Sporogenesis.pptx

SPOROGENESIS BIOL 3321K Diggs Cell division refresher • Mitosis Somatic cells • 2N -> 2N • 1 cell gives rise to two daughter cells • • Meiosis Sex cells • 2N -> 1N • 1 diploid cell gives rise to FOUR haploid daughter cells • Proceeds in two stages – • • • • 1N Meiosis I (1 2N cell -> 2 2N ce...

SPOROGENESIS BIOL 3321K Diggs Cell division refresher • Mitosis Somatic cells • 2N -> 2N • 1 cell gives rise to two daughter cells • • Meiosis Sex cells • 2N -> 1N • 1 diploid cell gives rise to FOUR haploid daughter cells • Proceeds in two stages – • • • • 1N Meiosis I (1 2N cell -> 2 2N cells) Meiosis II (2 2N cells -> 4 1N cells) cells can divide into either 2 (mitosis) or 4(meiosis) daughter cells. • Keep • in mind that plants can be polyploid. So “2N” for them really just means “full ploidy.” • ALSO keep in mind that this can be nuclear division – can take place without (or with delayed) cytokinesis. Alternation of generations • The characteristic life cycle of all plants (and green algae/charophytes). • Separated into a haploid (1N, gametophyte) and a 2N (diploid, sporophyte) generation. Keep polyploidy in mind. • These are SEPARATE PLANTS. • • Chromosomes by MEIOSIS. are doubled via FERTILIZATION (or fusion), or halved Homosporous alternation of generations Heterosporous alternation of generations • Flowering plants (angiosperms), gymnosperms, and several other major groups of plants are heterosporous. • In angiosperms, the male gametophyte is called pollen. • • A separate haploid plant made of exactly two (or sometimes three) cells The female gametophyte is called an ovule. • A separate plant (usually) reduced to exactly seven cells. • • •A Contains an egg cell, but is not itself an egg. Dependent on the parent plant – lives parasitically. few groups of plants (whisk ferns, etc.) are homosporous • “Micro-” is a prefix in plant reproduction implying the male, or sperm-bearing, life stage. • “Mega-” • The is the prefix for the female, or egg-bearing, stage. rest of this chapter is on Microsporogenesis – development of the sperm-bearing gametophyte • Megasporogenesis – development of the egg-bearing gametophyte • Pollination • Fertilization and seed development • Microsporogenesis • Development of the pollen – the spermbearing gametophyte • Occurs • in the androecium Stamens – specifically in the anthers • Anthers separated into four lobes, which develop into pollen sacs. Anthers have 2N microsporocytes • Surrounded by nutritive tapetum • Microsporocytes undergo nuclear meiosis • Form pollen tetrad (each is 1N) • Tetrads split and each is now a 1N microspore • Microspores • Develop intine (cellulose) and exine (sporopollenin) • And undergo mitosis (with cytokinesis) to form mature pollen • • Anthers then dehisce to release pollen. Microsporogenesis Megasporogenesis • Development of the ovule – the eggbearing gametophyte • Occurs • in the gynoecium Pistil – specifically in the ovary • An immature ovule consists of: Integuments – usually two layers of cells that wrap around the ovule for protection • Funiculus – and umbilicus connected to the placenta of the mother plant • Megasporocyte (2N) inside integuments and fed by a nutritive tissue called the nucellus • Micropyle is the opening in the integuments • Megasporogenesis • Megasporocyte megaspores undergoes meiosis to form 4 Three megaspores die • Remaining megaspore undergoes THREE nuclear mitoses resulting in 8 nuclei • • Cytokinesis separates these cells into • • • • • 2 synergids – produce pheromone-like chemicals to attract sperm to zone of greatest concentration – in front of egg cell 1 egg cell – will be fertilized Three antipodals – many functions depending on plant genus • Can help with nutrition • Can die • Can form defensive chemical gland 2 polar nuclei (or polar bodies) - Eventually unite and become basis for endosperm This is known as an mature embryo sac, or a mature female gametophyte. Pollination and fertilization • Mature pollen released from anther • Then vectored (by biotic or abiotic means) to the gynoecium of another plant • Plants can sometimes “self” (pollinate the gynoecium of the same flower or a different flower on the same plant to form a genetically clonal seed), but it’s not common. • Pollen • sticks to the stigma Then germinates • • Tube cell (1N) germinates and begins to dissolve a pollen tube down through the stigma/style tissue Generative cell (1N) germinates and enters pollen tube • • • Undergoes mitosis to form two 1N sperm cells. Both then travel down pollen tube, following tube cell Synergids attract the tube cell to the micropyle Tube cell dies, 2 sperm enter ovule (double fertilization) • • One sperm fertilizes the egg – forms zygote Other sperm unites with the polar nuclei to form triploid endosperm – food supply for growing embryo Mature seed structure • 2N embryo (but more like 1N + 1N) • 3N endosperm • 2N seed coat (derived from integuments, both sets of chromosomes from the mother plant)