Thursday, October 09, 2008

Parenthood: Males need not apply.

We've all heard stories of the "Virgin Mary" but did you know that in the realm of science it is actually possible for some species to become pregnant without having sex? Of course you have!

It's called 'asexual reproduction' and many simple species reproduce this way — bacteria, fungi and many plants. We just don't tend to see many female animals reproduce asexually, without the assistance of a male's DNA.

The Journal of Fish Biology reports that scientists have proof, through DNA testing, that a shark has managed to conceive without the help of a male - or even another female for that matter. It's not even the first time asexual reproduction has been documented in sharks either. The first known case occurred with a pup born to a hammerhead shark at an Omaha zoo years ago. The poor little thing got munched though — by the other sharks in the tank.

This newest case comes from a female blacktip shark at the Virginia Aquarium & Marine Science Center. Her tank never contained a male shark, yet she was carrying an almost full-term fetus when she died. As it so happens, pregnancy is risky for sharks just as it is risky for people. She died from late-term pregnancy complications, determined through an autopsy. The pup she was pregnant with died too. His DNA was analyzed — it contained no genetic material from a male — proof positive of parthenogenesis, asexual reproduction.

This so-called 'virgin birth' has been documented in some amphibians, reptiles, bony fish and birds!

What happens during conception via parthenogenis is that the newly forming cell acquires an extra X chromosome when the mother’s X chromosome duplicates itself during egg development. I found this graphic that sort of explains it visually, for those already familiar with normal meiosis.

The resulting offspring is always a female, unlike with asexual reproduction in birds which can result in male offspring because female birds carry an X and Y chromosome. I've always thought that was pretty cool about birds, and I suppose that means that means that female birds are the ones likely to show sex-linked traits like human males are more likely to express color blindness if the gene is inherited. But, back to the sharks — a shark mother only has X chromosomes to contribute, so 'virgin' conceptions alway result in a female.

Without the chromosomes provided by the other sex, the offspring of asexual reproduction have reduced genetic diversity — a major disadvantage. The greater genetic diversity produced by sexual reproduction (two parents) is significant. For one, it creates a healthier immune system — one that is able to recognize and kill off a wider variety of pathogens.

However, the ability to reproduce asexually can also be a significant advantage. If, for whatever reason, the sex ratio (number of males to females) becomes skewed, a females might not be able to find a mate to reproduce with. If she can reproduce without a male - the species may continue to live — but only as long as females can continue to reproduce asexually. If they can, the number of females will continue to rise, remember, because they can only produce other females. The sex ratio will then become even more skewed, making it even more difficult for the remaining females to reproduce sexually. Eventually, the population will plummet. This is likely to be the case when pathogens and congenital malformations are present.

The scientists, during interviews about their research which was picked up by the AP (how I found out about it) cautioned that these rare asexual births should not be viewed as a possible solution to declining global shark populations. And, now you know why =D

1 comment:

BackwoodSophisticate said...

Oh wow! How cool about the sharks.