Why Do Male Organisms Exist? | TS Digest


In recent years, more and more instances of parthenogenesis have emerged as “virgin births” in crocodiles, condors, and king cobras. While these animals can create babies without males in a pinch, a few other species have fully committed to parthenogenesis, going male-free for more than a million years.1

“There are a lot of advantages to getting rid of males and just reproducing by parthenogenesis,” said Michael Kearney, a physiological ecologist at the University of Melbourne. “The most powerful one is that everybody in the population is producing offspring, so the population growth rate doubles.” Parthenogenesis also protects animals from the dangers of sex, including exposure to predators and sexually transmitted infections.

At least in the short-term, this can be a highly successful strategy: Warramaba virgo, an all-female grasshopper species, hasn’t had sex in a quarter million years and appears to be thriving.2 Although single nucleotide polymorphism analysis revealed much less genetic variation in W. virgo compared to closely related sexual species, when Kearney and his colleagues investigated 14 fitness-related traits, including heat and cold tolerance, reproductive output, and longevity, they found that the parthenogens were on par with their sexually-reproducing relatives.

And yet, said, Kearney, “We don’t see really old parthenogens, so something gets them in the end.” 

“Sex mixes things in a particular way, bringing together new combinations of genes that might be advantageous,” he said. This might help animals cope with changing environments and rapidly evolving pathogens. And indeed older parthenogenic species may feel these costs: Some parthenogens have higher parasite loads, fewer positively selected genes, and a faster accumulation of deleterious mutations than their sexually-reproducing relatives.3,4 

However, Kearney noted that the mystery isn’t completely solved. “There are still interesting questions about how the genomes of parthenogens evolve, and to what extent that can help create diversity and maybe even meaningful phenotypic diversity.”

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References

  1. Schwander T et al. Curr Biol. 2011;21(13):1129-1134.
  2. Kearney MR et al. Science. 2022;376(6597):1110-1114.
  3. Moritz C et al. Proc Royal Soc B. 1991;244(1310):145-149.
  4. Jaron KS et al. Sci Adv. 2022;8(8):eabg3842.

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