Unisexual Ambystoma

Unisexual Ambystoma

Sunday, February 3, 2013

Hardy Kern: Parthenogenesis 101

An important component of our lab here at Ohio State is our group of undergraduate students/volunteers/researchers. Even though these students have schedules full of challenging classes, work, and other responsibilities, they still find time to help us take care of captive animals, meet once a week to talk about science, and even conduct their own research. For this blog post, one of our students, Hardy Kern, submitted an article that he wrote up about a topic that has recently captured his imagination. Here's Hardy:

Putting it plainly, I’m an animal nerd. For as long as I can remember I have been enthralled and captivated by all aspects of Kingdom Animalia and its inhabitants. Every single animal, whether cute and furry or creepy crawly, has something amazing about it; an adaptation, behavior, or physical trait which distinguish it from the rest and stop myself and fellow animal nerds in our tracks. What started as a starry eyed fascination with the natural world has become a drive to study it, taking every special adaptation or weird inconsistency into effect. Recently I have been focused on (or, truthfully, obsessed with) an amazing characteristic of reptiles which has received a lot of attention as of late: parthenogenesis

Parthenogenesis is a Greek word meaning “virgin origination.”  Basically, reproduction can happen by a female without a male intervening at all. Asexual reproduction is nothing new to most people; we know that bacteria can do it, we know it’s how our cells divide, and the more botanically inclined of us know plants can employ it as well. However, asexual reproduction in a higher level organism, like a reptile, is a big deal. It’s easy for us to imagine a minuscule bacteria or sedentary plant fertilizing itself, but for larger and more complex organisms, parthenogenesis is a phenomenon… or is it?

Partheno-whatasis? How It Works
Since that awkward birds-and-bees talk we had in middle school with our parents, we know the basics of reproduction:  

Male + Female = Offspring 

But parthenogens throw a wrench into this unflappable equation:

Female + Herself = Offspring

More or less, she clones herself. When an organism is preparing to breed, it first needs to make gametes. The female duplicates her genetic material during mitosis, doubling the original amount, resulting in a diploid cell which separates into two cells: the original and its copy. Meiosis then kicks in to actually make the eggs she’ll use; each of the two cells divide, creating four haploid cells, where each of the four cells contains all of her genetic information, but only half as much needed to manifest a new organism. 

These four cells are called polar bodies (above, orange ovals). In regular reproduction, one of them, the oocyte(1), will go on to become the egg and await fertilization from the male’s sperm cell (black oval). Sperm + Egg = Enough genetic material to constitute an offspring. Take the male out of the picture and you have a lonely, childless female… unless she’s parthenogenetic. For parthenogens, there’s a shortcut which removes the male entirely.  One of the female’s other polar bodies (2,3,4) will merge with the oocyte (called automixis), giving it enough genetic material to make a new organism.  Thus is born our parthenogenetic offspring.

Why is this the only result when you Google image search cloning?
Another way reptiles can reproduce parthenogenetically is through cloning, a process whereby an egg inside a female simply makes a copy of its own genetic information, allowing for enough genetic information to constitute an offspring. This cloning is not the same as artificial cloning, however, for the offspring are not always an exact copy of their parent. It is speculated that some parthenogenetic reptiles come from hybridization between two species which share adjacent habitats. Through a recognized but poorly understood process, a male and female lizard of two similar but not identical species will breed and produce an offspring with genetic information from both species. These individuals will have two complete sets of chromosomes, and are thus known as polyploid individuals. This offspring harbors the ability to reproduce asexually, but still incorporate some variation into its own progeny.   

Cells, Chromosomes, DNA: how they fit together.
Another factor which helps reptiles reproduce asexually comes from their sex chromosome assignments. In humans, and all other mammals, males have the XY chromosome combination (heterozygous), and females the XX (homozygous). In reptiles, this is switched.  Females are the heterozygous sex, WZ, and males are the homozygous sex, ZZ. Why is this important? For an organism to be parthenogenetic, its offspring must be able to reproduce without the intervening of a male. Seeing as only females are able to give birth, they are the more valuable gender to “make” in a parthenogenetic species. When the chromosomes separate during meiosis, there are two polar bodies with the “W” designation, and two with the “Z” designation. Speaking to probability, it is more likely that a WZ combination will be made than either a ZZ (male) or WW (nonviable) combination. As females have both the W and Z designations intrinsically, they can easily make more females.

