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Joseph H. Williams
Dept. of Ecology and Evolution, Univ. ofTennessee, Knoxville
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Progamic Phase interval betweenpollination and fertilization
Plesiomorphy - an ancestral or primitive
character Evolutionary novelty - any newly acquired
structure or property that permits the
performance of a new function, which, in
turn, will open a new adaptive zone
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Gymnosperms Angiosperms
non-flowering plant flowering plant
naked seed enclosed seed
have strobili or cones have flowers
male and female cones male pollen and female
ovules
haploid endosperm (n) triploid endosperm (3n)
mostly wind pollination mostly insect pollination
occupy land and water dwell in land only
of ancient evolutionary
origin
or more recent
evolutionary origin
include 600-750 species include 250,000 species(80% of all plant species)
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Angiosperms > GymnospermsFlower
Closed carpel
Highly reduced male and female gametophytes
Double Fertilization
Sexually formed polyploid endosperm
Long styles
Multi-seeded ovariesMuch faster pollen tube growth rates
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Parameters Gymnosperms Angiosperms
tip cellulose esterified pectin
wall cellulose microfibril-
based
amorphous callose-
based
plug esterified pectin callose
pollen tube growthrates
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Amorphous callose-based wall is faster and more
energy efficient than biosynthesis of a cellulose
microfibril-based wall.
Silencing of genes involved in callose synthesis
reduces pollen competitive ability.
Properties of Callose Walls and Plugs:
Walls: Impermeable
Plugs: Seal off the pollen tube
Reduce the risk of damage and allow pollen tubes
to grow longer distances
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Allows faster growth rates because
i. The pectic tip is more plastic and rapidly
extensible
ii. The mature tube cell wall has greater
resistance to tension stress due to secretion of
callose
iii. Callose plugs help maintain positive turgorpressure in the growing tip
Angiosperms pollen tube tips: esterified pectins
Lateral tube wall: with callose secondary wall
A f 900 di i di h h i
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A survey of >900 studies indicates that the timeinterval between pollination and fertilization(fertilization interval) ranges from:
10 hours to >12 months - Gymnosperms15 minutes to >12months - Angiosperms
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Early Cretaceous angiosperms and
ephedroids (Gnetales) were both diversifying
in similar habitats
Today 65 species ofEphedra are confined tosemiarid habitats whereas angiosperms have
radiated into virtually every environment on
earth
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Gymnosperm reproductive evolution isconstrained because their slow pollen tube
growth rates impose a trade-off between
pollen tube developmental time and pollen
tube pathway length.
pollen tube developmental time = pollentube pathways = pollen tube growth rate
pollen tube developmental time = pollen
tube pathways = pollen tube growth rate
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Gymnosperm pollen tube growth rate
evolution is thus severely constrained by the
tight linkage of developmental time and
pollen tube pathway length.
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A critical step in early angiosperm prehistory
was the origin of their unique pollen tube
wall growth pattern.
The callose wall and plug preceded the originof novelties such as true closed carpels, solid
styles, and deep multi-ovulate ovaries, as
well as the evolution of extreme traits such
as fertilization intervals as short as 15 min,pollen tube pathway lengths as longs as 500
mm, and pollen tube growth rates >1,000-
fold faster than those of gymnosperms.
Angiosperm pollen tube wall innovations gavepollen tubes the capacity for rapid and long-
distance growth, increasin the evolutionary
potential of both pollen tubes and the tissues
they interact with.
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Because angiosperms lack the gymnosperm
developmental constraint of slow and staticpollen tube growth rates, developmental
time and pollen tube pathway length became
evolutionary dissociated in derived groups.
Angiosperm fertilization biology is
distinguished not only by many novelties and
extreme traits, but also by much greater
independence (modularity) of their
developmental processes.
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Elevated reproductive trait diversity, and
perhaps increased modularity as well, are
strongly linked to the elevated taxonomic
diversity of flowering plants.
Rapid reproduction, due in large part to
accelerated pollen tube growth rates, is a
fundamental life history strategy shared bymany of the most diverse herbaceous clades
such as grasses and asters.
Moreover, the great developmental flexibility
of angiosperm pollen tubes expanded thepotential for pollen competition and
maternal responses to its effects, in turn,
speeding the evolution of prezygotic forms of
mating systems and reproductive barriers.
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Gymnosperms, generally lack strong
prezygotic barriers.
Pollen tube growth rate innovations truly lie
at the heart of the tremendousreproductive flexibility and opportunism
that Stebbins and others have described as
the critical factor in angiosperm success.
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Th ll f i l t ll t f lfill t
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The walls of growing plant cells must fulfill two
simultaneous and seemingly contradictory requirements.
First, they must expand to accommodate cell growth,
which is anisotropic in many tissues and determines organ
morphology. Second, they must maintain their structuralintegrity, both to constrain the turgor pressure that drives
cell growth and to provide structural rigidity to the plant.
These requirements are met by constructing primary cell
walls that can expand along with growing cells, whereassecondary cell walls are deposited after cell growth has
ceased and serve the latter function.One of the major constituents of both types of cell walls is cellulose, which exists as microfibrils composed of parallel
-1,4-linked glucan chains that are held together laterally by hydrogen bonds (Somerville, 2006). Microfibrils are 2 to 5
nm in diameter, can extend to several micrometers in length, and exhibit high tensile strength that allows cell walls to
withstand turgor pressures of up to 1 MPa (Franks, 2003). In vascular plants, cellulose is synthesized by a multimeric
cellulose synthase (CESA) complex composed of at least three types of glycosyl transferases arranged into ahexameric rosette (Somerville, 2006). After delivery to the plasma membrane, CESA initially moves in alignment with
cortical microtubules (Paredez et al., 2006), but its trajectory can be maintained independently of microtubule
orientation. For example, in older epidermal cells of the root elongation zone in Arabidopsis (Arabidopsis thaliana),
cellulose microfibrils at the inner wall face are oriented transversely despite the fact that microtubules reorient from
transverse to longitudinal along the elongation zone (Sugimoto et al., 2000), suggesting that microtubule orientation
and cellulose deposition are independent in at least some cases.
Depending on species, cell type, and developmental stage, cellulose microfibrils may be surrounded by additional
networks of polymers, including hemicelluloses, pectins, lignin, and arabinogalactan proteins (Somerville et al., 2004).
Hemicelluloses are composed of -1,4-linked carbohydrate backbones with side branches and include xyloglucans,
mannans, and arabinoxylans. Xyloglucan is thought to interact with the surface of cellulose and form cross-links
http://www.ncbi.nlm.nih.gov/pubmed/16824006http://www.ncbi.nlm.nih.gov/pubmed/12730269http://www.ncbi.nlm.nih.gov/pubmed/16824006http://www.ncbi.nlm.nih.gov/pubmed/16627697http://www.ncbi.nlm.nih.gov/pubmed/11115865http://www.ncbi.nlm.nih.gov/pubmed/15618507http://www.ncbi.nlm.nih.gov/pubmed/15642717http://www.ncbi.nlm.nih.gov/pubmed/15618507http://www.ncbi.nlm.nih.gov/pubmed/11115865http://www.ncbi.nlm.nih.gov/pubmed/16627697http://www.ncbi.nlm.nih.gov/pubmed/16824006http://www.ncbi.nlm.nih.gov/pubmed/12730269http://www.ncbi.nlm.nih.gov/pubmed/168240067/31/2019 Novelties of the Flowering Plant (140)
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