Symbiosis Lecture Notes

Definition and Concept

• Symbiosis: Derived from Ancient Greek, meaning "living together". It refers to a close and long-term biological interaction between two organisms of different species, known as symbionts.
• Can be mutualistic, commensalistic, or parasitic.
• Defined by Heinrich Anton de Bary in 1879 as "the living together of unlike organisms".

Types of Symbiotic Relationships

1. Mutualism: Both organisms benefit. E.g., Clownfish and sea anemone.
2. Commensalism: One organism benefits, the other is unaffected.
3. Parasitism: One organism benefits at the expense of the other.
4. Cleaning Symbiosis: One organism cleans parasites off another, often seen in marine environments.

Obligate vs Facultative

• Obligate Symbiosis: Necessary for survival. E.g., Lichens consist of fungi that cannot live alone.
• Facultative Symbiosis: Optional relationship, not necessary for survival.

Ectosymbiosis vs Endosymbiosis

• Ectosymbiosis: Symbiont lives on the host's surface. E.g., Barnacles on whales.
• Endosymbiosis: Symbiont lives within the host's tissues. E.g., Nitrogen-fixing bacteria in plant root nodules.

Evolutionary Importance

• Symbiosis is a key factor in evolution, promoting co-evolution and the development of complex traits.
• Mutualisms have evolved from parasitism or commensalism under certain conditions.

Examples of Symbiotic Relationships

• Acacia Ants and Acacias: Ants protect the acacia tree from herbivores and competitors; the tree provides shelter and food.
• Nitrogen Fixation in Legumes: Rhizobia fix nitrogen in the roots of legumes, essential for plant growth.
• Pollination: Mutualism between flowering plants and pollinators such as insects, bats, and birds.

Symbiogenesis

• Hypothesis that eukaryotic cells evolved from symbiotic relationships between different organisms.
• Mitochondria and chloroplasts in eukaryotic cells are thought to have originated from such relationships.

Notable Figures

• Lynn Margulis: Advocated for symbiosis as a major evolutionary force.

Other Concepts

• Amensalism: Asymmetric interaction where one species is harmed.
• Mimicry: A form of symbiosis where species evolve to resemble others for survival advantages.
• Hologenome Theory: Considers the host and its associated symbionts as a single evolutionary unit.

These notes provide an overview of the major concepts of symbiosis, its types, significance in evolution, and specific examples of symbiotic relationships across different species.