Origins of Angiosperms

Spring 2012

Outline

• Origin of the angiosperms • Characters of angiosperms• Brief history of angiosperm

classification• Major groups of angiosperms• ANITA grade

Origin of the Angiosperms

ConifersGinkgoCycads Gnetophytes Angiosperms

Divergence estimated at ca. 325 mybp

Progymnosperms(“seed ferns”)

AngiospermsGnetophytesConifersGinkgoCycads

Geologic Time

Timing of Angiosperm Divergences

• The timing of the origin of the angiosperms still is uncertain, but most would agree on a Triassic or Jurassic initial divergence, although there is no unequivocal fossil evidence• A demonstrable “burst” of

phylogenetic radiation is found in the fossil record beginning in the mid- to late Cretaceous, 140 - 100 mybp

Figure 7.1

Origin of the Angiosperms

•pollen grains from ca. 140 mya (early Cretaceous) but already major radiation!

•earliest flowers 130 mya

•likely no extant group of seed plants is very closely related to the angiosperms!

Origin of Angiosperms•Cycad-like plants: Bennettitales?•large, flowerlike strobili:•pollen-producing organs surrounding an axis bearing naked ovules/seeds

Origin of Angiosperms

Modification of a “seed fern” such as Caytonia?

Caytoniafossil: ovule

Origin of Angiosperms

•Archaefructus

•ca. 130 mya•ancestral flowering plant or extinct off-shoot of an extinct lineage?•aquatic plant (dissected leaves)•elongate reproductive axes:

-paired stamens below-several-seeded carpels above

Characters of Angiosperms

Fig. 6.1

What makes a plant an angiosperm?• Flower (usually with perianth)• Carpels with a stigmatic surface for pollen tube

germination; ovules enclosed within carpels; fruit• Ovules with two integuments• Reduced female gametophyte, usually 8 nuclei in 7 cells –

no archegonium• Double fertilization with the production of 3N endosperm• Stamens with two pairs of lateral pollen sacs

(microsporangia); pollen (male gametophyte) 3-nucleate• Xylem – most with vessels (evolved within angiosperms)• Phloem – sieve tube members with 1 or more companion

cells derived from the same mother cell

FlowerFigure 6.2

Spiral undifferentiatedperianth parts = tepals(plesiomorphic)

Magnolia

Magnolia

Differentiated sepals andpetals (each in whorls)(apomorphic)

Abutilon

connectivemicrosporangium

filament

Laminar stamensin basal angiosperms

paired pollen sacs

Early carpel with stigmatic crest…

…to the derived carpelwith a style and an apical stigma.

Figure 6.9 from the text

Female gametophyte in angiosperms

ovule

-no waiting time as in gymnosperms!-note 2 integuments (bitegmic; some angiosperm lineages have lost one integument) -gymnosperms have only 1 integument (unitegmic)

mature ovule

And it’s off to the races!

Indirect pollination(due to presenceof carpels)

-no waiting time as in gymnosperms!

Seed development in angiosperms

seeddouble fertilization

Avocado (Persea, Lauraceae)

seed

endocarpmesocarp

exocarppericarppericarpFlower

fruit

•are the water (solute) conducting cells of the xylem in most angiosperms•ends of cells have openings (perforation plate), cells shorter and wider•more efficient, faster rate of flow but more susceptible to air bubbles (embolisms) than tracheids are•may have arisen independently in two or more angiosperm lineages but may have had a single origin

Vessels in Angiosperms

Origin of vessels from tracheids

Figure 6.16B from the text

Angiosperm phloem

•stm = specialized sugar-conducting cells of the phloem of angiosperms; lack a nucleus at functional maturity

•cc = parenchyma cells associated with stm

-function to load/unload sugars into stm cavity-derived from the same mothercell as its stm

•sieve tube members + companion cells

stm

Brief history of angiosperm classification

Alternative ways of thinking about early angiosperm characters…

• “Old” School (German) - Engler - “Simple is primitive” (Few floral parts)- Ancestors are conifers- Pollination by wind- Modern relicts = “Amentiferae” (catkins)• “New” School (American) – Bessey

- “Flowers with many parts are primitive”- Ancestors are Cycad-like plants- Pollination by primitive insects- Modern relicts = Magnolias and allies

Heinrich Gustav Adolph Engler(1844-1930)

-German Botanist at Berlin Botanical Garden

-Was the primary European in interpreting the grouping of major angiosperm

assemblages-“Few simple flower parts primitive”-Small, unisexual flowers primitive

Englerian ‘Primitive Taxa’

“Amentiferae”

Quercus sp.

Juglans sp.

Betula sp.

Charles Edwin Bessey(1845-1915)

-Botanist at Iowa State University from 1869-1884 (left in 1884 to teach in Nebraska)-Was a “major player” in interpreting and understanding angiosperm evolution-“Many flower parts primitive”

Bessey HallIowa State University

Bessey’s “Cactus”(1915)

Placed plant groupswith many floral parts in a basalposition as the‘ancestral’ forms.

Outlined ‘dicta’ for the construction ofphylogenies using theevolutionary trends incharacter changes.

Polypetalous flowers,insect pollination, cycad-like ancestors

Besseyan ‘Primitive Taxa’

Nymphaeaceae

Magnoliaceae

Figure 6.1 from the text

Major Groups of Angiosperms

• Basal Angiosperms (ANITA grade)- Amborellaceae- Nympheaceae- Illiciaceae

• Magnoliid Complex- Magnoliales- Piperales- Winterales

• MONOCOTS• EUDICOTS (tricolpates)

Major Groups of Eudicots• Basal Eudicots

- Ranunculales and allied families • Caryophyllales & Saxifragales• Rosid Clade

- Fabids- Malvids

• Asterid Clade- Basal Asterids - Lamiids- Campanulids

ANITA grade

• Amborella (Amborellales)• Nymphaea (Nymphaeales)• Illicium (Austrobaileyales)• Trimenia (Austrobaileyales)• Austrobaileya (Austrobaileyales)

ca. 125 mybp

Fig. 6.1

Grade = a paraphyletic (orPolyphyletic) group whosemembers share a similar level of morphological orphysiological complexity.

ANITA Gradeor “basal”angiosperms

Major Angiosperm Clades

Amborellaceae

Nymphaeales

Austrobaileyales

MAGNOLIID COMPLEX

MONOCOTS

EUDICOTS [TRICOLPATES]

ANITA grade (basal

groups)

Soltis et al. 2000, APG II 2002, Judd et al. 2002

Basal Angiosperms: Amborellaceae

• New Caledonia• Understory shrub; plants dioecious• 1 species (monotypic): Amborella trichopoda• Leaves simple, evergreen• Flowers small, unisexual: ♀ apocarpous, with

stigmatic crests; ♂ with laminar stamens• Significant features: Most basal of all flowering

plants; no vessels in wood• Special uses: (none)

Basal Angiosperms:Amborellaceae (Amborella Family)

Amborella trichopoda

As we venture through the various major groups of

angiosperms…• Identify the plesiomorphic characteristics

associated with particular groups and note their apomorphies (if any) as well.

• Try to associate “syndromes” of characteristics with each group (make note of special characters occurring together).

• One good way to study is to write keys to the groups we cover in any given unit.

• Names of groups are important! Learn to spell and say them!

• Ask questions!!