The article on this particular case was published in BVA-International Journal of April 2009
By Dirk Van den Abeele
Ornitho-Genetics VZW
MUTAVI, Research & Advice Group
What would be the first thing that comes to mind when someone says that he has had a bird in his living room for years which has never been in contact with peers and which suddenly builds a nest, lays a few eggs and starts to brood. Nothing much you would think because we regularly see that females which are housed on their own, spontaneously start laying eggs and brooding. So far this is nothing new.
But if I were to add that one of these eggs hatched and that a young was born you might be surprised and you will probably be convinced that there must definitely be a ‘father’ involved somewhere.
Parthenogenesis in Agapornis roseicollis
On December 24th 2008 I received an email from the family van den Wijngaard from the Netherlands who were confronted by a similar phenomenon. Pivo, their tame A. roseicollis, of a year and a half old, had laid five eggs in her cage and had spontaneously started to brood. The bird had been owned by the family for a year and a half and had never been in contact with other birds. You can imagine their surprise when they saw on the 24th of December that a young had been born. This indeed goes against all logic because as far as we know it is impossible for birds to have fertilized eggs without the intervention of a male. And this is correct, in 99.999999999% of the cases a male is indeed needed. But there are a few exceptions. We know that nature is not easily controlled and that exceptions confirm the rule. This phenomenon does exist and even has a name: parthenogenesis.
Admittedly, when I received a message on my screen I first thought it was a joke because parthenogenesis is extremely rare. Yet I took the risk and called the person who sent me the message. The lady was, understandably, upset and could not fathom what had occurred. I tried to explain to her that this can indeed happen. I urged her to definitely not get rid of the chick should things go wrong. From my studies on genetics I know that a lot of the parthenogens die early on. And this turned out to be true. Unfortunately the chick died after four days. The dead chick was stored in the freezer at my request and I searched for possibilities to have it examined further. Unfortunately no researchers were available during the Christmas – New Year period and it was not until January 2009 when I was able to reach professor Tim Birkhead of the University of Sheffield.
Luckily he was prepared to investigate this case. The first examinations confirmed that this chick was definitely a case of parthenogenesis.
Parthenogenesis
I can imagine that most of us shrug our shoulders when we hear the term parthenogenesis and might think: is this a joke? And honestly, up until about ten years ago I was totally unfamiliar with the term parthenogenesis, until a teacher explained this subject during a course on genetics.
Parthenogenesis stands for ‘virgin reproduction or asexual reproduction’. With this phenomenon we see that in certain animal species females will have young without the intervention of a male. In some animal species this is even the norm. The Greek philosopher Aristotle (384 – 322 BC) already described a number of insects and fish which reproduced without copulation. This phenomenon has been discovered in more animal species over time. There are registered cases of parthenogenesis in snakes, lizards, beetles and various insects. In other species we have a mixture of parthenogenesis and normal reproduction but considering the complexity of this matter we will not delve deeper.
For a long time parthenogenesis has been described as ucina sine concubitu. Translated freely this means pregnant without copulation. Richard Owen was the first to define the term parthenogenesis as ‘reproduction without the immediate influence of a male’. Later on the definition of parthenogenesis has been revised a number of times, among other by Beatty and currently the definition is: ‘development of an embryo from a female gamete without a genetic contribution of a male gamete, which can grow into an adult individual’.
In birds parthenogenesis is less known, but this does not mean it has not been registered yet. The first description of this phenomenon dates back to 1872. At that time parthenogenesis was scientifically described for the first time in chickens [Gallus domesticus]. Later on it was also discovered in pigeons [Columba livia], turkeys [Meleagris gallopavo] and recently in zebra finches [Taeniopygia gutta].
I must immediately add that in the majority of these cases, the results were abortive. This means that the young died in the egg shortly after the first cell divisions. For instance in the zebra finch only the first cell divisions in the egg have been seen, but in chickens they have been able to hatch chicks. In tests during the 1960s – 1970s no less than 8532 – unfertilized – eggs were artificially brooded, out of these seven (parthenogenetic) chicks were born. Three of them died during the first days, four of them reached adulthood and one of them even turned out to be fertile.
