By Paul Donovan
Over the past few issues, I have covered reproduction in scorpions, spiders and insects. This month I want to look at a rather unusual mode of reproduction; so-called ‘virgin births’. This is where a female can give birth to offspring without the intervention, or genetic contribution of a male. This we call asexual reproduction, or parthenogenesis.
Parthenogenesis was first discovered by the naturalist and philosopher Charles Bonnet in 1740. Bonnet was studying aphids and observed how females could give birth without being mated, giving it the name virgin birth. Further observations discovered the trait in drone bees, silkworm moths and bagworm moths. It was not until the 1840s that the phenomena was given the name ‘parthenogenesis’.
In simple terms, parthenogenesis is defined as ‘the ability of an unfertilised ovum to produce a fully functional adult’. In other words, at cellular level, the difference between parthenogenesis and sexuals, is that in the latter, meiosis (cell division) is followed by fusion of a male and female gamete. In parthenogenesis, meiosis is changed so that only one particular set of chromosomes is transferred in a non-random fashion. This can lead to one hypothesising that unisex populations are literally producing virgin clones of themselves, but this is not the case; I will come back to this in a moment. A species can be obligate parthenogenic (reproduce exclusively through asexual reproduction) or, facultative (can switch between asexual, and sexual [requires a male] reproduction).
Parthenogenesis has only ever been reported in several species of fish, amphibians, some species of reptiles, but it is widespread in the invertebrate world. Interestingly, in the order Hymenoptera (wasps, bees, sawflies and ants), a male is born from unfertilised eggs, and females from fertilised eggs, a process called arrhenotoky. As hobbyists, the one group of insects which display high levels of parthenogenesis, are the stick insects (Phasmatodea). Although phasmids are often regarded as being obligately parthenogenic, the greater majority show high levels of facultative reproduction.
How did parthenogenesis evolve?
Although little is known of the origins of parthenogenesis, the ancestors of parthenogenetic species were unquestionably sexual ones, and must have come about as a result of genetic instabilities which accompanied the interference of foreign chromosomes. According to one study “all vertebrate parthenogens appear to have arisen from interspecific hybridisation, as shown by studies of chromosomes, protein variation, and DNA sequences”.
That being said, there is a chain of thought that parthenogenesis can occur without hybridisation. One of these is induced thelytoky (unfertilised eggs develop into females). Intracellular bacteria in the genera Rickettsia, Wolbachia and Cardinium, can induce thelytoky in a number of insect species. In other words, some bacteria can influence parthenogenesis, and every year strains are being found which can cause this.
Geographical distribution
A lot of parthenogenetic species are island forms, and that a single female, or an egg from a female that finds its way onto an island, may give rise to an entirely new colony. A good example of this is a species of New Zealand stick insect which found its way on the Isles of Scilly, where they are now thriving as an all-female colony. Parthenogenesis may also have allowed some species to have expanded their range, and become dominant over other forms, simply because males are not required for reproduction.
Is it like cloning?
One could be forgiven for thinking that as a female is giving birth to young without the intervention of a male, she is effectively producing clones of herself. But that is not the case. Parthenogenesis is a true form of reproduction, giving rise to genetically variable young based on the genetic material contained within the egg. Furthermore, as the genetic coding comes from a single individual, and not the combination of two parents, the offspring show genetic stability. It is this genetic stability which has allowed species to optimise the environment in which they live, by not introducing genes which would not be optimally suited to it.
The inheritance and subsequent duplication of the genes, involves only a single sex chromosome, meaning the unfertilised egg can be either male or female depending on the chromosome arrangement of that species; XY sex-determination system, the offspring will be female, and in ZW sex-determination system, the offspring will be male. As a consequence, some parthenogenetic species may be able to regulate the ratio of males to females in the group.
The question now arises, are asexually produced offspring, weaker than sexually produced offspring due to the lack of genetic variability? Much would seem to depend on the species. If a facultative species breeds sexually in the wild, subsequent offspring, due to variable genetic input, will be strong over several generations. If that same species were to be then be restricted to asexual reproduction, theoretically, due to the lack of genetic input, this would influence the strength of the offspring over a similar period, or the eggs viability during development.
