Ancestors of plants & alternation of generations Objectives of lecture Know possible origin of plants Understand Alternation of generations concept

Green algae as ancestors of land plants

Green algae as ancestors Green algae are considered to be ancestors of land plants This means that plants evolved from green algae Why? Biochemical evidence Food reserves- starch similar to plants Cell wall composition- cellulose similar to plants Photosynthetic pigments- chlorophyll a & b similar to plants Production of oxygen during photosynthesis

Green algae as ancestors to plants Molecular evidence rRNA &DNA sequence in some algal groups similar to plants Peroxisome contents Mechanisms of mitosis and cytokinesis structural Presence of plasmodesmata in cells Protection of young sporophytes Chloroplast structure- granna

Advanced algae- Chara spp

Alternation of generations To understand : Concept of alternation of generations  Events which separate the gametophyte & sporophyte Processes involved in alternation of generations  Fertilization & meiosis

Alternation of Generations Two types of generations (based on plant bodies) in the life cycle of a plant  Gametophyte (n)  Sporophyte (2n) Alternation of generations occurs in all plants (algae, bryophytes, pteridophytes, seed plants)

Gametophyte & Sporophyte Gametophyte (n)  Generation/phase during which organism produces gametes by mitosis Sporophyte (2n)  Generation/phase during which organism produces spores by meiosis Dominance of generation:  Depends on when meiosis & fertilisation occur during life cycle

Alternation of generations… A phenomenon which occurs in plants where a diploid spore-producing sporophyte generation alternates with the a gamete producing haploid gametophyte generation

Fertilisation & Meiosis Fertilization and meiosis are salient features/processes in sexual reproduction and alternation of generations Fertilisation  fusion of different genetic contributions from two parents to form new genetic identity  Involves fusion of gametes  Number of chromosomes is doubled (2n) Meiosis  reduction division process of two sets of chromosomes found in diploid parent to a single set of chromosomes  Number of chromosomes is single (halved) (n)

Fertilization & meiosis Fertilisation & meiosis occur at different points in the life cycle of different organisms: (a) In bryophytes, meiosis occurs immediately after fertilisation - most of the life cycle is spent in the haploid (n) state

Fertilization and meiosis… In tracheophytes, fertilisation & meiosis are separated by a longer period of time - organism has a diploid phase (sporophyte) & a haploid phase (gametophyte) (c) In animals – meiosis is immediately followed by fertilisation - organism is diploid (2n) most of its life

Alternation of generations Generalized life cycle involving alternation of generations

Alternation of generations Isomorphic alternation of generations Sporophyte and gametophyte generations look exactly the same eg alga Ulva Heteromorphic alternation of generations Sporophyte and gametophyte look different Example –ferns (show examples)

Alternation of generations… Evolutionary trend in the nature of two generations:  Gradual increase in dominance of sporophyte generation with evolutionary advancement