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Parthenogenesis – Virgin Births in Nature

Happy (belated) Christmas!

How do you really reproduce without sexual reproduction? Asexual reproduction, of course.  Simple, really… but not for the females of some species.

There are loads of links in this post, so click on them to learn more.

Parthenogenesis

Parthenogenesis

Some plants, insects, shark and lizard species are known to reproduce by parthenogenesis – embryo development is carried out without fertilisation by a male -so called  ‘virgin creations.’

Parthenogenesis can take a range of pathways :

  • Chromosomes in the egg can self-replicate, making up the diploid number and the embryo develops from there.

Other methods include suppression of male genotypes (technically still sexual reproduction?), or eggs cells dividing by meiosis.

The resulting offspring are going to be all the same gender. In some species, the XY system determines gender and parthenogenesis produces all females. In other, the ZW system dictates that they will all be male.

Parthenogenesis is a reproductive strategy that sacrifices the genetic variation (a driving force of evolution) of sexual reproduction for the simple ability to reproduce. Small invertebrates, such as aphids, can use it to produce large numbers of females very quickly.

Komodo Dragons

Komodo Dragons

Larger organisms, such as Komodo dragons (Indonesia link!), have been known to use parthenogenesis in the absence of males, producing an all-male clutch of eggs. It is thought that this might allow them to set up new populations on isolated islands, using just a single female. Here’s a quick video of a Komodo dragon parthenogen hatching:

Some interesting Komodo readers here from Richard Dawkins and Not Exactly Rocket Science.

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Parthenogenesis in sharks

Parthenogenesis in sharks

Parthenogenesis has also been observed in captive sharks – the female had no access to males, yet gave birth to live young (though only one, where the normal litter would be larger). Genetic tests confirmed parthenogenesis, rather than the alternative hypothesis of superfecundation (storing sperm for a long period of time). Read the full paper here, and another on hammerheads here. BBC audio explanation here.

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So can it work in us?

Let’s let House MD explain:

In short, no. Not naturally.

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Generally, we use mitosis to replace and repair damaged cells and tissues and for growth and development – filling in the gaps with copied cells. Along the way, our cells differentiate to their function and we end up with a body full of specialised cells – each cell’s structure and biochemistry reflect its function.

We don’t use mitosis for reproduction, as it narrows genetic variation – one of the driving forces of evolution. Instead, when sperm and eggs are produced, meiosis is used – producing daughter cells with half a set of chromosomes. During meiosis, crossing over occurs, giving some recombinants – or ‘mixed up’ chromosomes – leading to some varation. The greatest variation comes from the process of sexual reproduction itself – the gametes – sperm and egg – meet in fertilisation, combining their chromosomes to  make a new blastocyst, which becomes an embryo, then a fetus and out pops a baby.

All the offspring of organisms that reproduce sexually carry two copies of each chromosome – one from each parent –  and each chromosome carries different alleles – ‘versions’ of each gene. This leads to a great deal of variation and this genetic diversity keeps the the population going.

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What about uses in technology?

Funny you should ask that…

Induced parthenogenesis is being pursued as a method for obtaining embryonic stem cells. Read this New Scientist article to learn more.

The disgraced Korean scientist Hwang Woo-Suk, who shot to infamy after faking stem cell results, was actually and inadvertently pivotal in the use of parthenogenesis as a method to produce human embryonic stem cell lines:

Normally these parthenogenic embryos die after a few days, yet researchers are able to harvest them for stem cells for research. Ethically, these are considered engineered eggs, rather than human embryos. How do you feel about that?

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Questions to think about:

1. How does parthenogenesis differ from binary fission in bacteria, or vegetative reproduction in some plants?

2. How do the XY and ZW gender systems work?

3. How does sexual reproduction lead to genetic variation?

4. What are the costs of parthenogenesis in terms of evolution or resistance to disease?

5. How would the genetic fingerprint of a parthenogen differ from its parent?

6. How would researchers use genetic fingerprinting to determine whether the offspring were parthenogens or were the product of sexual reproduction?

7. What are the ethical considerations of using parthenogenic human ambryonic stem cells?

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References:

Chapman et al. Parthenogenesis in a large-bodied requiem shark, the blacktip <i>Carcharhinus limbatus</i>. Journal of Fish Biology, 2008; 73 (6): 1473 DOI: 10.1111/j.1095-8649.2008.02018.x

Chapman et al. Virgin birth in a hammerhead sharkBiol Lett. 2007 August 22; 3(4): 425–427. Published online 2007 May 22. doi: 10.1098/rsbl.2007.0189.


Mice cloned from frozen bodies

So cloning is nothing new, but for the first time we see healthy lab mice cloned from the cells of dead mice – some articles are calling this a ‘resurrection’, although it is nothing of the sort.

It is, however, a big step forward for genetic reserach – lifting the possibility of cloning long-extinct species out of the realms of pure science fiction and into the ‘almost there’ zone. A team of Japanese researchers from Kobe, Japan, used a modified method to clone these mice from tissues that had been frozen for up to 16 years.

Here’s a clip from Japanese news, with a really clear section showing what looks like the insertion of the nucleus into an egg:

For a useful reader, here is the NewScientist article. Here’s another one from the Guardian.

As ever, one of the best reading sources is the Not Exactly Rocket Science blog, where Ed Yong tackles the research paper directly in his article and makes it easy to understand. Check it out here.

Learning idea: ‘dub’ the Japanese clip into your own language, using the information from the articles above.

How did they achieve it? How does it differ from normal cloning? What surprising results did they encounter?

Try to get your story to complement the images in the video clip.

I can just see Jeff Goldblum‘s gurning face as runs for his life from a cloned dodo…

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