Hypotheses of Asexual Reproduction

It seems that a large portion of the animal kingdom engage in some type of sexual reproduction, and this percentage increases if you look at the more complex species other than those of the amoeba and others.  Sexual reproduction has certain advantages such as recombination of genes, increasing genetic diversity, and allowing natural selection to act on individual genetic traits instead of the entire organism.  Seems like good stuff.  Two individuals with good survival traits can pass them on to their offspring together, thereby giving that offspring both sets of traits and increasing their likelihood of survival.  That’s the most common hypothesis for sexual reproduction, at any rate, and seems to make sense.  So, why is asexual reproduction still around?

Well, one of the benefits of asexual reproduction is the speed at which that reproduction could occur.  Instead of two individuals being required to produce one offspring, each individual can produce their own.  With sexual reproduction, assuming everyone can and does reproduce, with 100 individuals you may get 50 offspring, bringing the population to 150.  If everyone reproduces again, that’s 225 individuals in three generations.  However, with asexual reproduction, again assuming everyone can and does reproduce, if you have 100 individuals you may get 100 offspring, bringing the population to 200.  If everyone reproduces again, that’s 400 individuals in three generations.   This is oversimplified, of course, but you get the idea.

Asexual reproduction is also considered to be less costly than sexual reproduction.  What does that mean?  Well, for sexual organisms they have to produce an egg or a sperm of some sort.  And then they have to seek out another individual they can mate with.  Each
individual must meet whatever criteria the other has for a suitable partner before they mate.  Organisms also compete against others for mates.  Sometimes this comes in the form of physically fighting, which often requires bigger “tools” such as horns to win these matches.  Others compete by being the best singer, or dancer, or having the brightest colors.  Each of these traits are “expensive” in terms of the energy that must be used.  Asexually reproducing animals need to expend the energy to reproduce on their own, but they do not need to use extra energy to seek and win mates.

Since many of the more complex animals that are capable of asexual reproduction are also capable of sexual production, it could be seen as a benefit to have the best of both worlds.  A hypothesis that surrounds the Komodo dragons’ asexually reproductive ability is that an adult female would be able to start a new colony in a place where she may not have any potential mates.  In the dragon’s case all of the offspring would be male, and the mother could then mate with one of the son’s in order to produce more offspring.  Not the best way to get a diverse population, but a quick way to start one.  Other dragons then may come in from the outside and add new genetic material to the already established colony.

For other species, like certain types of sharks, their ranges can be so large that it may not be very easy for them to find mates.  When they do, great, they can reproduce sexually.  However, when they aren’t able to do so, instead of just waiting to reproduce, some species may be able to reproduce on their own.  Many sharks are slow growers.  It takes them several years to reach their reproductive potential and when they do reproduce, their offspring are few.  Being capable of asexual reproduction may allow certain shark species the ability to keep their numbers up in hard times.

Species that are capable of both types of reproduction do seem to have a bit of an advantage, or at the very least we may be able to see why they are capable of both.  But
what about those rare complex species like the New Mexico Whiptails (Cnemidophorus neomexicanus) that only reproduce asexually?  That’s trickier.

It would be my personal guess that their environment has been fairly stable, first and foremost.  It is believed by most scientists that sexual reproduction has a distinct advantage over asexual reproduction in terms of a population’s ability to evolve and adapt more quickly to change.  So, in situations like that of the little Whiptails, it is believed that their isolated communities didn’t give them much choice in terms of exchanging genes.
Perhaps, over time, their males became less suitable, less able to survive, due to this genetic bottlenecking.  Perhaps, over time, males just became fewer and fewer.  The end result would have been to go extinct, at least locally, or find an alternative.

The New Mexico Whiptails have a lot more chromosomes than their sexually reproducing neighbors do.  And their offspring are not genetic clones; the mother’s genes are recombined to produce each new Whiptail.  How they ended up with these extra chromosomes is still a guess at this point, but it’s interesting, yes?  What I also find interesting in all of this is that no matter the advantages, no mammals that we are aware of have the asexual reproduction ability, even in tough times.  Life is a constant mystery, and with science and curiosity, it’s sure fun closing doors and opening new ones.

Complex Asexuals

What do you think of when you read the title of this post?  Those of you that may have heard of the fairly recently recognized human sexual orientation called asexual (according to the Asexual Visibility and Education Network (AVEN) the most common definition of an asexual person is an individual that does not experience sexual attraction) might be thinking of that when I mention complexity and asexuals.  I’m betting many other people aren’t sure what to think.  Still others, you biologists out there, are probably thinking of things like amoeba.

Excluding the human variety, asexual animals are those that do not need a partner in order to reproduce.  Believe it or not, though, this process doesn’t necessarily mean the offspring are clones; often times genetic shuffling mixes things up enough so that the kids are a little different from mom.   There are four classifications of asexual reproduction:

Budding is just what it sounds like.  A bud forms on the side of the parent, grows, and eventually breaks off, forming a completely independent organism.  Some jellyfish may breed like this.  With corals, the buds do not break off.

Gemmules are internal buds.  They are usually more tolerant of hostile environments than the parent may be.  This is a common way for sponges to breed.

Polyembryony is when a single egg results in more than one individual.  They are then released from the body to develop more fully.  Think flatworms here.

Parthenogenesis is when a female’s eggs develop without being fertilized by a male.  This is fairly common for aphids depending on the season.

But wait a minute… why would there be different sexes if the species breeds asexually?  Ah, well now we’re getting to the interesting part.  Parthenogenesis species can sometimes breed sexually, and on occasion these species are pretty complex animals you probably hadn’t thought would breed without mating.  Here are some cool examples:

The first confirmed case of a bonnethead hammerhead shark (Sphyrna  tiburo) reproducing asexually took place in 2001 in a Nebraska zoo.  The mother had not had any contact with a male for at least three years and zoo keepers were not sure how the baby had developed until DNA tests confirmed that the little shark didn’t have a daddy.  (http://www.msnbc.msn.com/id/18809674/ns/technology_and_science-science/t/female-sharks-capable-virgin-birth/)

The monstrous Komodo dragon (Varanus komodoensis) has been confirmed to be capable of reproducing asexually after several eggs hatched at London’s Chester Zoo.  Interestingly enough, the mom, Flora, had all sons.  (http://www.livescience.com/9460-female-komodo-dragon-virgin-births.html)

Let’s not forget the New Mexico Whiptails (Cnemidophorus neomexicanus).  Not only are they capable of asexual reproduction, it’s actually their only method of reproduction.  Whiptails are all females and have been given the nickname the “lesbian lizards.”  (http://www.scientificamerican.com/article.cfm?id=asexual-lizards)

And believe it or not, turkeys have been known to reproduce asexually as well.  Some research suggests that up to 40% of eggs that are unfertilized by a male may still be capable of hatching.  And maybe that’s why most people eat chicken eggs instead…

Those species aren’t the only animals that reproduce by way of parthenogenesis.  It’s relatively common in some reptile, fish, insect, and amphibian species, but pretty rare in birds and nonexistent in mammals outside a laboratory (not withstanding any religious dogma).  Yet why do these animals reproduce this way?  If they can breed sexually, or their close relatives can, what would be the benefit?  Next week we’ll find out.