Sexual vs. Asexual Reproduction

Sexual Reproduction

• Involves two parents, leading to offspring with genetic variation.

Asexual Reproduction

• Involves a single parent producing genetically identical offspring (clones).

Asexual Reproduction Mechanisms

1. Fragmentation:

   • Separation of a parent plant into parts that can regenerate into whole plants.

   • Commonly seen in many species where adventitious shoots develop.

2. Apomixis:

   • Production of seeds without meiosis or fertilization.

   • Involves a diploid cell in the ovule forming an embryo, leading to the maturation of seeds (e.g., dandelions).

Adaptation and Benefits

Adaptation Role:

• Both sexual and asexual methods contribute to plant population adaptation to environments.

Benefits of Sexual Reproduction:

• Generates genetic variation, which can be advantageous in changing environments.

• Seeds that are produced can disperse to new locations, enhancing colonization potential.

Benefits of Asexual Reproduction:

• Effective in stable environments, allowing rapid cloning and expansion of successful traits.

• Progeny are mature fragments, unlike seedlings that might be fragile.

Sexual Compatibility & Incompatibility

Cross Pollination:

• Promotes genetic diversity via outbreeding; often arises from self-incompatibility.

• Mechanisms can include timing issues or complexity in pollen tube formation and fertilization.

• Example: Incompatibility among various apple varieties.

Self-Incompatibility

• Description of how pollen and stigma can recognize genetic similarity, blocking fertilization.

Self-Compatibility

• Plants capable of self-pollination leading to offspring from one plant without cross-fertilization.

Parthenocarpy

• Development of fruit without fertilization (e.g., seedless fruits like bananas, cucumbers).

• Can be induced using plant growth regulators (PGRs).

Polyploids

Definition:

• Plants with more than two sets of chromosomes.

• Diploid (2n) is the basic form in higher plants formed from the fusion of gametes. Haploid (n) is restricted to gametes.

Evolutionary roles:

• Hybrids from species crossing can often be sterile due to meiotic chromosome pairing challenges.

• Polyploidy might overcome hybrid sterility by providing extra chromosome sets for pairing during meiosis.

Significance:

• Over 40% of existing flowering plants have arisen via polyploid processes.

Aneuploid Plants

• Aneuploidy involves plants with extra or missing chromosomes, leading to phenotypic variations due to genetic composition differences.

Mitosis and Meiosis

Mitosis Stages:

• Prophase, Metaphase, Anaphase, Telophase: key stages with specific events like chromosome alignment and separation.

Meiosis Overview:

• Halves the chromosome number through two divisions (Meiosis I and II).

• Differences from Mitosis:

  • In Meiosis, DNA duplication does not occur during the second division.

Mutations and Plant Genetics

• Over time, mutations occur within chromosome pairs.

• Beneficial mutations improve survival chances; deleterious mutations may have negative effects.

• Mutations can lead to dominant and recessive gene expressions.

Chromosome Counts in Plants

Examples of chromosome variation:

• Radish: 18 chromosomes

• Machaeranthera gracilis: 4 chromosomes

• Tropical adder’s tongue fern: over 1000 chromosomes

• Humans: 46 chromosomes.

Polyploid Characteristics

• Polyploid plants tend to be larger and higher-yielding compared to diploid counterparts.

• Cultivated polyploid plants include potato, cotton, peanut, wheat, oats, and sugar cane.

• Notable features: bigger flowers, ornamental qualities, and seedless fruits (e.g., bananas).

Meiosis Stages in Detail

Prophase I:

• Chromosomes coil, pair up, and swap segments; nuclear envelope disintegrates.

Metaphase I:

• Homologous chromosomes align at the cell equator; spindle fibers form.

Anaphase I:

• Chromosomes migrate to poles without chromatids separating.

Telophase I:

• Two nuclei form; cells can partially revert to interphase or proceed to Meiosis II.

Final Stages of Meiosis II

• Prophase II to Telophase II: Similar to mitosis with continued chromosome segregation and formation of new nuclear envelopes, resulting in four haploid cells.