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“Killer Sperm” Prevents Mating Between Worm Species, and sheds light on evolution.

Normally different species are unsuccessful at interbreeding, and if they do, the hybrid offspring is usually sterile. The vast majority of the time, mating across species is merely unsuccessful in producing offspring. In this way, species are kept separate and the diversity of life is maintained. Recently scientist from University of Toronto and University of Maryland discovered that when female worms belonging to the Caenorhabditis genus mate outside their species, they end up with reduced lifespans and fewer offspring than usual. The findings were published in the journal PLoS Biology.

The aim of this project was to solve the genetic causes of Charles Darwin’s ‘mystery of mysteries’ how new species are formed. The research findings of this project may lead to exploring the possible reasons; or they may discover a mechanism underlying the formation of new species

To survive, females evolve to become increasingly resistant to aggressive sperm. Researchers added that this feature is beneficial to males is not optimal or even harmful to females, and vice versa.

Within species, this “sexually antagonistic co-evolution” is usually in-synchronization so the sperm cells do not end up harming their own females.

This image depicts an instance of cross-species breeding gone awry. Fluorescence microscopy reveals sperm, in red, invading a female worm's body.
This image depicts an instance of cross-species breeding gone awry. Fluorescence microscopy reveals sperm, in red, invading a female worm’s body.

When the researchers observed the sterile and dying female worms under a microscope using a fluorescent stain to visualize sperm in live worms, they discovered that the foreign sperm had broken through the sphincter of the worm’s uterus and invaded the ovaries. There, the sperm prematurely fertilized the eggs, which were then unable to develop into viable offspring. The sperm eventually destroyed the ovaries, resulting in sterility. The sperm then traveled farther throughout the worm’s body, resulting in tissue damage and death.

Research:-

Team of scientist tested their theory by mating hermaphrodite worms (self-mating) of one species to males of another species. As team expected, the hermaphrodites, are used to the gentler sperm they produce themselves. Hence after receiving other species sperm they were found to be prone to sterility and death.

The research findings predicted that fertility problems could be a by-product of sexual conflict among gametes in species with an evolutionary history of intense polygamous mating. Team concluded that females typically just select sperm from males of their own species during fertilization, an action that does not lead to long-term consequences because there is no gene flow between the species. 

The results suggest the interaction between sperm and the female reproductive tract as a novel reason for failed mating in worms. These findings could provide new mechanistic insights into how the sexual antagonism evolves.

The team now plans to study rare worm hybrids to understand how a single ancestral population happily interbreeding split into descendant populations that are averse to doing so.

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