本期的內(nèi)容是生命周期。本文集的這一部分是遺傳、進(jìn)化與生態(tài)學(xué) Genetics, Evolution, and Ecology. 這門課理論上建議在閱讀完文集的第一部分的內(nèi)容之后再開始學(xué)習(xí)，但基礎(chǔ)不足的朋友也可以嘗試閱讀喔~

這一部分的主要內(nèi)容均來(lái)自 Prof. Angela J. Roles 的課程，因此本文集的這一部分均不會(huì)標(biāo)記為原創(chuàng)。但由于文本來(lái)源不清晰，UP主還是一個(gè)字一個(gè)字碼出來(lái)的文章，本文禁止非授權(quán)的轉(zhuǎn)載，謝謝！

Lesson 7: Life Cycles

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[1] Life Cycles - General

?A life cycle describes the series of changes a living individual undergoes?from birth to death?- The cell cycle describes the life cycle of a cell.

?Usually includes:

????- Naming of stages (based on growth/age/size);

????- Ploidy and method of growth (e.g., mitosis, meiosis, binary fission);

????- Stages that are important for dispersal or those that are non-mobile;

????- Changes in habitat or other important environmental conditions related to?growth/reproduction;

????- Point at which reproductive maturity is achieved;

????- Methods of reproduction (may be multiple);

????- Which stages are unicellular or multicellular.

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[2] Bacteria

Bacterial life cycle - Remember Bacillus licheniformis?

?In this unicellular prokaryotic organism the cell cycle is the life cycle.

?Good growth conditions → produces haploid spores via binary fission.

?Bad conditions →?produces dormant?endospores

?Spores can survive?extreme conditions (e.g.,?high heat or radiation);

?Conditions improve →?germinate, develop into?regular vegetative cells.

?Enables dispersal to new?habitat patches.

?

Remember aquatic Caulobacter crescentus?

?Model system for?prokaryotic life cycle,?maintenance of cellular?asymmetry;

?Start with motile?swarmer cell can’t?reproduce until develops?into stalked cell;

?Daughter cells are?physiologically and?morphologically?different.

????- Stalked cell reinitiates?process.

[3] Sexual Life Cycles (plants, animals, fungi, some protists)

?How do these life cycles differ from each other? From prokaryotes?

????- Identify ploidy at each stage

????- Which stage(s) are multicellular??

????- Are gametes produced via mitosis or meiosis?

????- In which stage does meiosis occur?

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(1) Animals: Consider the life cycle of Culex, a mosquito

?Fertilized diploid eggs (zygotes)?are laid on water, hatch into?larvae;

?Larvae are omnivorous and grow?by mitosis, developing into pupae;

?Pupae do not feed, they develop?into the adult stage.

?Adults fly (disperse!!), produce?haploid gametes via meiosis.

????-?Males feed on nectar, produce?haploid sperm, seek mates.

????- Females require a blood meal to?produce haploid eggs.

????- Males provide the sperm to?fertilize a female’s eggs, which she?then deposits on water.

?Diapause to survive stressful?conditions

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Animal life cycles

?Only certain body cells?undergo meiosis to?produce gametes.

?Haploid cells (gametes)?usually are short-lived and?not independent.

?Mitosis is responsible for?development, growth,?maintenance of the whole?organism.

????Note that there are some cases where an adult animal is haploid and?produces gametes via mitosis. Example: male honeybees.

?

(2) Plants: Angiosperm life cycles

Solanum lycopersicum (tomato)

?Diploid seeds (zygotes) germinate, grow via mitosis into seedlings.

?Mature plants produce?flowers containing?structures that produce?spores via meiosis.

?Haploid spores undergo?mitosis to produce?multicellular gametophytes?which make gametes via?mitosis.

?A new diploid zygote is?formed when female?gametes are fertilized by?male gametes.

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Plants: Fern life cycles

?Meiosis in the diploid?(sporophyte) produces?haploid spores which develop?into gametes (gametes?produced via mitosis).

?Haploid spores develop into?a multicellular structure?(gametophyte) that?produces gametes using?mitosis.?

????Alternation of generations = both the haploid and diploid parts of the life?cycle are multicellular.

(3) Fungal life cycles

?Fungi can form heterokarya, cells with?multiple haploid nuclei. Eventually, nuclei?fuse to form a diploid nucleus (karyogamy).

?Meiosis produces?haploid spores which?may enter an?asexual OR sexual?reproductive cycle.

?In the sexual cycle,?plasmogamy occurs?when two haploid?cells fuse their?cytoplasms.

[4] Non-sexual

- Eukaryotic alternatives to sexual (meiotic) reproduction?- Vegetative reproduction

(1) Vegetative reproduction: the parent organism develops mitotic?structures that detach to become independent. These offspring are?morphologically different from sexual offspring and do not require any?fertilization.

(2) Fragmentation: small pieces (fragments) of the adult parent organism?can become detached?and develop into a full, mature individual.

?This occurs in many aquatic?animals such as sponges and?annelid worms.

?Plants and fungi also often employ?fragmentation for reproduction.

?How does this work? Why don’t they need to start from gametes?

(3) Parthenogenesis: Parent organism uses the sexual machinery to?produce offspring that do not require fertilization. This may or may not?involve reductive divisions (from diploid to haploid).

?Parthenogenesis is common in?plants and animals

?Many organisms can switch?between normal meiosis and?parthenogenesis

?Often, meiotic offspring can?withstand bad conditions or are?very good at dispersing to new?habitats.

[5] Genetic Variation: Mitosis versus Meiosis

?Mitosis does NOT reduce genetic variation;

?Meiosis does not produce more fit genotypes ON AVERAGE, but does?increase how?many different genotypes may occur.

- Thus, meiosis does NOT increase fitness of the individual...

- But, the increased variation of meiotically produced offspring may increase persistence of the lineage in changing environments.

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Facultative parthenogenesis

- Facultative parthenogenesis is the best of both worlds!