遗传 进化与生态学 7 - Life Cycles

本期的内容是生命周期。本文集的这一部分是遗传、进化与生态学 Genetics, Evolution, and Ecology. 这门课理论上建议在阅读完文集的第一部分的内容之后再开始学习，但基础不足的朋友也可以尝试阅读喔~

这一部分的主要内容均来自 Prof. Angela J. Roles 的课程，因此本文集的这一部分均不会标记为原创。但由于文本来源不清晰，UP主还是一个字一个字码出来的文章，本文禁止非授权的转载，谢谢！

Lesson 7: Life Cycles

[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.

[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?

(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

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.

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.

Facultative parthenogenesis

- Facultative parthenogenesis is the best of both worlds!