trends in evol of plant life cycle adaptations to a dry environment reduction in size of gametophyte loss of antheridium, archegonium increase in size of sporophyte gametophyte retained on sporophyte

preview: major events in plant evolution Fig 29.7grade vs clade

Fig 29.4 what are plants?

share derived characters of plants & charophycean green algae way of making cellulose walls enzyme glycolate oxidase (recovery when rubisco grabs O 2 instead of CO 2 ) cell division mechanism sperm ultrastructure DNA sequences, genomic architecture

shared derived characters of plants are life cycle features (alt of gen) 1) gametophytes producing gametes in multicellular gametangia 2) multicellular diploid embryo (young sporophyte) retained on parent plant –embryo protected, nourished –plants called “embryophytes” (Fig 29.4) 3) spores in multicellular sporangia –sporopollenin (very resistant) in spore walls

origin of plant life cycle? Coleochaete –retains egg and zygote –protects and nourishes charophyceans have haploid life cycle (only zygote is diploid)

2n n meiosissyngamy zygote gametes sporophyte spores gametophyte kelps, plants Fig Life Cycles Fig 13.6 n 2n zygote Chlamydomonas Fig n 2n gametes humans mitosis

hypothesis: plant life cycle arose from delay in meiosis zygote undergoes mitosis –produces multicellular sporophyte many cells undergo meiosis –advantage on land (less water for fertilization)

life on land: plenty of light, CO 2 but dessication, UV light

preadaptations to life on land 1) resistance to desiccation resistant compounds incl. sporopollenin (Coleochaete zygotes, plant spores & pollen) 2) protection from UV light surface layer

strategies for life on land 1) H 2 O level variable = poikilohydry –same as environment stay in wet places OR dry & rehydrate 2) maintain constant internal H 2 O level = homeohydry (type of homeostasis) cuticle, stomata, lignified H 2 0 cond. cells

physiology and anatomy of “bryophytes” (grade not clade) poikilohydry small, attach via rhizoids –long tubular cells or filaments some have water conducting cells –but not lignified stomata only on sporophytes of hornworts & mosses cuticle on some sporophytes, and parts of gametophytes (leaves, pores)

1) liverworts (leafy OR thalloid) 2) hornworts (horn-like sporophyte) 3) mosses (most obvious & diverse) diversity of “bryophytes”

liverwort sperm release, when sprayed with water thalloid liverwort—spots are pores for gas exchange gemma cups—asexual reproduction gemma cup with gemmae a single gemma a single gemma germinating sexual reproduction

Physcomitrella patens genome moss model organism

“bryophyte” life cycle gametophyte is prominent –sperm require water sporophyte smaller, dependent –spores dispersed by wind sporophyte female gametophyte male gametophyte

diversification for spore dispersal in bryophytes height facilitates dispersal gametophyte or sporophyte may be tall

“bryophyte” ecology diverse habitats, especially mosses 1) Andreaea (habitat like early Earth) thick walls resist UV, store C 2) Sphagnum (peat moss) stores CO 2 assoc. methanotrophs CH 4 -->CO 2 used for gardening –dead cells hold water

evidence of early plants molecular data: plants est. 700 my old fossil evidence: at least 475 mya Mystery: where is fossil record? Answer1: microfossils spores sheets of cells (from sporangia) lower epidermis with rhizoids Answer 2: macrofossils?