trends in evol of plant life cycle
adaptations to a dry environment
•
•
•
•
reduction in size of gametophyte
loss of antheridium, archegonium
increase in size of sporophyte
gametophyte retained on sporophyte
preview: major events in plant evolution
Fig 29.7
grade vs clade
Fig 29.4
what are plants?
share derived characters of plants &
charophycean green algae
• way of making cellulose walls
• enzyme glycolate oxidase (recovery
when rubisco grabs O2 instead of
CO2)
• cell division mechanism
• sperm ultrastructure
• DNA sequences, genomic architecture
shared derived characters of plants
are life cycle features (alt of gen)
• 1) gametophytes producing gametes in
multicellular gametangia
• 2) multicellular diploid embryo (young
sporophyte) retained on parent plant
– embryo protected, nourished
– plants called “embryophytes” (Fig 29.4)
• 3) spores in multicellular sporangia
– sporopollenin (very resistant) in spore walls
origin of plant life cycle?
• Coleochaete
– retains egg and zygote
– protects and nourishes
• charophyceans have haploid life cycle
(only zygote is diploid)
3 Life Cycles Fig 13.6
gametes
gametophyte mitosis
spores
gametes
n
meiosis
syngamy
2n
mitosis
n
2n
n
2n
sporophyte
zygote
humans
Chlamydomonas
Fig 28.22
zygote
mitosis
kelps, plants
Fig 28.16
hypothesis: plant life cycle arose
from delay in meiosis
• zygote undergoes mitosis
– produces multicellular sporophyte
• many cells undergo meiosis
– advantage on land
(less water for fertilization)
life on land:
plenty of light, CO2
but dessication, UV light
preadaptations to life on land
• 1) resistance to
desiccation
• resistant compounds
incl. sporopollenin
(Coleochaete zygotes,
plant spores & pollen)
• 2) protection from
UV light
• surface layer
strategies for life on land
• 1) H2O level variable = poikilohydry
– same as environment
• stay in wet places OR dry & rehydrate
• 2) maintain constant internal H2O level =
homeohydry (type of homeostasis)
• cuticle, stomata, lignified H20 cond. cells
physiology and anatomy of
“bryophytes” (grade not clade)
• poikilohydry
• small, attach via rhizoids
– long tubular cells or filaments
• some have water conducting cells
– but not lignified
• stomata only on sporophytes of
hornworts & mosses
• cuticle on some sporophytes, and parts of
gametophytes (leaves, pores)
diversity of “bryophytes”
•1) liverworts (leafy OR thalloid)
•2) hornworts (horn-like sporophyte)
•3) mosses (most obvious & diverse)
thalloid liverwort—spots are pores for gas exchange
gemma cups—asexual reproduction
gemma cup with gemmae
a single gemma
a single gemma germinating
sexual reproduction
liverwort sperm release, when sprayed with water
http://www.youtube.com/watch?v=ALGDLzWcvnU
http://www.palaeos.com/Plants/Bryophyta/Bryophyta.html
Physcomitrella patens genome
moss model organism
http://moss.nibb.ac.jp/what.html
“bryophyte” life cycle
• gametophyte is prominent
– sperm require water
• sporophyte smaller, dependent
– spores dispersed by wind
sporophyte
female
gametophyte
male
gametophyte
diversification for spore dispersal in bryophytes
height facilitates dispersal
gametophyte or sporophyte may be tall
“bryophyte” ecology
•
•
•
•
•
•
diverse habitats, especially mosses
1) Andreaea (habitat like early Earth)
thick walls resist UV, store C
2) Sphagnum (peat moss) stores CO2
assoc. methanotrophs CH4-->CO2
used for gardening
– dead cells hold water
evidence of early plants
•
•
•
•
•
•
•
•
molecular data: plants est. 700 my old
fossil evidence: at least 475 mya
Mystery: where is fossil record?
Answer1: microfossils
spores
sheets of cells (from sporangia)
lower epidermis with rhizoids
Answer 2: macrofossils?