A World Without Sex? by Libby Pool

Zookeepers at Louisville Zoo, USA, were astounded when they found Thelma the python had laid 61 eggs in her enclosure. But why were they so amazed? Laying eggs is quite normal for pythons. What made this event so miraculous was that no male had slithered anywhere near Thelma. How did she produce offspring without an act of copulation?

Thelma had become the first reticulated python in the world to have a ‘virgin birth,’ triggering new genetic research into this peculiar phenomenon. DNA evidence confirmed that Thelma really was the sole parent of the hatchlings. In the words of Bill McMahan, the zoo’s curator of ectotherms, “I guess sometimes truth is stranger than fiction.”

Despite this, such ‘virgin births’ where females produce offspring from an ovum without fertilisation are actually seen throughout nature. Scientifically termed ‘parthenogenesis,’ virgin births have been recorded in species from bees to Komodo dragons and new cases are being recorded all the time. But how can it be possible to fertilise an ovum without a sperm?

The nuts and bolts…

Parthenogenesis may occur in two ways, which you can read about here. In the case of Thelma, she replaced the fertilising sperm with a ‘polar body:’ a by-product of her dividing cells. All offspring are half clones of their mother.

Other examples of parthenogenesis in nature…

Two particularly interesting examples of parthenogenesis occur in species of stick insect and sawfish. Many scientists have predicted that females turn to parthenogenesis as a last resort. When population sizes are low, it becomes less likely that a female will find a mate. This is especially troublesome for shark species that lead solitary lifestyles, roaming open ocean waters and only occasionally bumping into others. One species, the smalltooth sawfish, was found to commit parthenogenesis in the wild in 2015. This discovery came completely by chance. Ecologists studying genetic variation in the population came across young, healthy sawfish created through virgin births. This was quite the revelation! The species is likely using parthenogenesis as a survival strategy against their declining populations. As ecologist, Kevin Feldheim, described, “if they can’t find a mate, it’s possible this mechanism kicks in as a last-ditch effort for these females to pass on their genes.”

Despite this, female stick insects actually choose to go solo in reproduction even when presented with ample mating opportunities. Reproduction can be costly for females, so they’d rather go at it alone. In studies, female giant prickly stick insects have fought off advancing males by kicking their hind legs. They also produced anti-aphrodisiac chemicals to put the boys off.

Do we really need males?

These virgin births beg the question: what does the future hold for males? Male giant prickly stick insects could be at risk of dying out, as females will quite happily reproduce via parthenogenesis, only produce female offspring when they do so. However, in many species including stick insects, evidence suggests males will force females to mate with them. Typically, males win these sexual conflicts more often than females do, giving them a fighting chance at survival. This is thought to be one of the reasons why parthenogenesis remains rare. Another reason is that it typically requires optimal conditions to occur. Thelma, for instance, was kept at perfect temperature with an unlimited food source. She had “the optimal conditions to make the biological leap into solo parenthood” claimed McMahan. Obviously, such perfect conditions rarely occur naturally.

Parthenogenesis does have its advantages. For example, if a species became isolated on an island, it would be able to increase its numbers without the need for sexual reproduction. This has been seen in species of whiptail lizard. However, virgin births can carry huge costs for the offspring they produce. Parthenogenesis is essentially inbreeding, leaving individuals vulnerable to diseases and malformations associated with decreased genetic diversity. For this reason, scientists recommended that Komodo dragons were not kept in captive isolation for fear they may begin cloning themselves. This would reduce genetic diversity in the already vulnerable population. Similarly, a captive pit viper gave birth to one stillborn snake and four undeveloped ova through parthenogenesis. This indicates that ‘virgin births’ are far from ideal and often end in early mortality or maldevelopment.

For now, it seems the boys are safe. Species do require males to maintain viable populations as the health costs of decreased genetic diversity are just too great. Evolution is a clever phenomenon. It has given species a way to reproduce asexually when the chances of mating sexually are dire. This helps populations persist through times of stress. However, it has created mechanisms to control virgin births, preventing them from becoming so common that species suffer from inbreeding depression. Thanks to evolution, we won’t see a world without sex anytime soon.

References:

http://www.bbc.co.uk/earth/story/20141219-spectacular-real-virgin-births

http://www.bbc.co.uk/earth/story/20151216-virgin-births-are-happening-everywhere

https://news.nationalgeographic.com/news/2014/10/141023-virgin-birth-pythons-snakes-animals-science/

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