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Ulva lactuca thallus A thallus is a plant- like multicellular body lacking any vascular tissue (xylem or phloem).

Ulva life cycle There are three isomorphic thalli: Some species of Ulva are anisogamous. Since they are motile, they are also called zoospores. The meiospores made by the sporophyte are of two mating types. + + – – 1N 2N zygote isogametes + – + – sporocyte GametophytesSporophyte syngamy meiosis settles on substrate settle on substrate holdfast The life cycle is: diplohaplontic diplontic haplontic

Codium

Codium Life Cycle The life cycle is: diplohaplontic diplontic haplontic 1N 2N syngamy anisogametes zygote sporangium sporocytes zoospores Sporophytes 2 sexes holdfast meiosis

Oedogonium thallus

Oedogonium Here the filaments are out of focus. The asexual zoospore is in focus.

Oogonium

Oedogonium This is the zoospore, or maybe it is the sperm. Both have the same look! When both oogonium and antheridium appear on the same filament the alga is homothallic.

Oedogonium Life Cycle The zygote divides meiotically to release 4 zoospores. Since antheridia and oogonia are on the same filaments, this species is homothallic. This life cycle is ? 1N 2N syngamy meiosis zygote (hypnospore) (in oogonium) Zoospores settle down with a holdfast and grow into a filament. sperm chemotaxis egg (in oogonium) Vegetative zoospores can clone the gametophyte empty antheridia

Evolution of Gametes for Syngamy: Isogametes - Anisogametes - Oogametes Evolution of Life Histories: multicellularity and sex Unicellular ( neither truly diplontic nor truly haplontic…but taking both “shortcuts” maybe with asexual reproduction added on diploid and/or haploid sides ) Diplontic (no multicellular haploid) Haplontic (no multicellular diploid) Diplohaplontic (both haploid and diploid multicellular forms) + – Sequence unclear: elaborations vs reductions? Multiple pathways in different groups?

SYNGAMY zygote SporophyteGametophyte sporangium sporocyte spores gametangia gametes germination mitosis differentiation mitosis germination mitosis 1N 2N differentiation MEIOSIS Life Cycle of Flowering Plants The flowering plant is a multicellular diploid sporophyte. The sporophyte encloses the multicellular haploid gametophyte. The life history is thus diplohaplontic. The gametes are oogamous but the sperm lack flagellae. Gender expression has moved from gametophyte to sporophyte.