evolution of angiosperms

• What are the closest relatives to angiosperms among the extinct gymnosperms? Among the living gymnosperms?

TWO HYPOTHESIS

• Gnetifer hypothesis: all gymnosperms are equally divergent of angiosperm

  • Thought this is closest to the truth

• Anthophyte hypothesis: gnetophytes 

• Closest relatives to angiosperms among extinct gymnosperms: bennettitales

When do angiosperms first appear in the fossil record?  How does that differ from the age of the split with the closest living relatives according to DNA sequence data?

• First appeared as pollen in fossil record around 125 to 130 million years ago

• By middle cretaceous, many lineages of angiosperms had become dominant plants in many terrestrial environments

• Archefructus, oldest widely accepted fossil angiosperm, ~125 (mya)

• Eudicot pollen ~135 mya

• DNA studies: amborella diverged

• What groups of angiosperms are there other than the monocots and eudicots? Which lineages diverged prior to the common ancestor of the monocots and eudicots?

• Monocots: flowering plants with cotyledon in the seed and parallel veins in leaf

  • One cotyledon, parallel leaf venation, vascular bundles in ring, vascular cambium absent, sieve-tube plastids with dense proteinaceous inclusions

• Eudicots: having two seed leave

• Basal angiosperms→ diverged prior to common ancestor of monocots and eudicots

  • Amborella trichopoda

  • Nymphaeales

  • Austrobaileyales

• What are some distinctive features of the earliest angiosperm flowers?

• Perianths did not have distinct sepals and petals

• Stamens were diverse in structure and function

• Carpels were unspecialized

• Various members of basal angiosperms have flowers with 

  • Undifferentiated perianths (sepels/petals)

  • Undifferentiated stamens (no filament)

  • Pistils lacking style/stigma, carpels seldom fused

  • 4-nucleate female gametophytes

• According to the book, what are the four principal trends seen in flower evolution over time?

• 1. Parts: indefinite to definite in number

• 2. Axis: from spital to whorls

• 3. Ovary: form superior to inferior, with perianth differentiated into calyx and corolla

• 4. Symmetry: from radial to bilateral

• What are some features that have allowed flowers to coevolve and diversify with animal pollinators?

• Bisexual flowers that have both carpels and stamens in a single flower offer advantage by making visit by pollinator more effective, pollinator can pick up and deliver pollen to each stop

• If plant species by only one or few kinds of visitors, selection favors specializations related to characteristics of visitors

• Many modifications evolved in flowers promoted consistency of specific type of visitor to that kind of flower

• What features differentiate flowers pollinated by different types of animals (e.g. bees, flies, beetles, butterflies, moths, bats, hummingbirds)?

• Ex: many modern angiosperms are pollinated solely or chiefly by beetles, others by flies

• Both depend on floral odors that are fruit or resemble dung or carrion

• In beetle-pollinated flowers, essential floral parts are often covered and thus protected from their gnawing visitors

• Ex: bees need nectar, females collect pollen to feed the larvae, have characteristics to carry nectar

• Bird pollinated flowers have copious thing nectar but usually little odor, birds have poor sense of smell

• How do flowers not pollinated by animals differ from those?

Do not produce nectar, have dull colors, relatively odorless, petals of flowers are either small or absent, sexes often separated on same plant

• What pigments are responsible for the color of flowers?

• Flavonoids ← most important pigments, compounds with 2 six ringed carbon rings linked by a three carbon unit

  • Probably occur in all angiosperms

    • (specifically) Anthocyanins: major determinants of flower color, major class of flavonoids, most red and blue pigments → water soluble and found in vacuoles

  • Carotenoids → oil soluble and found in plastids

    • When pigments () reflect yellow

  •  Cyanidin → red in acidic solution, violet in neutral, and blue in alkaline solution

  • Flavonols → very commonly found in leaves and many flowers

    • Many colorless or nearly so, may contribute to ivory or white hues of certain flowers 

  • Betacyanins:

    • Goosefoot, cactus, portulaca families and in other members of order caryophyllales red pigments

  • Betalains (specific classes of plants)

    • Example: beets

• Many red, orange, or yellow flowers owe their color to presence of caretenoids

• What are the types of fruits? (this part of the chapter reviews material from lab) [swollen inflorescence]

• Parthenocarpy: fruit develop without fertilization and seed development

  • Simple fruits: develop from a single carpel or from two or more untied carpels

    • Most diverse

    • Berries, drupes- stone fruits (coconut), and pomes– apple, pear, quince(develop from a compound inferior ovary)

  • Aggregate fruits: formed from a gynoecium

• Fruitlets: individual matured carpels or ovaries

  • Multiple fruits: derived from inflorescence, from the combined gynoecia of many flowers

• Accessory tissue: any fruit that contains accessory tissue (simple, aggregate, multiple)

• Dehiscent fruit: split open at maturity and commonly contain several seeds?

• Indehiscent fruit: do not split open at maturity and usually originate from an ovary in which only one seed develops

gynoecium

reproductive part of flower, typically consist of one or more carpels

inflorescence

arrangement of flowers on floral axis of plant

• What features are seen in fruits that have different modes of dispersal (i.e. animals, wind, water?

• Some plants have light fruits or seeds that disperse by wind

• Shoot their seeds aloft

• Fruits and seeds adapted for floating are dispersed by water

• Fruit and seeds that are fleshy or have adaptations for attachment are dispersed by animals

• Chory is a mode of dispersal, many kinds [includes via animal, wind, gravity, etc.]

• Refer to table in slides for examples

• Autochory (blast seeds out): expulsion

What are a few general patterns in the coevolution of insects and secondary metabolites? [EXAMPLES]

• When a given family of plants is characterized by a distinctive group of secondary plant products (metabolites), those plants are apt to be eaten only be insects belonging to certain families

  • Ex: mustard family, characterized by the presence of mustard-oil glycosides, as well as associated enzymes that break down these glycosides to release pungent odors associated with cabbage, horseradish, and mustard

• Plants in genus plassiflora produce cyanogenic glycosides, which makes them poisonous to most insect herbivores, but heliconius caterpillars feed exclusively on passiflora

• ~600 species in passiflora in the new world, the exact chemical defenses vary from species to species

• 40 species of heliconius

• Heliconius use secondary compounds to ward off their own predators, and are aposematic to advertise their unpalatability

• Good illustration of the gradual nature of coevolution between plant secondary metabolites and insect herbivores

• Passifloras produce:

  • Faux eggs on the leaves to repel gravid heliconius females

  • Sugary glands to attract ants that predate on heliconius caterpillars