L12- Origin of the Angiosperms

• angiosperms are flowering plants→ reproduce using flowers

• have many different ecologies e.g. tree, shrubs, small, aquatic

• over 90% of plants today are flowering plants

Why were angiosperms difficult to study?

• uninformative fossil record- flowers are difficult to preserve

• difficult to understand evolutionary relationships of extant flowering plants (until recently)

• morphological gaps between angiosperms and other seed plants e.g. ginkgos, cycads

Why is it easier to study angiosperms now?

• new exceptionally preserved fossils are being discovered (charcoalified flowers)

• cladistics revolution→

  • used to use only morphological data

  • can now use molecular data and sequence plants

• evo-devo studies

  • looking at genes e.g. those involved in flower formation

What are angiosperms?

• are seed plants that have all these characteristics (other seed plants that are not angiosperms have some of these, but not all):

  • enclosed ovary (carpel)

  • flowers

  • specialised conducting tissues

  • ovules with a double-layered seed coat (two integuments)

  • pollen with a tectate wall

  • double fertilisation

• reproduction:

  • one male hits the egg and fertilises it, one male hits the embryo sac (wall) and produces a triploid endosperm storage tissue- nutrients for the seed to survive

    

• pollen grain:

  • has distinctive tectate walls

  • the holes in the wall have recognition compounds→ makes sure right species of pollen are fertilising

    

• fossil record:

  • earliest fossils are from the Cretaceous

    

  • first pollen is in the Valanginian, Cretaceous e.g. TDM section shows they have the tectate wall:

    

  • first flowers are in the Barremian, Cretaceous e.g. charcoalified and put under SEN:

    

  • first leaves are in the Barremian, Cretaceous, get two types:

    • monocots e.g. palm trees, grasses, parallel veins

    • dicots e.g. interconnected veins

    

  • first fruits and seeds are in the Aptian, Cretaceous e.g. leaf attached to a seed- can do whole plant reconstruction

    

• nature of the earliest angiosperms:

  • there are 3 hypotheses of what the angiosperms looked like:

    • trees e.g. magnolia

    • water plants e.g. lilies

    • herbaceous small shrubs→ is most likely:

      • angiosperm wood is rare

      • early seeds were small

      • early leaves were small

  • leaves change in structure over time in a successional gradient (small shrub leaves to tree-like leaves)

    

Palaeogeography:

• most continents separated, high sea levels, shallow epicontinental sea weaves

  

• mapped earliest angiosperm pollen from around the world→ angiosperms evolved in the equatorial region (at the equator) but then moved north and south

  

• timeline→

  

• species abundance→ still get the other species but angiosperms are the most dominant

  

Origin of angiosperms:

• molecular clock evidence-

  • maps differences between genomes of different species, works out the rate of change and when they divided but assumes the same rate for all

  • is not good with big events→ suggests far back origin

• phylogeny-

  • are sister group to all the seed plants

  • diverged in the late devonian (but no fossils have been found from then)

    

• hypotheses of origin-

  • sister group to the gnetales (modern group of seed plants)

    • similar wood, are seed plants, some are close to double fertilisation, some have beetle fertilisation

  • sister group to the bennettitales (extinct group)

    • similar wood and flower-like structures

    

• cladistic analysis evidence-

  • are actually sister group to the conifers

    

• most basal angiosperms-

  • morphological data had three hypotheses (all rejected)

    

  • molecular data

    • split in the devonian

    • the most basal of living flowering plants is the amborella

      

• evo-devo research-

  • theories to address the problem of how angiosperms evolved

  • e.g. ‘mostly male' theory→

    • gymnosperms have 2 paralogues (male and female) but angiosperms have lost their female paralogue (can’t produce female structures)

    • angiosperms have ectopically evolved their female cone from the male paralogue

    • now all angiosperms have this characteristic

  • more theories are coming out

Why did angiosperms evolved?

• insects→ did the diversification of insect pollinators lead to angiosperm diversification?

  • no→ main insect groups diversify much earlier than the angiosperms

    

• dinosaurs→ did the change from high browsing sauropods/stegosaurs to low browsing ornithischians cause the diversification of angiosperms?

  • is difficult to test

• most likely the fact that angiosperms are good at speciating and adapting to different ecologies