December’s bank holidays and Christmas’s holidays have in common in that the Immaculate Conception is celebrated in both. The biological phenomenon in which a female animal reproduces without mating with a male is called parthenogenesis and, even if there isn’t any proof that this could happen to human beings, virginal birth is a widely distributed thing throughout the animal kingdom. In this entry we’ll see how this incredible phenomenon happens and some species in which it appears.
WHAT IS PARTHENOGENESIS?
Parthenogenesis is a type of asexual reproduction in which the offspring comes from a non-fertilized ovum. Without fertilization (union of the oocyte’s and the sperm’s genetic material) the offspring won’t have any part of the father’s DNA (if there is a father). The resulting babies will be genetic copies (clones) of their mother.
During fertilization, when the ovum and the sperm fuse together (both haploid cells, with just one copy of chromosomes, n chromosomes) a new individual is formed with a unique genetic combination, with DNA from its father and its mother (diploid, with two copies of each chromosome, 2n chromosomes in each cell). Triploid (3n) or tetraploid (4n) individuals only appear in asexual hybrid species, and most cases are non-viable. Images by Ehamberg.
In parthenogenetic animals, the lack of paternal genetic material must be compensated because in many species haploid foetuses are non-viable. In these species diploidy (2n chromosomes) is usually re-established through a process called automixis. Yet in some species, haploid individuals with parthenogenetic origins are viable and have no problems in surviving.
It is impossible to pose a general example for asexual reproduction, as it is widely distributed through very different animal groups and there are many cases with many differences among them. Bellow, we’ll present you some examples of different strategies used by animals to reproduce asexually.
HAPLODIPLOIDY IN BEES AND WASPS
Haplodiploidy is a phenomenon that appears in two insect orders, hymenopterans (bees, ants and wasps) and thysanopterans (thrips or stormbugs). In this sexual determination system, if the ovum is fertilized it will develop into a female while, if it isn’t fertilized a haploid male will be born.
Colony of Carniolan honey bees (Apis mellifera carnica), a subspecies of hony bee from Eastern Europe. Photo by Levi Asay.
In the honey bee, when the queen bee mates with a drone (male bee), all the diploid individuals (2n) will became females, with DNA combined from the queen and the drone. By contrast, drones are born by parthenogenesis, in which an egg from the queen will develop into a haploid drone (n). This means that the individuals in a bee colony, descendants from the same queen, are much more closely related to each other than regular siblings (drones have 100% of their mother’s DNA). It is believed that this helped to the development of eusocial behaviours in different hymenopteran groups.
This kind of parthenogenesis is found in different invertebrate groups that can alternate between asexual and sexual reproduction during its life cycle depending on the environmental conditions.
Diagram about the life cycle of a rotifer, in which parthenogenetic asexual reproduction during good environmental conditions is alternated with sexual reproductions with a haploid male during adverse conditions. Image extracted from Hanson et al. 2013.
Some invertebrate groups like aphids, present asexual parthenogenic reproduction from spring until early autumn, when conditions are favourable. During this stage in many populations we find only females that give birth to more females.
Fast motion video in which we can see how the aphids take advantage during good weather conditions to increase fast and efficiently the number of individuals asexually. Video by Neil Bromhall.
When autumn approaches, parthenogenetic females start giving birth to sexual males and females. Both sexes are born by parthenogenesis and have 100% of their mother’s DNA. Sexual winged individuals then disperse to avoid mating with their own siblings. These will mate and females will lay resistant eggs that will survive winter. In spring these eggs will hatch and give rise to a new generation of parthenogenetic females that will start the cycle again.
TRUE PARTHENOGENESIS IN SQUAMATES
The only vertebrates that show true parthenogenesis are the squamates, with about 50 lizard species and one snake being obligate parthenotes. These are unisexual species, all individuals being females that reproduce asexually without the intervention of any male. Also, there are many other species that, even if they usually reproduce sexually, are also able to reproduce asexually when there are no males available (facultative parthenogenesis).
