The Wonderful World of Green Plant Diversity and

Evolution

Biol 366

Spring 2012

Tree of Life: The Big Picture

EukaryotesArchaeaBacteria

ca. 4 bya

now

>3.5 bya

>2 bya

membrane-boundnucleus, organelles, etc.

Fig. 1.1 from Simpson

Green plants share:

• Chlorophylls a (ancestral) and b

• Starch storage

• Stellate flagellar structure

• Certain gene transfers from the chloroplast to the nucleus

• And other features (see Ch. 3)

Green plant diversity:

• Ca. 350,000 species

• Two major groups: 1) chlorophytes (marine and other green algae) and 2) streptophytes [freshwater green algae and embryophytes (= land plants)]

• A major branch (clade) in the eukaryotic Tree of Life

Fig. 3.1 from Simpson

Some definitions

• Clade = branch on an evolutionary tree, a lineage, includes an ancestor and all its descendants. Ex.: Green plants, chlorophytes, land plants.

• Paraphyletic group = a group that includes an ancestor and some (but not all) of its descendants, indicated by double quotation marks. Ex.: “Green algae”

Chlorophytes

Fig. 3.1 from Simpson

Basal streptophytes

desmids

Spirogyra

CharaNitella

(Judd et al. 2008)

Conjugation inSpirogyra

Haplontic life cycle (haploid dominantor zygotic meiosis)

The only diploid cellIs the zygote

zygote (2n)

haploid body

biology.unm.edu

mason.gmu.edu

CharalesHaplontic but some havemulticellular gametangia(gamete-producing structures)

♀

♂

Embryophytes (land plants) share:

• Cuticle• Alternation of generations (multicellular

sporophyte and multicellular gametophyte)• Multicellular gametangia (gamete-producing

structures)• Multicellular sporangium (spore-producing

structure)• Embryo (young sporophyte)• Parenchyma? (more likely ancestral)

Generalized embryophyte life cycle:Alternation of generations

Bryophytes

• Hornworts, liverworts, mosses• Gametophyte-dominant• No vascular tissue (except conducting

cells in a few mosses)• Separate male and female

gametophytes• Sperm must swim to the egg, therefore

need water for fertilization and therefore must remain small

hornworts

liverwortsmosses

Plant Tree of Life: Embryophtes

Tracheophytes(vascular plants)HornwortsLiverworts

ca. 450 mya

now Mosses

“Bryophytes”

Liverwort gametophyte

Liverwortthallus showing air pores

LiverwortMulticellular gametangia(male = antheridia)

LiverwortMulticellular gametangia (female = archegonia)OogamyRetention of zygote within the female gametophyteMulticellular embryo

Hornworts

G

S

Moss male gametangia(= antheridia)

Capsule = sporangium of the sporophyte

Generalized embryophyte life cycle:Alternation of generations

Tracheophytes (vascular plants)

• Vascular tissue (tracheids) present• Include lycophytes (quillworts, clubmosses,

spikemosses), monilophytes (ferns, horsetails, whisk ferns), and spermatophytes (seed plants)

Fig. 4.1 from Simpson

Lycophytes & Monilophytes

• Quillworts, clubmosses & spikemosses (= lycophytes); ferns, whisk-ferns, & horsetails (= monilophytes);

• Independent gametophytes and sporophytes• Sperm must still swim to the egg• Most are homosporous; a few evolved

heterospory• Many homosporous ferns have means of

avoiding self-fertilization

Lycophytes

Selaginella (spikemoss)

Lycopodium and friends (clubmosses)

Isoetes (quillwort)

Whisk-fern (Psilotum)

Ferns (Leptosporangia)

Monilophytes (ferns, horsetails, whisk ferns)

horsetails

1n

2n

2n2n

2n

1n spores

gametophyte

sporophyte

Nutritionally independentsporophytes andgametophytes

Fern Life Cycle,Fig. 4.32, Simpson

Lignophytes (woody plants) & Spermatophytes (seed plants)

• Secondary xylem (wood) & bark, heterospory, seeds, eustele, pollen (also pollen tube, pollination droplet)

• Includes gymnosperms and angiosperms

Fig. 5.1 from Simpson

Gymnosperms

• Conifers, gingko, cycads, Gnetales

• Molecular data support this group as having a single common ancestor

• No obvious defining character (see characters for Lignophytes & Spermatophytes)

Female cone with each scalebearing usually two ovules; directly exposed to pollen

Male cones with eachscale bearing two or more microsporangia

biology.ualberta.edupine pollen

pine microsporangia

male

female

Fig. 5.7 from Simpson

Angiosperms

• “Dicotyledons”, monocotyledons• Heterosporous (ancestral)• Sporophyte-dominant (ancestral)• Pollen = male gametophyte (ancestral)• Archegonia lost; embryo sac = female

gametophyte; ovules enclosed in carpels (indirect pollination)

• Double fertilization produces zygote + primary endosperm nucleus

Flower = a short, determinate shoot bearing highlymodified leaves, some of which are fertile (i.e.,bearing either microsporangia or megasporangia),with the megasporangia in carpels

Animal pollination syndromes

Wind pollination

A wide range of fruit types…

Fig. 5.7 from Simpson

over 300,000 species of angiosperms

The wonderful world of land plant diversity