Today is Wednesday, February 25 th, 2015 Pre-Class: What land plant seems the best adapted to actually being an aquatic plant? Put your lab notebooks in.
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Today is Wednesday,February 25th, 2015
Pre-Class:What land plant seems the best adapted to actually
being an aquatic plant?
Put your lab notebooks in the cardboard box next to the Turn-In Box and get a paper towel for you/your
partner.
In This Lesson:Unit 8
Plant Evolution(Lesson 1 of 2)
Today’s Agenda
• Evolution of Land Plants– vs. Zombies?
• Dead bodies.– Human bodies.
• Where is this in my book?– Chapter 29.
By the end of this lesson…
• You should be able to distinguish between primary and secondary endosymbiosis.
• You should be able to describe some of the early adaptations that allowed algae and aquatic plants to adapt to life on land.
“Lifeless.”
• That’s how you would have to describe Earth’s land areas for around 3 billion years.
• As we know, life evolved in the ocean first.• And what else evolved in the ocean?– Photosynthesis.
• Recall that the first photosynthetic organisms were cyanobacteria – photosynthetic bacteria.
Secondary Endosymbiosis
• You already know that chloroplasts evolved through serial endosymbiosis – the same way mitochondria evolved.
• What we didn’t previously discuss is the concept of secondary endosymbiosis.– Secondary endosymbiosis is just like “regular”
endosymbiosis, except a photosynthetic eukaryote gets engulfed by a larger eukaryote.
• Secondary endosymbiosis is a process that occurred after primary endosymbiosis (what we already learned).
Secondary Endosymbiosis: So?
• The reason secondary endosymbiosis is important is because it explains some weird stuff observed in modern photosynthetic organisms.
• Some plastids (chloroplasts and related organelles) have three membranes and some have four.– These are sometimes accompanied by lots of extraneous
genomes that appear to have unique evolutionary origins.• The most parsimonious way to explain this
phenomenon is that endosymbiosis occurred between eukaryotes.– Sure enough, laboratory data confirms this.
Case-in-Point: Hatena arenicola
• H. arenicola is a marine…protist? No one really knows…it’s some kind of single-celled eukaryote.
• It’s normally colorless but uses a “feeding apparatus” to “eat” a green alga (singular of algae).
• After it eats the alga, it doesn’t digest it and the feeding apparatus withers away, at which point the formerly heterotrophic cell becomes autotrophic.
• Two related articles:– Marine Microorganism Plays Both Host and Killer– First Known Photosynthetic Animal
http://www.scientificamerican.com/article/marine-microorganism-play/
Origins
• As you can see from the previous image, red algae and green algae (and descendants of green algae – true plants) emerged from primary endosymbiosis.
• Other life forms emerged from secondary endosymbiosis:– Heterokonts (diatoms, other algae), some
dinoflagellates, euglenoids.• Still others underwent tertiary endosymbiosis:– Other dinoflagellates.
Evolution of Land Plants• Oceanic plant precursors were
generally unicellular.• Land plants are multicellular,
but their evolutionary roots (see what I did there?) are connected to green algae.– Specifically, the evolutionary
ancestor to land plants is a class of green algae known as charophyceans.• Class Charophyceae includes an
order known as coleochaetales, which is most closely related to the embryophytes, modern land plants.
Charophyceans
Coleochaetales
Evidence for Plant Evolution
• Plants evolved from charophyceans over 500 million years ago.
• Today, each group has homologous structures:– Chloroplasts with similar DNA genomes.• Yes, biologists used the molecular clock technique.
– Cell walls are made by similar structures.• Rosette (ring-shaped) cellulose-synthesizing protein
complexes in the plasma membrane make the cellulose-based cell wall.
– Sperm has a flagellated structure like plant precursors.
Differences In Land Plants
• Obviously, land plants have evolved a number of unique structures/life history elements that differentiate them from charophyceans:– Apical meristems– Alternation of generations– Walled spores in sporangia– Multicellular gametangia– Multicellular, dependent embryos
• We’ll discuss each of these further later, although “alternation of generations” should sound familiar.
Other Differences
• Plants vs. Algae– Roots vs. Holdfast– Stem vs. Water Support– Leaf Photosynthesis vs.
Whole Alga Photosynthesis– Differences in gas/water
exchange
Alternation of GenerationsSummary Slide
• A diploid sporophyte produces haploid spores with its sporangia through meiosis.
• Haploid spores undergo mitosis to develop into a haploid gametophyte.– Asexual reproduction by the sporophyte.