Desert grassland whiptail lizard (Cnemidophorus uniparens) which, as its scientific name implies, is a parthenogenic species in which all specimens are female. Photo by Ltshears.
There are isolated cases of captive female sharks, snakes and Komodo dragons that have reproduced without fertilization or mating with a male. Yet, this is known as accidental parthenogenesis, because the high mortality of the offspring (surviving between 1/100.000 and 1/million) shows that it is probably due to a failure of the organism, more than an adaptive phenomenon.
Baby Komodo dragon (Varanus komodoensis) born by accidental parthenogenesis at Chester Zoo. Photo by Neil.
Females from the true parthenogenetic species produce haploid eggs (with n chromosomes) which eventually become diploid (2n chromosomes) by two consecutive division cycles during meiosis (automixis). In species with facultative parthenogenesis, diploidy is achieved by the fusion of the ovum with a haploid polar body that forms during meiosis.
Scheme of the formation of polar bodies during oogenesis, which may help parthenogenetic reptiles to regain their diploidy. Scheme by Studentreader.
True parthenogenesis is especially well-known in the Brahminy blind snake (Ramphotyphlops brahminus) and many species of lizards. In these species females generate clones of themselves. Parthenogenetic lizard species (like in amphibians) probably originated from a hybridization event between two sexual species. Many whiptail lizards (genera Cnemidophorus/Aspidoscelis) present unisexual species in which no males exist, from a hybridation process.
Brahminy blind snake (Ramphotyphlops braminus), the only known unisexual ophidian, in which all specimens found to date are females. Photo taken from Kaiser et al. 2011.
The species Cnemidophorus uniparens is a parthenogenic unisexual species, which appeared asa result of the hybridization between C. inornatus and C. burti. The resulting hybrid reproduced again with C. inornatus, forming the triploid (3n) parthenote C. uniparens. The presence of triploid, tetraploid, etc. genomes is a common phenomenon between unisexual reptiles, as its hybrid origins sometimes prevents the mixing of genomes. Also, a greater chromosomal variability compensates the lack of genetic recombination.
Despite being unisexual, sexual behaviours have been observed in this species similar to bisexual species. In C. uniparens there are documented sexual behaviours in which one female takes the role of a male and “mounts” another female contacting their cloacae. It is known that mounted females increase their egg production after this fake copula. It is believed that from one year to the other females shift their roles of mounting or being mounted, varying from year to year the number of eggs laid.
Three species of whiptail lizards. The middle one, Cnemidophorus neomexicanus is an unisexual parthenogenic species, originated from the hybridization of two bisexual species, C. inornatus (left) and C. tigris (right). Photo by Alistair J. Cullum.
Even if they are true parthenogenetic species, many of these squamates keep their ability to add new DNA to their offspring. This is due to the fact that if there’s no genetic recombination by the fusion of the ovum and the spermatozoon, there’s a high risk of accumulating genetic mutations detrimental for the species. Yet parthenogenesis allows these species to quickly colonize new habitats, because it is not necessary for two individuals to find each other to procreate, and 100% of the population is able to reproduce.
As you can see, there is a great number of animals that don’t need males nor sex to reproduce. The existence of a similar process in human beings is pretty much improbable (no to say impossible). Besides, if 2000 years ago a woman would have given birth to a baby without fertilization, probably this would have been a girl, because it wouldn’t have been able to acquire the Y chromosome from anywhere. Yet, this doesn’t mean we cannot enjoy the upcoming holidays. Merry Christmas and Happy New Year to everyone!
The following sources have been used during the elaboration of this entry:
- Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
- Masó & Pijoan (2011). Anfibios y Reptiles de la Península Ibérica, Baleares y Canarias. Ediciones Omega.
- Cover image by Pepper M, Doughty P, Fujita MK, Moritz C, Keogh JS.