Download ij 0 1 0 1 0 1 0 1 0 1 0 1

IB 168: Systematics of Vascular Plants
Spring 2006
Key concepts -- Lecture 3 (1/25/06; end of Phylogenetics/Lycophytes)
Conclusion of phylogenetic concepts
Not all shared characteristics are informative about phylogenetic relationships
Hennig (1966): Shared, derived character-states are useful for resolving phylogeny, but
shared, ancestral characteristics are not.
Apomorphy (= apomorphic character-state): A derived state (either shared or unique).
Autapomorphy: A unique, derived state; diagnostic for a terminal taxon but not
informative about its relationships to other taxa.
Synapomorphy: A shared, derived state; diagnostic of phylogenetic relationships.
Synapomorphies are the basis for resolving monophyletic groups.
Plesiomorphy: An ancestral state.
Symplesiomorphy: A shared, ancestral state; not diagnostic of relationships.
Symplesiomorphic similarities often have been the basis for recognition of paraphyletic
groups.
out1
A
out2
B
C
1
0
g
1 e
0
?
a
d
?
b
0
h
0
1
i j
f
0
c e 1
0
1
1
In the tree above, character state 1 of char. B is synapomorphic for clade A+B+C but
symplesiomorphic for clade B+C. Similarly, char. state 1 of char. h is synapomorphic for
clade B+C but plesiomorphic for taxon B. Char.-state 1 of char. j is autapomorphic for
taxon C, as is char. state 1 of char. g for taxon A.
Possession of the same character-state by different taxa may be attributable to common
ancestry of those taxa or independent evolution of the character-state
Homology (in systematic sense): Similarity due to common ancestry; a homologous
characteristic is one that was passed along to different groups from their common
ancestor (both synapomorphies and symplesiomorphies represent homologous
characteristics). For example, char. state 1 of character b in the above tree is homologous
in taxa A, B, and C, as is presence of char. state 0 of character h in out2 and A (even
though symplesiomorphic).
"Bryophytes"
Rhyniophytes
Homoplasy: Similarity due to independent evolution; a homoplastic or homoplasious
characteristic is misleading about relationship, even if it has the same genetic basis in
groups where it evolved independently. Homoplastic characteristics include those that
arise as evolutionary parallelisms or reversals. For example, presence of char. state 1 in
character e in both taxa A and C in the tree on the previous page is an example of
homoplasy.
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Early vascular-plant evolution
Phylogenetic data have demonstrated that the sporophyte-dominant, tracheophyte lifecycle evolved from a gametophyte-dominant life-cycle. We know this based on two lines
of evidence: (1) Paraphyly of "bryophytes" (mosses, liverworts, hornworts) and (2)
recognition that the earliest tracheophytes (Rhyniophytes, now extinct) had sporophyte
and gametophyte generations that appeared similar morphologically. The grade of
"bryophyte" lineages below the base of the tracheophytes illustrates that gametophytedominance came first; Rhyniophyte fossils suggest that the two phases of the life cycle
were co-dominant prior to the transition to sporophyte dominance in early tracheophyte
evolution.
Modern
tracheophytes
(vascular plants)
sporophyte dominant;
tracheids arise
sporophyte/gametophyte
share dominance
alternation of generations/
gametophyte dominant
Why sporophyte dominance? Many possible reasons, for example, (1) benefits of
masking deleterious mutations in the diploid (sporophyte) phase; (2) truncation of
gametophyte generation, with the vulnerabilities of requiring a medium for freeswimming sperm.
Tracheophyte phylogeny consists of three principal branches:
1) Lycophytes: earlier thought to be "fern allies" but ferns are more closely related to
seed plants than to Lycophytes.
2) Ferns, including two families (Equisetaceae and Psilotaceae) that until recently were
thought to fall well outside the fern lineage.
3) Seed plants: monophyletic -- all modern seed plants from one origin of seed.
All three major lineages of tracheophytes have evolved *heterospory (see below)
independently:
Lycophytes (heterospory probably evolved once in common ancestor of Selaginellaceae
and Isoëtaceae, after branching off of Lycopodiaceae, which is homosporous).
Ferns (heterospory evolved in common ancestor of two families of water ferns -Marsileaceae and Salviniaceae; all other ferns are homosporous).
Seed plants (all are heterosporous; heterospory had to evolved prior to origin of the seed,
as will be discussed in lecture on origin of seed plants).
*Heterospory: Production of two different kinds of spores (megaspores &
microspores; each from different sporangia -- megasporangia and microsporangia) by the
sporophyte; each type of spore matures into a unisexual gametophyte, either male or
female. In contrast, spores of homosporous plants mature to be bisexual gametophytes,
with both male and female gametangia.
Sporophyte
Embryo
Microsporangia
Megasporangia
Diploid (2n)
fertilization (zygote formed)---------------------------------------------meiosis---------Haploid (2n)
eggs
sperm
archegonia
Megaspores
Microspores
Megagametophyte
antheridia
Microgametophyte
Why heterospory? Unclear. Some have suggested selection for outcrossing, although
homosporous plants have mechanisms to promote outcrossing. Heterospory is associated
with reduction and protection of the gametophyte phase, which develops inside the
protective spore-wall.
Lycophytes - An ancient lineage dating from the Devonian (>400 million years ago) and
sister-group of other living tracheophytes.
Today, moss-like or onion-like plants; prehistorically, included dominant trees of swamp
forests of the Paleozoic (e.g., Lepidodendron).
Diagnostic (synapomorphic) features of Lycophytes:
(1) microphylls (leaves with single strand of unbranched conducting tissue); (2) kidneyshaped sporangia that open transversely at maturity; (3) sporangia produced in
microphyll axils (just above leaves; sometimes sporophylls (= leaves associated with
sporangia) arrayed in cones (= strobili).
2ndary xylem
heterostyly;
ligule
Lycopodiaceae
Selaginellaceae
Isoetaceae
(Lepidodendrales)
Lycophytes (ca. 1200 spp.)
Ferns (ca. 12,500 spp.)
(Pteridophytes)
Seed plants
>270,000 spp.
microphylls
axillary kidney-shaped
sporangia
transverse
dehiscence of
sporangia
Other (plesiomorphic) features of Lycophytes (all five features below resolved as present
in the most recent common ancestor of all living tracheophytes):
(1) sporangia are eusporangia (sporangial outer wall > 1 cell layer thick); (2) spores
globose with trilete scar; (3) free-living gametophytes (above or below ground), not
attached to sporophyte at maturity; (4) stems dichotomously branching; (5) sperm with 2
flagellae (sperm of ferns and those seed plants with motile sperm have numerous
flagellae).
Three extant families of Lycophytes (ca. 1,200 species):
(1) Lycopodiaceae: "Club mosses" -- homosporous, without ligules, with underground
gametophyte; worldwide, rhizomatous moss-like plants, often in forest understories.
(2) Selaginellaceae: "Spike mosses" -- heterosporous, with ligules (scale-like outgrowth
near base of each microphyll); mostly tropical but includes "resurrection plants" of dry
areas (can dehydrate and rapidly rehydrate and resume photosynthesis); moss-like in
appearance, sometimes grow on (but do not parasitize) other plants (that is, are often
epiphytes).
(3) Isoëtaceae: "Quillworts" -- heterosporous, with ligules, sporangia embedded in leaf
base; onion-like appearance; associated with water (often along edges of lakes, ponds).
Lepidodendron and other ancient (extinct) tree lycophytes were heterosporous, with
ligules, and therefore thought to be closely related to Selaginellaceae and Isoëtaceae.