Origin of Photosynthesis, Evolution and Life Cycle

Origins of Photosynthesis, Evolution, and Diversity of Life Cycles

1. Introduction & Example Case: Triantha occidentalis (Western False Asphodel)

• Triantha occidentalis is a plant known since 1879, but its carnivorous nature was only discovered in 2021 by researchers from the University of British Columbia and UW–Madison.

• Carnivorous Mechanism:

  • The plant has small glandular hairs (trichomes) on its flower stalks that secrete a sticky substance.

  • This substance traps small insects, which are later digested using phosphatase enzymes.

  • Unlike typical carnivorous plants (e.g., Venus flytrap, pitcher plants), this plant does not trap pollinators (e.g., bees and butterflies), allowing for pollination while still feeding on insects.

• Evolutionary and Ecological Role:

  • Found in nutrient-poor, bog environments.

  • Researchers conducted a nitrogen tracer experiment, where fruit flies labeled with nitrogen-15 were fed to the plant.

  • Results showed that over 50% of the plant’s nitrogen came from digested insects, confirming its carnivory.

  • The study highlights the evolutionary advantage of carnivory in plants living in nutrient-poor conditions.

• Cultural Aspects:

  • In Malaysia, a similar plant, the pitcher plant, is used for food (rice cooked in pitcher plants with coconut milk).

2. Eukaryotic Diversity & Endosymbiosis

• The Endosymbiotic Theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Two Main Hypotheses for Eukaryotic Cell Evolution:

  • Hypothesis 1: An ancestral archaeon later incorporated a bacterium, which became the mitochondrion.

  • Hypothesis 2: An archaeon and a bacterium formed a symbiotic relationship, evolving together into a eukaryotic cell, with the bacterium becoming the mitochondrion.

• Why Is This Important?

  • Explains the evolution of complex cells.

  • Shows how energy-producing organelles (mitochondria, chloroplasts) were once free-living bacteria.

  • Helps understand the diversity and relationships among eukaryotic organisms.

3. Archaea & Their Role in Evolution

• Characteristics of Archaea:

  • Unicellular prokaryotes (no nucleus or membrane-bound organelles).

  • Reproduce asexually, similar to bacteria.

  • Unlike bacteria, archaea have sugar-rich cell walls instead of protein/sugar-based walls.

  • This unique cell wall composition allows archaea to thrive in extreme environments.

• Ecological Importance:

  • Archaea are not limited to extreme environments but are abundant in marine plankton.

  • Many archaea live in symbiosis with other organisms.

  • They represent a third domain of life, separate from Bacteria and Eukarya.

4. Spread of Photosynthesis in Eukaryotes

• Photosynthesis in eukaryotes arose multiple times through endosymbiotic events.

• Primary Endosymbiosis:

  • A cyanobacterium was engulfed by a eukaryotic cell, becoming a chloroplast.

  • This led to the rise of Archaeplastida (the group containing red algae, green algae, and land plants).

• Secondary Endosymbiosis:

  • Some eukaryotic cells engulfed photosynthetic eukaryotes, gaining additional chloroplast membranes.

  • This explains why some algae have four membranes around their chloroplasts.

• Why Does This Matter?

  • Helps explain the evolutionary relationships between different photosynthetic organisms.

  • Shows how photosynthesis spread across different superkingdoms.

5. Life Cycles & Reproduction in Eukaryotes

• Haploid (1n) vs. Diploid (2n) Cells:

  • Haploid (1n): One set of chromosomes (e.g., gametes like sperm or egg).

  • Diploid (2n): Two sets of chromosomes (e.g., fertilized zygote).

  • Sexual reproduction involves haploid gametes fusing to create a diploid zygote.

• Types of Cell Division:

  • Mitosis: Asexual reproduction, results in identical daughter cells.

  • Meiosis: Sexual reproduction, reduces chromosome number by half to form gametes.

• Genetic Diversity & Life Cycles:

  • Some single-celled eukaryotes exist primarily as haploid cells and reproduce by mitotic cell division.

  • Sexual reproduction occurs periodically, introducing genetic variation.

  • Example: Diatoms:

    • Normally diploid (2n) but reproduce asexually.

    • Their silica cell wall limits growth, so when they get too small, they switch to sexual reproduction.

    • Meiotic division produces haploid gametes, which fuse to form a new diploid cell.

6. Summary & Key Takeaways

• Triantha occidentalis is a newly identified carnivorous plant that evolved to trap insects while allowing pollination.

• Endosymbiosis played a crucial role in the origin of mitochondria, chloroplasts, and eukaryotic diversity.

• Archaea, once thought to live only in extreme environments, are widespread and form symbiotic relationships.

• Photosynthesis spread through primary and secondary endosymbiosis, leading to multiple photosynthetic eukaryotic lineages.

• Eukaryotic life cycles involve haploid and diploid stages, with reproduction varying between mitosis and meiosis.

• Diatoms provide an example of how life cycles shift between asexual and sexual reproduction in response to environmental constraints.

This lecture highlights the interconnections between evolution, symbiosis, photosynthesis, and life cycle diversity, shaping how life has adapted and diversified over time.