Discovering Biology in a Digital World

Her name is Flora and she is a single parent. Born in Miami, Flora moved to Chester, UK, as a toddler. Now, she's almost 8 years old and starting a family, all on her own.

Literally.

Four eggs have hatched and another eight are ready to go. Yet, Flora has never gotten cozy with another male dragon.

How did Flora accomplish this feat and how do we know that she's not just good at keeping secrets?

Flora's not the only one
Parthenogenesis -- reproduction without the need for fertilization by a male -- has been observed before in about 70 vertebrate species, including snakes and monitor lizards (2). But this phenomenon hasn't been observed in Komodo dragons until now. Even stranger, Flora has company. Sungai, another female Komodo, at London Zoo, UK, was also thought to have produced offspring, on her own.

How we do we know that there wasn't a male dragon involved somewhere?
Of course, there's the obvious problem of hiding a male Komodo dragon somewhere in the UK and helping him sneak into and out of the zoo. That isn't enough to convince biologists, though. The females of some species are able to store sperm for many months and use it when they're good and ready. This possibility didn't seem likely with Flora, but Sungai had lived with, and engaged in courtship behavior with her brother Kimaan, so she might have stored some sperm for future use.

What did the researchers do to show that the baby lizards didn't have a daddy?

They genotyped the babies and did some fun tricks with math.

What is genotyping?
Genotyping is the process of finding and using genetic information to distinguish between individual chromosomes. A genotype can consist of a DNA sequence, it can be the number of repetitive sequences at certain DNA position, or it can be the size of a DNA fragment. It can be any bit of genetic information that differs between individuals.

How was genotyping used in this study?

The researchers isolated DNA from dragon eggs and from Flora, and they looked at genotypes at different chromosomal positions. (We call the positions "loci" which I always considered to be synonymous with "location." A locus is kind of like a chromosomal address, complete with city (chromosome number), street (chromome arm and distance), and zip code (genus and species). The kinds of sequences that were examined are called "microsatellites." The name of the sequences isn't important, but it is important toknow that these sequences are quite variable, so that if you find one microsatellite sequence at a certain position, chances are, the microsatellite sequence at the same position on a different chromosome, is different.

How would this work?
This image below (modified from the supplemental data from ref.2) shows the loci that were examined in Flora's DNA. The different colors are used to identify loci that differ between her maternal and paternal chromosomes. If the two loci are different, we say that she's heterozygous for that locus.

As you can see from the image, five of the loci that were examined in Flora were heterozygous. Only two were homozygous.

Flora's children could inherit either locus from the pairs of loci that you see represented in this picture. At K03, they could have gotten either the blue or the purple version. If her children had a father, it's likely they would be heterozygous, at some of the loci, just like Flora.

Time for a math moment
We can even follow the same logic used by Watts, et. al. and estimate the probability that Flora's offspring would be heterozygous at one or more loci - just like their mom.

We can draw a square like this to show how the inheritance works at any one of the sites. Dragons are like humans apparently, with two copies of each chromosome. We're only considering the five heterozygous sites in this analysis.

The variability at each of the five loci is high enough, that the likelihood of having a homozygous locus is one of out four or 0.25 (1 divided by 4).

We can caluculate the probability of having homozygotes (two identical loci) at all five positions using the product rule. This probability of having all five = (prob. of locus 1) x (prob. of locus 2) x (prob. of locus 3)x (prob. of locus 4) x (prob. of locus 5). Since the probability is 0.25 for having homozygotes at any of the loci, we end up with probability of having homozygotes at all five sites = 0.25 x 0.25 x 0.25 x 0.25 x 0.25 = 0.00098, a chance of 1 out of 10,000, or a 0.1% chance.

Pretty low.

The chance of having heterozygous offspring, however, from a male dragon mating with a female dragon, is pretty high. This equals 100% - 0.1% = 99.9 %.

So, we have a chance of 99.9% of having heterozygotes.

Enough with the math! What were the results?

The genotyping experiments showed that all of Flora's babies were homozygous at every site.

The best explanation, is the one the zookeepers must have thought was the most obvious. The four dragon babies are indeed the result of a virgin birth.

Dragons are indeed miraculous!

References:

1. Kerri Smith, "Dragon births startle zoo keepers." Nature News Published online: 20 December 2006; | doi:10.1038/news061218-7.

2. Watts, Phillip., et al. "Parthenogenesis in Komodo dragons." Nature, 444 . 1021 - 1022 (2006).

technorati tags: Komodo dragons, virgin birth, genotyping, genetics