Charles Darwin’s “Abominable Mystery”

A recent paper published in Nature Plants by the CSIR-Centre for Cellular and Molecular Biology (CCMB), Hyderabad sheds light on the molecular innovations in flowering plants that could solve Charles Darwin’s “abominable mystery” question.

The study described the role of a newfound gene called SHUKR (meaning ‘sperm’ in many Indian languages) and its effect on the plant Arabidopsis thaliana.

About SHUKR Gene

SHUKR is a gene found in the sporophyte (the dominant stage in flowering plants).
The SHUKR gene first emerged in eudicots (The group comprises 75% of all flowering plants) about 125 million years ago.
It controls the development of pollen, which contains the sperm needed for reproduction.
Earlier, scientists believed that the development of pollen (gametophyte) was independent of the sporophyte.
But this study shows that in flowering plants, the sporophyte directly influences how the pollen develops.

Key Highlights of the Study

Subject: To Solve Darwin’s Mystery in flowering plants what he termed as “abominable mystery”.
Findings:
- The study shows that the sporophyte controls gametophyte development in flowering plants and not vice versa as was previously assumed.
- Pollen Control: The SHUKR gene was seen regulating the F-box genes in pollen, affecting the development of pollen. It was found that when a functional SHUKR gene is absent, the flower fails to produce viable pollen.
  - Both the SHUKR gene and F-box genes under SHUKR’s control are evolving rapidly.
- Faster Adaptation: SHUKR and the F-box genes, evolving fast, allowed the eudicot plants to explore, adapt to, and successfully reproduce in various environmental conditions through variations in pollen.
- Environmental Resilience: The Gene adaptation led to other forms of adaptation in land plants like developing a robust root system, a vasculature allowing water and minerals to move from the roots to different cells of the plant body, and the evolution of many forms of flower-pollinator strategies.

Significance of the Study

Solving Darwin’s Mystery: Charles Darwin once described the rapid rise and spread of flowering plants about 130 million years ago as an “abominable mystery”, because evolution is usually slow and gradual.
The research can help identify new mechanisms to ensure plants can survive increasingly harsh environments.
Study Plant Science: SHUKR opens a new path to plant fitness as there is ongoing research exploring genes responsible for plants’ physical sturdiness, immunity, and/or tolerance to salinity and drought.
Plant Engineering: It may be possible to naturally improve environmental resilience in plants using preconditioned pollen as the sporophyte of an eudicot may create pollen fit for those conditions by modulating the protein composition in the pollen when exposed to a specific environmental condition.

Life-Cycles of Land Plants

Phases: A plant’s life-cycle has two distinct phases which dictate their anatomies and functions.
- Gametophyte (gamete-making plant): Gametophyte cells contain one set of genes and make either sperm or egg. The gametophytes differentiate into male and female gametophytes.
  - The male gametophytes develop as sperm-containing pollen that delivers sperm to the egg cells in female gametophytes through the wind, insects or other animals that come in contact with the flowers.
  - Seed Development: The union of sperm cells and eggs gives rise to seeds, which germinate to make new sporophytes of flowering plants.
- Sporophyte (spore-making plant): The fusion of a sperm and an egg gives rise to a sporophyte. Each sporophyte contains two sets of genes, one from each contributing gamete.
  - When it matures, the sporophyte cells divide to make new cells called spores. The spores have novel combinations of a single set of genes.

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