species of different ancestry that is the result of
convergent or parallel evolution.
• We will see these terms often in later chapters.
• Rapid evolution of angiosperms (angiosperm origins less certain)
– Evolution and elaboration of flowers as reproductive organs
– Self-incompatibility genes
– Evolution of flower structures that enable insects or birds to
pollinate them and to disperse their seeds are prominent
evolutionary changes.
– Here be sure to recall that this involves the concept of
coevolution and pollination syndromes—read box 10.3 (also
covered in gen bio).
Figure 14: Cycad gymnosperm,
Williamsonia sewardiana
Figure 15: Range of flower types and their animal pollinators in phlox plants
Adapted from Grant, V., and K. A. Grant. Flower Pollination in the Phlox
Family. Columbia University Press, 1965.
Fungi
• Fungi first appeared along with the first vascular plants in the
Silurian, 440 Mya—other forms maybe as far back as 1.4 Bya.
• Fungi lack chlorophyll
• The weight of evidence is that fungi:
– share a choanoflagellate ancestor with animals;
– are more closely related to animals than they are to plants;
– are not simple plants but a sister group to animals.
• Fungi are either saproprobes (heterotrophs that derive nutrition
from dead decaying organic material-absorptive nutrition) or are
parasites deriving nutrition from living host (i.e. athlete’s foot).
• Note here the concept of arm’s race—a competition between host
and parasite fungi or predator and prey—we will see this term
often.
Recall that Fungi was a chapter covered in gen bio just as plants were.
Key Concepts
• Plants, fungi and animals are the three kingdoms of multicellular organisms.
• Multicellularity evolved multiple times, either by cells failing to separate after
division or by aggregation of two or more cells.
• Land plants evolved around 460 Mya from ancestors with some features found
in green algae alive today.
• Horizontal gene transfer, endosymbiosis, and the separation of somatic and
germinal tissues were important processes in land plant evolution.
• Many of the adaptations of land plants reflect the transition from water to land.
• Leaves evolved multiple times independently in different groups.
• The first large leaves appeared in tree ferns during the Carboniferous Period.
• Plants and the insects that pollinate them co-evolved.
• Fungi are not simple plants but a sister group of animals.