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The Sex Paradox

• Sexual reproduction is widespread across organisms, but it comes with significant costs, leading to the question: why is it so common?

• 99% of multicellular eukaryotes reproduce sexually.

Historical Context

• Sarah “Sally” Otto's Perspective (1988):

  • Observed a discussion about sex as "a big puzzle" during a lab meeting.

  • Argued that sexual reproduction promotes variation that aids adaptation to changing environments.

• August Weismann (1886):

  • Proposed that sexual reproduction reshuffles genes, allowing for natural selection to act on individual differences.

  • Other hypotheses include:

    • Sex purges deleterious mutations (Muller’s ratchet).

    • Introduces beneficial mutations (Fisher-Muller hypothesis).

    • Helps organisms evade parasites (Red Queen hypothesis).

Costs of Sexual Reproduction

• Time and energy spent on finding a mate.

• Only half of an individual's genes are passed to the next generation.

• Genetic reshuffling can break apart favorable gene combinations.

Experimental Evidence for Theoretical Hypotheses

• Recent studies have provided insights:

  • Freshwater rotifers and engineered yeast studies:

    • Indicated that sexual reproduction is advantageous under environmental stress, supporting Weismann’s hypothesis.

    • Parasitic snails suggest that sex evolved as a defense against infections.

Model Organisms and Research Challenges

• Traditional model organisms (like mice, flies) are not useful for comparing sexual and asexual reproduction, as they typically reproduce only sexually.

• Yeast Studies:

  • Yeasts reproduce asexually in resource-rich environments, but switch to sexual reproduction during starvation.

  • Experiments revealed that sex enhances adaptation in harsh conditions, aligning with Weismann’s hypothesis.

Insights from Rotifer Research

• Brachionus calyciflorus Rotifers:

  • Asexual at low densities, shift to sexual reproduction in crowded conditions.

  • Rotifers show genetic variation due to sex but also exhibit periods where asexual offspring perform better short-term.

  • Overall, sexual populations tend to adapt better over time.

Lively's Research on Mud Snails

• Potamopyrgus antipodarum in New Zealand:

  • Studies led by Curt Lively exploring several hypotheses around the maintenance of sex.

  • Supported the Red Queen hypothesis showing that sex may protect against parasites by producing resistant offspring.

Bdelloid Rotifers: An Asexual Success

• Bdelloid rotifers have thrived asexually over millions of years without going extinct.

• Their survival strategies include:

  • Evading parasites by desiccating and blowing away in the wind.

  • Incorporation of foreign DNA (from bacteria or fungi) into their genome, allowing for genetic variability.

Cryptococcus Fungi: Unisexual Reproduction

• Unusual reproduction in Cryptococcus neoformans where identical clones undergo meiosis to recombine DNA, leading to unexpected diversity benefits.

• Suggests that mechanisms of sexual reproduction might not always require two distinct mating types.

Conclusion: The Mystery of Sex

• Despite various benefits proposed for sexual reproduction, the evolutionary origins and persistent prevalence of sex remain unclear.

• Current research advances towards understanding why sex persists in nature amidst its high costs, focusing on adaptability to environmental changes and resistance to diseases.

Key Quotes

• "We still really don’t know the answer to this very basic question: why sex exists." - Mark Welch

• "At the end of the day, we’d like to know why sex is maintained in nature." - Aneil Agrawal.