Download origin of Angiosperms

Mechanisms of Plant Evolution
Special topic:
the origin of Angiosperms
Special topic: origin of Angiosperms
Two concurrent models for the origin of Angiosperms
Paleoherb Hypothesis: primitive angiosperms with short life
cycle and mixed monocot- / dicot-traits as origin (Taylor
und Hickey).
Pro: 18S, flower morphology
Woody Magnoliid Hypothese: Magnolia like trees as origin
(Doyle und Donoghue).
Pro: non-fused petals, no sepals, whorls of flower organs,
living fossiles with few species
Special topic: origin of Angiosperms
•Missing links: Fossile records for the
transition
between
Gymnoand
Angiosperms are lacking. How did the
first flower look like?
•Gnetales: Primitive gymnosperms that
are almost extinct, but display at the same
time typical Angiosperm traits:
Archaefructus. In 1998 the so far oldest Angiosperm fossile was
discovered. Archaefructus (Clusiaceae) – Sun et al. In the Search of the
First Flower, Science 282, 1692ff. This plant was very close to the Gnetales
– primitive gymnosperms that are almost extinct.
•Angiosperm traits in Gnetales: Double
fertilization, reticulate leaves, flower-like
sporangiophores, xylem with trachea.
Special topic: origin of Angiosperms
•Gnetales - advanced: reticulate leaves,
double fertilization, „flowers“, xylem with
trachea.
•Gnetales - primitive: seeds not
surrounded by maternal tissue (gymnosperms), anthers leaf like, no separation
theca-filament, carpels open, not fused
into a node.
Gnetum gnemon. Male and female flowers of Gnetum gnemon –Note the
reticulate leaves that are typical for Angiosperms. Evolutionary series for
anthers and carpels (primitive left, advanced right). Gnetum is a
gymnosperm with primitive anthers and carpels. By the way: this
primordial plant exists in Germany only in the Botanical Garden of
Karlsruhe!
•Missing link: Gnetum gnemon combines
advanced and primitive traits. It might
therefore be near the branching point
between gymno- and angiosperms.
Special topic: origin of Angiosperms
•Archaefructus: The flower (e.g. fused
anthers and carpels) is much more
advanced than that of Gnetum, but the
morphology places Archaefructus in the
Gnetales.
•Gnetales: By
Gnetum
falls
gymnosperms.
Archaefructus. According to molecular traits, the Gnetales are clearly in
the gymnosperms (right), according to morphological traits, they are close
to the angiosperms. Archaefructus is closer than Gnetum gnemon –
molecular data do not exist in fossiles!
molecular phylogeny,
clearly
into
the
•Archaefructus is the oldest known
angiosperm: Flower organs partially more
modern than Magnoliidae, other traits are
more primitive.
1:0 for Woody Magnoliid!
Special topic: origin of Angiosperms
OLD
conifers
club
dicots
mosses
paleoherbs
ferns
cycads Magnoliids
monocots
Shocker: Monocots First
loss of cambium, herbs
cambium
•Monocot-first hypothesis: no common
ancestor for gymno- and angiosperms.
Semiaquatic club mosses yield monocots,
thus no selection for cambium, cambium
was invented a second time giving rise to
the rapid development of the dicots
NEW
conifers
club
dicots
mosses
ferns
Magnoliids paleoherbs
cycads
monocots
•Traditional: seed plants monophyletic,
cambium in common ancestor of gymnoand angiosperms, tree-habit basal. Later
lost in monocots and paleoherbs.
Alisma (monocot)
•support: genetics, embryology Isoetes /
monocots,
dicot
cotyledons
not
homologous to „monocot“, primitive
monocots amphibic, linear leaf venation
from pin-formed telomes, primitive order
of vasculature in monocots
cambium
twice
Isoetes (club moss)