Reproductive strategies: successful without sex

For reproduction to work, two sexes usually have to be involved.

But why actually?

After all, the search for suitable partners is often quite tedious, and the production of offspring can also work without sex.

For example, in parthenogenesis, also known as virgin birth, the offspring develop from unfertilized egg cells.

A few reptiles use this uncomplicated method, water fleas and some insects use it more often.

With good reason, however, parthenogenesis has only rarely proven to be a recipe for success in evolution.

A study on stick insects recently confirmed that, like inbreeding, it greatly reduces genetic diversity.

Representatives of the particularly primitive genus

, which is predominantly native to California, served as research objects.

Of the twenty or so species of these rather inconspicuous insects, five reproduce by parthenogenesis.

Scientists led by Kamil Jaron and Darren Parker from the University of Lausanne examined the genome of these five stick insects and used the most closely related species that reproduces sexually for comparison.

In sexual reproduction, the offspring inherits half their genetic makeup from their mother and half from their father.

The two parents often contribute very different genetic variants.

As Jaron and colleagues report in Science Advances, these differences are within normal limits in stick insects that reproduce sexually.

In contrast, in species that practice parthenogenesis, the offspring inherits both editions of its DNA from the mother.

It was to be expected that these two versions would be quite similar.

The differences turned out to be so small that they were difficult to quantify.

Jaron and colleagues estimate that variations that are limited to individual building blocks of DNA occur at least 140 times less frequently than in relatives who are sexually active.

More drastic changes, such as duplicated or upside-down stretches, were also almost always found in both editions of DNA in parthenogenic species.

Success story in a highly flammable habitat

During cell division, from which the egg cells emerge, the two versions of the DNA can change by exchanging parts with each other.

Similar to massive inbreeding, however, the genetic diversity that can result is narrowly limited.

After all, there is never a partner in the game who could introduce new variants.

In stick insects that practice parthenogenesis, the vast majority of the sparse genetic variants appear to have arisen through mutations only after consequent abstinence from sex.

If stick insects always reproduce without sex, they not only have less potential to adapt to changing living conditions.

Harmful mutations also accumulate in their genome more rapidly than in species that reproduce sexually.

But why has parthenogenesis developed despite all the disadvantages in some stick insects of the genus

proven as a reproductive strategy?

This success story is probably related to the fire-hazardous habitat in which the wingless insects cavort: Since Californian scrubland is often ravaged by fire, pioneers are often required to quickly repopulate burned-down areas.

In this situation, the genetic disadvantages that parthenogenesis entails are likely to be outweighed by the advantage of self-sufficiency in terms of reproduction technology: even those who crawl around alone in the open air can produce offspring.

Unlike stick insects, aphids can take advantage of parthenogenesis and sex alike.

These delicate little insects are notorious among gardeners and farmers for their ability to build up large populations in a short space of time: By means of virgin production, aphids produce offspring non-stop, which in turn reproduce at lightning speed.

In between, however, most species occasionally produce a generation that is not only on the move with females, but also has males to offer.

This opportunity for sexual reproduction allows aphids to maintain their genetic diversity—and with it, their adaptability.