Evolution of Plants David Baum

Game plan What are plants and how did they evolve? Differences between plant and animal evolution Some stories of plant evolution

What are the three most important * events in the evolution of life on earth? 1.Oxygenic photosynthesis (cyanobacteria) 2.Invasion of land (plants) 3.Human agriculture and technology * Profoundly affecting the globes chemistry and ecology

Early land plants were low to the ground

They became larger, more complex, and acquired a vascular system Time

Multiple origins of trees Crane and Leslie (2013)

Why?

An evolutionary arms race The Red Queen principle Now, here, I see it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that! (Through the Looking Glass, Lewis Carroll)

Competition for light is a very important driver of plant evolution

Problems that plants faced Gain light, water, nutrients Escaping predators (once animals invaded land) Sex! Fern sperm cell If you want to know more: Botany 130, 300, 305, 401, 500

Are there differences between plant and animal evolution? Very few – plants are excellent model systems But.. –Greater diversity in sexual systems Abundant asexuality –More chemistry less behavior –Maybe more evolution by hopeful monsters

Examples of hopeful monsters? Rudall PJ, Bateman RM Trends Plant Sci. 8(2): Rudall PJ, Bateman RM Biol Rev Camb Philos Soc.77(3):

Are flowers monsters? Pollen cone Seed cone Living gymnosperms have unisexual cones

Are flowers monsters? Microsporophyll (stamen) Megasporophyll A flower is a bisexual cone (although unisexual flowers have evolved in many groups)

If so: quite successful! ~300,000 species of flowering plants Dominate all land ecosystems (and several aquatic ones) Provide all food resources for humans

Are there differences between plant and animal evolution? Very few – plants are excellent model systems But.. –Greater diversity in sexual systems Abundant asexuality –More chemistry less behavior –Maybe more evolution by hopeful monsters –Polyploidy

Why is polyploidy common in plants? Tolerance of different ploidy levels during development (dosage compensation?) Often make unreduced gametes (rescues meiotic problems) Chester et al Genes 1(2),

Why is polyploidy common in plants? Tolerance of different ploidy levels during development (dosage compensation?) Often make unreduced gametes (rescues meiotic problems) Allows for instant speciation

Example of instant speciation Spartina maritima grew in British salt marshes A different species, S. alterniflora arrived ~1816, probably from boats arriving from America –This was figured out partly by talking to thatchers in the 1830s who noted that the new form of cordgrass had appeared some 20 years earlier

Example of instant speciation In 1879 two botanist brothers, H. & J. Groves, found a plant that confused them: –Spartina which...would be rather more S. alterniflora… yet which we now consider to be [S. maritima] In 1880 they concluded they had found a new species, which they called S. townsendii It was not fertile Examination of the herbarium records showed that it had appeared by 1870

Example of instant speciation Fertile S. townsendii appeared and was eventually named as a new species S. anglica. Stapf (1910) noted: towards the end of the eighties (c. 1890) something occurred that favoured the spreading of the grass – this is now presumed to represent the origin of S. anglica. Stapf (1926) hypothesized that S. anglica and S. townsendii are hybrids of S. maritima X S. alterniflora

Example of instant speciation S. anglica spread quickly around Southampton harbor It gradually reached other estuaries – especially during World War II – perhaps because of training with landing craft k/2011/11/uk-military- bridging-world-war-ii-africa- and-northwest-europe/

Example of instant speciation But if this is true: how come …it is fertile and breeds true to type (Hutchins, 1930)? [S. anglica]

Hypothesis S. maritima 2a chromosomes S. alterniflora 2b chromosomes S. townsendii a+ b chromosomes (sterile)

S. townsendii a+ b chromosomes (sterile) Hypothesis Chromosome doubling S. anglica 2a + 2b

This hypothesis explains a few things Why the S. townsendii arose suddenly Why it was successful (hybrid vigor) Why it was sterile Why S. anglica arose suddenly Why it was successful Why it is fertile But is it true?

C. Leonard Huskins (1930) Professor of Genetics at McGill (Became Professor of Botany at UW- Madison in 1945) S. maritima, 2n = 56 (2a) S. alterniflora, 2n = 70 (2b) S. anglica, 2n = Huskins (1930) Genetica 126

There are many other examples of polyploid species in plants Often generate competitively superior species Many crops have arisen by hybrid polyploidy (including wheat and potato) Means that plants have lots of extra genes that can acquire new function All vascular plants have undergone multiple rounds of polyploidy in their ancestry

Pollination Stories

Pollination (only occurs in seed plants) avoids the need for motile sperm Pollen is a minute male plant Can be carried by wind (rarely water) More commonly animals do it –Insects –Birds –Mammals

Pollen needs to deliver the gametes to the egg cells Stigma

Pollen tubes grow through plant tissues – navigated chemically

Plants have evolved diverse ways to get pollen from one flower to another Wind Water (rare) Animals –Mutualistic (give a reward) –Parasitic (trick the animal)

How do you think this evolved? What else would you like to know?

figs are tomb blossoms At the other extreme: Figs and fig wasps

Implications There is a one-to-one relationship between a fig species and its wasp pollinator species Predicts cospeciation: that the figs and wasp

Prediction: One-to-one species association Cospeciation A B C D E F a b c d e f Fig phylogeny Wasp phylogeny

Actual result Host switching Weiblen and Bush (2002)

Feel free to contact me: Plant evolution is similar to other multicellular eukaryotes Arms race for light Polyploidy is especially important Coevolution with animals for pollination (and dispersal) is important (Also, abundant coevolution with fungi and bacteria)