Green plants viridophytes

1
Plant phylogenies

• Green plants viridophytes
• Land plants embryophytes
• Vascular plants tracheophytes
• Seed plants spermatophytes
• Flowering plants - angiosperms

2
Spermatophytes seed plants
By far the most diverse lineage within the
vascular plants. Most of this diversity is
accounted for by just one subclade the flowering
plants (angiosperms). The gynosperms also are
included in spermatophytes but ferns and fern
allies are not.
3
Extant seed plant lineages
Cycads (Cycadales)-150 sp Ginkgos (Ginkgoales)-1
sp Conifers (Coniferales)-550 sp Gnetophytes
(Gnetales)-70 sp (Gnetum, Ephedra and
Welwitschia) Flowering Plants (Angiospermae) -250
000 sp
4
Diversification of seed plants
5
Darwins abominable mystery the origin of
angiosperms
6
Cycads

• Squat trunks, with limited secondary xylem, and
  large compound leaves resembling those of ferns
  or palms.
• Plants are dioecious

7
Ginkgo
There is just one surviving species (Ginkgo
biloba) within ginkgoales. Species is hardly
known in the wild, but it has been maintained for
centuries around temples in China.
Ginkgoales are well-known in the fossil
record. Ginkgos are dioecious.
8
Conifers
Shrubs or trees with well-developed wood and
often needlelike leaves.
Leaves are borne singly along stem, but in Pinus
are clustered in short shoots. Many conifers are
monoecious. Some, like Juniperus are dioecious.
9
Gnetophytes
Woody plant group Vessel elements and double
fertilization as in angiosperms
Gnetum are generally woody climbers Welwitschia
has one species with only 2 strap like
leaves Ephedra have long branches with tiny scale
like leaves at their nodes
Welwitschia
Ephedra sp.
10
Angiosperm characteristics
Flowers present Closed carpel Endosperm
develops with zygote formation Gametophyte
generation is reduced in size
11
The relationship of among seed plant lineages
At some point almost every living and fossil
group of gymnosperms has been proposed as a
possible ancestor of the angiosperms The
relationship between angiosperms and Gnetales has
received much attention Determining the
relationships is important for systematics
Critical for investigations of character state
evolution
12
Changing views of the relationships of Gnetales
and other seed plants
molecular phylogenetics
morphological cladistics
morphology
1985 Crane Angiosperms and Gnetales are sisters
2000 Bowe et al. Chaw et al. Gnepines
1907 Wettstein Angiosperms derived from Gnetales
1986/1992 Doyle Donoghue Anthophyte hypothesis
1908 Arber Parkin Gnetales simple flowers
are reduced
13
Changing views of the relationships of Gnetales
and other seed plants
14
Evidence from multi-gene studies
Studies generally place Gnetales with the
conifers Some analyses place them within the
Conifers A structural mutation places Gnetales
as sister to conifers
15
Morphology pros and cons

• PROS
• can incorporate fossils

• CONS
• difficult for distantly related taxa
• subjective character scoring
• fewer characters

16
Molecules pros and cons

• PROS
• less difficulty with distantly related taxa
• objective character scoring
• many more characters

• CONS
• cannot incorporate fossil taxa

17
Morphology vs molecules
Key feature uniting angiosperms and Gnetales
1) thin vs thick megaspore membrane 2)
microspores aggregated in a whorl (different
than other gymnosperms) 3) lignin chemistry
4) tunica layer in the apical meristem 5)
double fertilization 6) microsporangia fused
basally 7) embryo derived from uninucleate cell
via cell divisions 8) siphonogamy (fertilization
via pollen tube) 9) granular exine structure
18
Why are morphology and molecules incongruent?
Several putative synapomorphies for angiosperms
and Gnetales may not be homologous Tunica layer
- 2 cells thick in angiosperms 1 cell thick in
Gnetales Megaspore wall - absent in angiosperms
thick in Gnetales Granular exine structure - not
ancestral in angiosperms Vessels - derived from
different structures Double fertilization -
synapomorphy for all seed plants?
19
Some problems encountered when constructing seed
plant phylogenies
Rapid cladogenesis Rooting Long branch
attraction-uneven rates of evolution (with
rapidly evolving sequences sites saturate and
homoplasy is thought to be synapomorphy) Poor
taxon sampling Sampling error - weak bootstrap
support
20
Conclusions

• Gnetales are sister to all conifers or just to
  Pinacea
• The relationship of cycads and Ginkgo is still
  uncertain
• Important to incorporate fossil taxa especially
  if no modern relatives of angiosperms exist
• A better understanding of morphology of seeds
  plants is needed (i.e. determining homologous
  characters)

Angiosperms
Cycads
Ginkgo
Other conifers
Gnetales
Pinacae