This thing called life

Sex cells: virgin birth in the hammerhead shark

No daddy? But where did it come from? Powerdinky/Flickr

I was listening to Triple J on Tuesday afternoon when comedian David O’Doherty shared what he called a show stopping fact: female hammerhead sharks can produce baby hammerhead sharks without the benefit of a male. A lively discussion ensued, at the end of which the interviewer, known mysteriously as “The Doctor”, attempted to enlighten us by saying, “It’s called parthenogenesis.”

Even the Doctor stumbled on that last word. But I am here to tell you that Parthenogenesisis a lovely word, from the Greek. Partheno means virgin and genesis is about birth, or beginnings. To remember it, think of the Parthenon being generated from the barren soil of the Acropolis in Athens.

As you think of the Parthenon, imagine that the columns are chromosomes: long and grooved with an ornate base and top. The columns hold up the whole structure of the temple. In a similar way, chromosomes (which are homes for our DNA), provide structure to this thing called life.

The truth is that parthenogenesis itself comes in several forms, all having to do with how the chromosomes arrive. Whether or not they are complete depends upon the nature of their journey from a previous cell. So I had to look into it, to learn what happened in the hammerhead shark.

I was delighted to discover that the baby hammerhead shark was born at the Henry Doorly Zoo in Omaha, Nebraska, because I grew up in Nebraska and have visited the zoo many times.

There were three female sharks on display, all of which had been collected as babies and had never met a male shark. Nevertheless, since female sharks can store sperm, the scientists had to rule out the possibility of an insemination event.

DNA tests revealed that the baby shark contained only it’s mothers DNA. Not only were there no new genes which would have pointed to a potential father, there were actually fewer genes in the baby than in the mother. More precisely, there were fewer forms of each gene.

Genes come in various versions, consisting of short stretches of DNA with slight variations that we call alleles. Think of all these little eels, twined around the columns in the Parthenon. Chromosomes are simply swarming with alleles.

There’s more than one way to knit together a species’ genes. Patti Haskins

When the sex cell of the shark went through a process called meiosis, every column swapped some eels with another column, and then distributed themselves under four new parthenons. Four cells with different sets of chromosomes, each carrying different alleles were produced.

One of those cells was the egg, and the other three were her smaller sisters, or polar bodies. Like bridesmaids at a wedding, their job is to support and care for the bride. In the hammerhead shark which had waited too long for a sperm, the egg decided to fuse with a sister polar body, and the baby embryo was formed.

Unfortunately, in this scenario, some alleles got swapped away. Unique eels were left behind in the unchosen bridesmaids. This means that the baby shark lost genetic diversity, and if this went on for generations, evolutionary potential would be lost.

The sharks know this. Even the baby who had never met a male would prefer to join her sex cells with another shark than be forced to join her own. Creating and maintaining diversity is the whole reason for sex, even though many animals and plants are able to do without it at times. Having the ability to produce a baby shark without a male may mean that an isolated group of sharks can survive long enough to find a mate.

Other sharks seem to have this ability. An Aquarium in Detroit stopped throwing away the eggs laid by their spotted bamboo sharks, which they thought were sterile, once they heard about the virgin born hammerhead. Lo and behold, the bamboo shark eggs hatched even though there were no males in the tank.

During the interview on Triple J, the Doctor and Dave were worried that men might become redundant. In the short term they were worried about themselves, about whether human females could find men unnecessary. In the longer term they may have wondered that evolution might choose a simpler paradigm, with many species deciding to produce offspring without the need for sperm. There is good news on both counts.

Parthenogenesis has never been observed in a mammal. This is not to say that it may not be discovered in some isolated mammal in the future, but the human species is not at risk of virgin births on a grand scale. And in the longer term, the need for genetic diversity is so strong that sex cells and the need to combine them from unrelated individuals will never go out of fashion.

Swapping alleles is too beneficial for any species to give it up without a very good reason.