What Good Is It?
Sexual reproduction has its obvious advantages, namely the chromosome shuffling which leads to genetic variation within individuals. When environmental conditions change, the easiest way for an organism to successfully adapt is to have mutated genes which may create an individual better suited for the new environment. Asexually reproducing organisms do not, for the most part, experience any sort of genetic recombination; what you see is what you get. An offspring will have the same genetic makeup as its parents. So, why keep it around?
Asexual whiptail lizard in Arizona. Photo by Rob Denton.
For one thing, being a non-recombinant individual can be advantageous. If an organism is well adapted for one environment, having offspring which are equally as well adapted will ensure the progeny’s survival. The babies are born into a world they are perfectly suited for, all because their mother was. Parthenogenetic individuals also experience less competition for resources. In a sexually reproducing species, it can be assumed that both males and females of that species will require the same resources to self-sustain. This can create a natural battle of the sexes, as one gender needs to feed, clean, house, and move itself just as much as the other does.  With no males in a species, parthenogens have the advantage of decreased resource competition; a whole sector of potential dinner-stealers is virtually eliminated.

The most successful cases of parthenogens are typically species which are facultative. Facultative parthenogens are species which can reproduce either sexually or asexually depending on the circumstance. This method of reproduction is ideal for a species which lives in an unstable or developing habitat, or which has much opportunity for habitat expansion. Though asexual reproduction is largely an unconscious decision by the species in question, certain environmental pressures can trigger either reproductive process.

Komodo dragons (Varanus komodoensi) provide an excellent example of a reptilian facultative parthenogen (try saying that 5 times fast). The prevalent method of reproduction is sexual; males and females are able to mate and produce offspring whose genes are a jumble of both of the parents. The habitat of Komodos is made up of a series of islands. If a female dragon successfully swims to a new, uninhabited island where there are plentiful resources, her body will know this is an ideal place to raise young.  With no males around to mate with her, her inherent asexual superpower kicks in, and she lays a clutch of eggs. Because female reptiles have both the male and female sex chromosomes, she is able to lay male eggs. Once these males grow and become sexually mature, they can mate with their mother (whose pendulum will swing back to the regular reproduction side) and found a sexual population of dragons on the new island. Virgin Komodo births have occurred in two zoos in the United Kingdom, and quite possibly several times in the United States as well.   

The obvious downside to exclusive parthenogenetic reproduction is a lack of variation. When a species becomes extremely specialized for its environment, even the smallest of changes can disrupt the entire population.  For this reason, males become extremely hot commodities in facultative parthenogenetic populations. In species whose environment is largely unstable, only females are produced.  These parthenogenetic females can create multitudes more of females with no genetic cost to the species; if 10 females with the same DNA go out into an unstable environment and 5 of them do not live to reproduce, no genetic information has been lost from the pool.  It is preserved in the other identical females. Only when an environment has stabilized will males begin to appear and stay – the genetic variation they provide is too valuable to risk otherwise.

Is Parthenogenesis Common?
So far, parthenogenesis has been confirmed in roughly 70 vertebrate species, most of them reptilian.  It has been observed in pythons, rattlesnakes, monitor lizards, rock lizards, whiptail lizards, and dozens of others. Most recently it was confirmed that a wild female copperhead, Agkistrodon contortrix, reproduced parthenogenetically – the first observation for that species. Her successful litter of wriggling young snakes posits and interesting question: just how common is parthenogenesis? It is entirely possible that it has been an extremely natural and normal process for millions of years, but is just now getting our attention. With the wide range of reptiles the process has been seen in, it is understandable to think that it is common in many more species than just those which have been extensively studied. There are even some non-reptilian parthenogens out there; chickens and turkeys in large scale poultry farms have been known to occasionally produce viable offspring without ever being in contact with a male.
Copperhead from Daniel Boone National Forest, Kentucky. Photo by Rob Denton
While we may still be getting our feet wet in the genetic pool comprised of parthenogens, one thing is entirely certain: there are always new and amazing things to be found in nature. As an avid animal fanatic, I for one can’t wait to see what other secrets reptiles, and all other creatures, have in store for us.

Until next time,

Hardy Kern

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