Learning Objectives
- Understand the types of reproduction — asexual and sexual
- Learn about different modes of asexual reproduction
- Understand the events in sexual reproduction
- Study the significance of reproduction in organisms
Key Concepts
Asexual Reproduction
Involves a single parent; offspring are genetically identical (clones). No fusion of gametes. Methods: Binary fission (Amoeba — simple; Leishmania — longitudinal; Plasmodium — multiple fission), Budding (yeast, Hydra), Sporulation (Penicillium, Mucor — zoospores, conidia), Fragmentation (Spirogyra), Vegetative propagation in plants (runners, rhizomes, tubers, bulbs, offset, leaves — Bryophyllum). NEET Note: Vegetative propagation preserves genetic identity and is useful in horticulture.
Sexual Reproduction
Involves two parents (usually) and fusion of male and female gametes (syngamy). Offspring are genetically different due to genetic recombination. Events in sexual reproduction:
1. Pre-fertilization events: Gametogenesis (formation of gametes) and Gamete transfer. Isogamy: Gametes morphologically similar (Cladophora). Anisogamy: Gametes dissimilar in size (Fucus). Oogamy: Large non-motile egg + small motile sperm (most organisms). Plants may be monoecious/homothallic (both sexes on same plant — cucurbits, coconut, maize) or dioecious/heterothallic (sexes on separate plants — papaya, date palm). Animals may be bisexual/hermaphrodite (earthworm, leech, tapeworm) or unisexual (cockroach, mammals).
2. Fertilization (Syngamy): Fusion of male and female gametes. External fertilization: In aquatic environment (frog, bony fish — produce many gametes to overcome losses). Internal fertilization: Inside female body (reptiles, birds, mammals). Product of syngamy = zygote (always diploid).
3. Post-fertilization events: Zygote development into embryo (embryogenesis). Oviparous: Lay eggs (birds, reptiles, frog). Viviparous: Give birth to young ones (most mammals). Zygote undergoes cell division (cleavage) and differentiation to form embryo.
Special Cases
Parthenogenesis: Development of organism from unfertilized egg. Example: honeybees (drones are haploid males from unfertilized eggs), rotifers, some lizards. Meiocyte (2n) → Gametes (n) → Zygote (2n). Chromosome number is restored at fertilization.
Summary
Reproduction is essential for species continuation. Asexual reproduction produces clones through methods like fission, budding, and fragmentation. Sexual reproduction involves gametogenesis, fertilization, and embryogenesis, producing genetically diverse offspring. Organisms can be oviparous or viviparous based on embryo development patterns.
Important Terms
- Clone: Genetically identical offspring produced by asexual reproduction
- Syngamy: Fusion of male and female gametes to form zygote
- Oogamy: Fusion of large non-motile egg with small motile sperm
- Monoecious: Both male and female reproductive organs on same organism
- Dioecious: Male and female reproductive organs on separate organisms
- Parthenogenesis: Development from unfertilized egg
- Viviparous: Organisms giving birth to live young
- Embryogenesis: Development of embryo from zygote
Quick Revision
- Asexual = single parent, clones; Sexual = two parents, variation
- Binary fission: Amoeba; Multiple fission: Plasmodium; Budding: yeast, Hydra
- Vegetative propagation preserves genetic identity
- Isogamy → Anisogamy → Oogamy (increasing specialization)
- External fertilization: aquatic; Internal fertilization: terrestrial
- Oviparous = eggs; Viviparous = live young
- Parthenogenesis: honeybee drones (haploid, from unfertilized eggs)
- Bryophyllum reproduces vegetatively from leaf notches