• The haploid gametophyte produces haploid gametes with its gametangia.
• Two haploid gametes fuse to form a diploid zygote, restarting the process.– Sexual reproduction by the gametophyte.
Walled Spores in Sporangia
• Sporangia are the spore-producing organs in some land plants.– Key: Spores are haploid cells that
develop asexually into gametophytes without becoming diploid.
– Key: Sporangia are diploid and produce haploid spores by meiosis.
• In many plants, the walls of the spores are toughened by a polymer called sporopollenin.
Multicellular Gametangia
• The first land plants produced gametes in structures known as gametangia.– Female gametangia are archegonia.– Male gametangia are antheridia.
Multicellular, Dependent Embryos
• In land plants, zygotes, which are multicellular, stay within the parent tissue.– In other words, algal
embryos are released to the wild, whereas the plant zygote develops within the parent plant.
– Obviously, algae have single-celled offspring while plants have multicellular zygotes.
Embryo
Adaptations to Land
• Many adaptations of land plants are…well…just for that. Life on land.
• For many plants:– To avoid desiccation (drying), they
evolved a waxy cuticle.– Gas exchange – mainly CO2 – takes
place through stomata.– Water and nutrients are moved
through xylem and phloem.– Seeds protect embryos.– Secondary compounds protect
against herbivory and UV radiation.
First Land Plants• After all of these adaptations, the first
land plants emerged: bryophytes.– They include mosses, liverworts, and
hornworts.– They’re non-vascular (so no xylem/phloem
and no roots).• Rhizoids are their root-like structure.
– They have swimming (flagellated) sperm to fertilize through water.
– They are dominated by the haploid stage of life (gametophyte).• That includes what you think of when you
hear “moss.”
– They form spores for reproduction.• Haploid cells that sprout and stay haploid.
Haploid Diploid
Bryophytes
• So despite being the first land plants, bryophytes are still dependent on damp environments.
• They don’t really have a developed root structure, so that’s why you’ll find mosses growing in damp, shaded places.– Including, but not limited to, the north
side of stuff.• For the first 100 million years,
bryophytes were the only land plants.
Peat Bogs
• In large quantities and wetlands, bryophytes may form peat bogs, which are large carbon sinks in which mosses and dead/decaying plant material build up.– They’re excellent as sources of fuel
because of the carbon that has built up (although it’s not so great for climate change).
– Aside: They also have a weird ability to preserve bodies.
Aside: Peat Bodies
http://www.pbs.org/wgbh/nova/bog/iron-nf.html
Gallagh Man (25 y.o.?)(400-200 BC)
Staked to the ground and probably strangled. Found in Ireland, 1821.
http://images.nationalgeographic.com/wpf/media-live/photos/000/817/cache/bog-bodies-new-research-01_81702_990x742.jpg
Tollund Man(400-300 BC)
Also probably killed. Found in Denmark, 1950.
Aside: Peat Bodies
http://s3.amazonaws.com/medias.photodeck.com/c32f8d2a-e3df-11e0-9b5a-2d8bf7f2a331/1111_iblock_732_xgaplus.jpg
Grauballe Man(100 BC-100 AD)
Throat cut, and remnants of hallocinogenic fungal
soup found inside.Found in Denmark, 1950. http://img.morgenpost.de/img/wissen/crop100261854/7468726035-ci3x2l-
w620/115460.jpg
Windeby “Girl” (teenage)(1-200 AD)
Actually a boy, found next to an older man.
Found in Germany, 1952.
From here…
• Okay, this might seem a little strange as we’re only about 33-ish slides into the lesson but…
• …we’re done.– For reals.
• Even to this point we’ve been starting to discuss plant structures, so that’s the cue to move on.
• All we need now is closure…
Closure
• After the bryophytes, which are in many ways like land algae, plants underwent a great deal of diversification which gave rise to a great deal of the anatomy we’re going to discuss.
• Think of bryophytes like the transitional step, whereas vascular plants are the real land plants.
• Oh, and if you’re wondering where aquatic plants like lilies fit in, well, they appear to have “gone back” to life in the water, in the same way whales went back to the sea.– That’s their niche, not their origin.– They have too many homologous structures with land plants.
ClosureA Look Ahead
• From bryophytes, plants evolved into pteridophytes (ferns), which possess the first leaves known as fronds and the first vascular tissue.
• Ferns continued to evolve, first to gymnosperms and then to angiosperms.
• The Theme:– In our next lesson, watch for a progressive reduction of
the gametophyte stage.• For now: CrashCourse – The Sex Lives of
Nonvascular Plants
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