The four chicks which reached adulthood, were all males. Three of them (one of them turned out to be fertile) were normal diploid and had two normal sex chromosomes – ZZ. The fourth chick was intersexual and had a W and two Z chromosomes (ZZW). Individuals with this genotype are usually sterile. This phenomenon also exists in mammals and therefore also in humans. There are men who are genotypically XXY, this is called the Klinefelter syndrome.
In turkeys similar results have been seen and parthenogens – the name for individuals developed from an unfertilized gamete – were born which later turned out to be fertile. The number of cases of parthenogenesis is higher than in chickens. It is estimated that in about 17% of the unfertilized turkey eggs, if they are brooded, there is a certain form of parthenogenesis. This can range from the first cell divisions to fully developed chicks.
The examination of 67 parthenogenetic turkey embryos proves that all parthenogenetic chicks were a mixture of haploid, diploid and aneuploidy cells. Simply put: a normal genome – cell – contains a number of chromosome pairs whereby, in birds, each chromosome, except for the sex chromosome, occurs double. Haploid means that of each pair only one specimen is present, diploid means doubly present, aneuploidy: too much or not enough of 1 type of chromosome due to an error during the meiosis.
The cause of this phenomenon is not yet known. For some reason or another an unfertilized egg will start to spontaneously divide. There are various studies and theories but a clear answer cannot be provided yet. In most it can be assumed that this concerns normal haploid gametes of the female which spontaneously start to divide. In this case the parthenogenetic chick is simply diploid and in principle has all chromosomes necessary to develop normally. Yet this does not appear to be sufficient considering the small number of chicks which remain alive and the even smaller number of fertile chicks. When a parthenogenetic chick is born which is intersexual (has ZZW chromosomes) it might be assumed that, in the female, the start was a diploid gamete – ZW – and the single Z chromosome spontaneously divided. Of course these are all just assumptions and my own personal interpretations. To confirm them we need solid scientific research.
I can image that the question on everyone’s mind is whether this is also possible in mammals (and therefore in humans). And the answer is yes. This has already been seen in mice, all young were female. In mammals, the female has, contrary to birds were it is the opposite, two active sex chromosomes – XX. Also in humans it has been seen on occasion that an egg cell spontaneously started to divide in the uterus, without prior fertilization, but no further cell divisions occurred. Parthenogenesis has also been seen a few times in sharks.
The main question remains: is this simply a coincidence or is it a new, significant step in the evolution of nature or a remnant from the distant past? Fact is that asexual reproduction is beneficial to the species especially when mates are scares – slow moving species or species with a large habitat they might never reach another mate. It is also beneficial for the individual because it doesn’t have to share its genes. With reproduction any individual stand the chance to lose some of its “good” genes in its offspring. It might happen that its mate doesn’t have such good genes and the offspring might be “average”. This is not necessarily in human terms like show quality, but for survival of the species and the ability to thrive. Therefore in some cases asexual reproduction is better for the species. However if the individual has “bad” genes then it is more beneficial for the species that it does get “better” genes for its offspring to improve the gene pool.
Dirk
Literature
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- Darcey, K. M., Buss, E. G., Bloom, S. E., & Olsen, M. W. (1971). A Cytological Study of Early Cell Populations in Developing Parthenogenetic Blastodiscs of the Turkey. Genetics, 69(4), 479–489.
- Graham, C. F. (1974). The production of parthenogenetic mammalian embryos and their use in biological research. Biological Reviews, 49(3), 399-422. doi: 10.1111/j.1469-185X.1974.tb01085.x.
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- Oellacher, J. (1872). Die veränderungen des unbefruchteten Keimes des Hühneries in
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- Schut, E., Hemmings, N., & Birkhead, T. R. (2008). Parthenogenesis in a passerine bird, the Zebra Finch Taeniopygia castanotis. Ibis, 150(1), 197-199. doi: 10.1111/j.1474-919X.2007.00755.x.
