Evolution of Plants David Baum

Game plan What are “plants” and how did they evolve? Differences between plant and animal evolution Why were angiosperms so successful? Pollination biology

What are the three most important What are the three most important* events in the evolution of life on earth? Oxygenic photosynthesis (cyanobacteria) Invasion of land (plants) Human agriculture and technology (plants and humans) *Profoundly affecting the globe’s 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? http://www.earthhistory.org.uk/recolonisation/vegetation-in-devonian

An evolutionary arm’s 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! If you want to know more: Botany 130, 300, 305, 401, 500 Fern sperm cell

Are there differences between plant and animal evolution? Very few – plants are excellent “model systems” But.. More abundant polyploidy and allopolyploid speciation

Why is polyploidy common in plants? Tolerance of different ploidy levels during development (dosage compensation?) Often make unreduced gametes (“rescues” meiotic problems) Or? Chester et al. 2010. Genes 1(2), 166-192.

Are there differences between plant and animal evolution? Very few – plants are excellent “model systems” But.. More abundant polyploidy and allopolyploid speciation Greater diversity in sexual systems

Terminology (from Wikipedia)

Self-pollination is important in some species Reproductive assurance Viola (Some flowers self in bud) Arabidopsis (pollen shed onto stigma) Hibiscus (active selfing; Ruan et al. 2010)

Some species produce seeds asexually May still require (self) pollen Dandelion - Taraxacum Blackberry- Rubus

Mechanisms that promote cross-pollination Spatial separation of anthers and stigma in a flower Wild buckwheat

Mechanisms that promote cross-pollination Spatial separation of anthers and stigma in a flower Temporal separation of pollen release and stigma receptivity Female phase Male phase Aeonium

Mechanisms that promote cross-pollination Spatial separation of anthers and stigma in a flower Temporal separation of pollen release and stigma receptivity Self-incompatibility (SI)

Mechanisms that promote cross-pollination Spatial separation of anthers and stigma in a flower Temporal separation of pollen release and stigma receptivity Self-incompatibility Tsuchimatsu et al. (2010) Nature Self pollen Cross pollen

Consequences SI taxa speciate slower than SC taxa But they also go extinct slower SI has higher net rate of diversification Emma E. Goldberg et al. Science 2010;330:493-495

Mechanisms that promote cross-pollination Spatial separation of anthers and stigma in a flower Temporal separation of pollen release and stigma receptivity Self-incompatibility Dioecy (800-5000 origins!) Varied mechanisms/genetics Salix (Willow)

Actually it is more complicated Gynodioecy: Females + hermaphrodites Androdioecy: Males (top) + hermaphrodites Wild strawberry Datisca http://raven.iab.alaska.edu/~dewolf/datiscaPics.html

Are there differences between plant and animal evolution? Very few – plants are excellent “model systems” But.. More abundant polyploidy and allopolyploid speciation Greater diversity in sexual systems More chemistry less behavior

Are there differences between plant and animal evolution? Amazing diversity of chemicals produced Still evolving Be thankful! Opium Poppy Hops Natural rubber Coffee

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

Examples of “hopeful monsters?” Rudall PJ, Bateman RM. 2003. Rudall PJ, Bateman RM. 2002. Trends Plant Sci. 8(2):76-82. Biol Rev Camb Philos Soc.77(3):403-441

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

Are flowers monsters? A flower is a bisexual “cone” Megasporophyll Microsporophyll (stamen) 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 human Why so successful?

Why are the angiosperms so successful? Better competitors? Reticulate venation (better in low light) Vessels (grow faster in moist environments) Reduced gametophytes and double fertilization (more rapid seed formation)

Why are the angiosperms so successful? Elevated speciation rate Animal pollination (sister lineages become reproductively isolated quickly) Fruit (allows for long distance dispersal) Reduced extinction rate Animal pollination allows for persistence at low densities

Animal pollination

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

Plants have evolved diverse ways to get animals to visit Mentzelia involucrata Mutualistic (give a reward) Nectar Pollen Solid food Perfume Heat Sex Parasitic (trick the animal) Mohavea confertiflora

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

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

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 a C c B b D Wasp phylogeny Fig phylogeny C c B b D d E e F f

Actual result Host switching Weiblen and Bush (2002)

Feel free to contact me: dbaum@wisc.edu Plant evolution is similar to other multicellular eukaryotes Arm’s race for light Plants (may) differ in frequency of polyploidy, chemical evolution, sexual lability, hopeful monsters Angiosperms very successful Coevolution with animals for pollination Feel free to contact me: dbaum@wisc.edu