Life Cycles Observing Evolutionary Trends in the Life Cycles of Protists, Plants and Animals

Generalized Life Cycle Fusion of haploid nuclei Diploid Adult Mitosis Diploid Phase in specialized structures Diploid Cell Fertilization/Syngamy Meiosis Gametes Haploid Cells Haploid Phase Mitosis Mitosis Fusion of gametes Haploid Adult

Categories of Life Cycles Haplontic Haploid phase is predominant Chlamydomonas Ulothrix Diplohaplontic Haploid and diploid phases are equally represented Ulva Diplontic Diploid phase is predominant Fern Echinoderm Ascaris Human Animals

Haplontic Life Cycle Diploid Phase Haploid Phase Fertilization/Syngamy same size Diploid Phase Meiosis Haploid Cells Isogametes Fertilization/Syngamy Diploid Cell Mitosis in poor conditions, cells of opposite mating type can fuse Haploid Cells Haploid Phase

Haplontic Life Cycle of Chlamydomonas Resting spore withstands poor conditions Haplontic Life Cycle of Chlamydomonas Photosynthetic protist showing an early eukaryotic life cycle No mitosis in diploid phase

Haplontic Life Cycle of Ulothrix Isogametes fuse to form diploid zygote Gametangium: cell that produces gametes by mitosis Photosynthetic Protist No mitosis in diploid phase Zygote produces spores by meiosis Haploid gametophyte grows from spore by mitosis Gametophyte can produce gametes or zoospores Motile spores (zoospores) formed by mitosis or meiosis Sporangium: cell that produces spores by mitosis Spore: haploid cell that can grow by mitosis Gametophyte: multicellular haploid structure that produces gametes Haplontic Life Cycle of Ulothrix

Applying Your Knowledge Mitosis Meiosis Both Mitosis and Meiosis For Ulothrix, spores are produced by Growth of a gametophyte occurs by For Chlamydomonas, spores are produced by For Ulothrix, gametes are produced by

Diplohaplontic Life Cycle Diploid Sporophyte Mitosis Meiosis Haploid Cells in specialized structures Diploid Phase Zygote Gametes Fertilization/Syngamy Mitosis Haploid Gametophyte Mitosis Haploid Phase

Diplohaplontic Life Cycle of Ulva Photosynthetic Protist Introduction of Mitosis in Diploid phase Gametophyte produces isogametes Isomorphic gametophyte and sporophyte Sporophyte: multicellular diploid structure that produces spores by meiosis

Applying Your Knowledge Diploid Phase Haploid Phase Both Diploid and Haploid Phases Where is the sporophyte found? Zoospores are part of which phase? Where is the gametophyte found? Where is the zygote found?

Diplontic Life Cycle in Fern Diploid Sporophyte Mitosis in specialized structures Diploid Phase Zygote Meiosis Haploid Cells Mitosis Gametes Fertilization/Syngamy Haploid Phase Haploid Gametophyte Mitosis

Diplontic Life Cycle of Fern Archegonium: organ that produces eggs by mitosis Plant Smaller gametophyte Larger, longer-lived sporophyte with tissues and organs Spores produced by meiosis in sporangia on the underside of sporophyte leaves Antheridium: organ that produces sperm by mitosis contain sporangia (singular: sorus)

Sperm swim to archegonium and one sperm fertilizes the egg Archegonium: organ that produces eggs by mitosis Antheridium: organ that produces sperm by mitosis

Applying Your Knowledge If a fern species has 5 pairs of homologous chromosomes, how many chromosomes would be found in a(n) Sporophyte Archegonium Spore Gametophyte Sporangium (not including spores inside)

Diplontic Life Cycle in Animals Diploid Organism Mitosis Diploid Phase in sex organs Fertilization/Syngamy Zygote Gametes Meiosis Haploid Phase

Evolutionary Trends Reduced length of haploid phase Reduced amount of mitosis in haploid phase Increased amount of mitosis in diploid phase Increased size of diploid adult organism Role of mitosis changes from reproduction in haploid phasegrowth in haploid phasegrowth in diploid phase Single cells  Multicellular organisms Increased tissue differentiation Longer-lived species Note that the role of meiosis and fertilization remains the same: to generate new combinations by sexual reproduction