A classic example which illustrates this, is the Giant Prickly Stick Insect, or Macleay’s Spectre Extatosoma tiaratum. Females produced over several generations will result in smaller, weaker individuals. Whereas if a male mates with a female, the offspring show higher levels of strength and size.
Why in some species, and not in others?
Parthenogenesis can be inherently important in populations of animals which are isolated from one another, or where sexes are isolated. It reduces the females need to rely on a male for reproduction, and limits the amount of time she expends in energy searching for one. It also ensures in isolated species, that they will not suffer as a consequence of low male numbers, or weak males, as producing only females (except in lizards and snakes where parthenogenesis produces only males) bolsters population numbers. Each female is capable of contributing to the next generation ensuring population numbers remain stable or at higher levels than species whose reproduction produces both males and females. Such females are also better placed to recover from natural disasters more quickly. In this respect, parthenogenetic reproduction offers distinct advantages.
Disadvantages
Of course parthenogenesis is not without its faults. We can see from the limited number of species which practice it, that it is not a desirable mode of reproduction. One of the biggest disadvantages, is that it limits genetic diversity that would otherwise occur from the input of a female mating with different males. This allows a species to strengthen individual traits in the long term, which are advantageous to adapting to specific biological changes.
Are males ever needed?
As I have mentioned, some parthenogenic species have the ability to switch between asexual and sexual reproduction. Although many species of stick insects, for example, have negated the need for males entirely, (males have never been found in some species), others do produce males, albeit on a limited scale. What is interesting, is how some species can exhibit asexual or sexual reproduction depending on whether they are in the wild, or in captivity. From the few studies which have been undertaken, those species which are obligate parthenogenic reproduce sexually, producing both males and females; if kept in captivity they show high levels of facultative asexual reproduction.
It is difficult to say why this should be, but having worked in zoological collections, and now work with invertebrates and reptiles in their natural habitats, I am of the strong opinion, that many of the behaviours exhibited by captive individuals are borne from the constraints of being kept in captivity, and are not a true representation of how they function in the wild. In other words, those behaviours we see in captivity are artificial.
Parthenogenesis in reptiles
Parthenogenesis is widespread through the insect order, but reproduction solely through obligate parthenogenesis is only limited to a few species of reptiles – predominantly lizards. That being said, in the absence of males, it is uncertain how many reptilian species are capable of facultative reproduction. It may be more widespread than we are currently aware of.
Lizards
Amongst the most notable reptiles to exhibit parthenogenesis are the Caucasian rock lizards of the genus Lacerta, and Whiptail lizards in Cnemidophorus. There are between 13 and 15 species in this genus which are considered truly parthenogenetic.
Given that these lizards do not require the services of a male to produce young, they do show the need to engage in some sort of courtship to stimulate ovulation. And this is achieved in female-female stimulation. Two females will come together, where one assumes the role of the male. Although no penetration of any type will take place, the behaviour is necessary to induce ovulation.
This could be an indication that these lizards are still evolving asexuality, but have yet to lose the courtship behaviour element, which is not present in fully fledged parthenogenetic species?
Geckos
Quite a few gecko species have been reported to be parthenogenetic, including representatives of Heteronotia, Rhacodactylus, Lepidodactylus, Lepidophyma, Hemidactylus, Nactus and Hemiphyllodactylus. In a number of these species, as with Cnemidophorus, female-female courtship takes place.
Komodo Dragons
Watts, in 2006, reported a case of a Komodo dragon Varanus komodoensis showing asexual behavior. Unlike in other parthenogenetic species, where the offspring are females, this Komodos’ offspring were male. This is because Komodo dragons use the W and Z chromosomes; females have one W and one Z, males have two Zs. The egg from the female carries one chromosome, either a W or Z, and when parthenogenesis takes place, either the W or Z is duplicated. This means the eggs are WW or ZZ. WW eggs are not viable, but ZZ eggs are, meaning only males are born.
There is an intriguing hypothesis behind why only males are born as a result of asexual reproduction, and that is, it may be used as an advantage in island colonisation. A theory has been put forward, that this could enable a single female to have male offspring asexually, and then switch to sexual reproduction to maintain a higher level of genetic diversity than asexual reproduction alone could produce.
Two cases of parthenogenesis were reported in captive Komodo dragons in 2006. One was from Chester Zoo, and the other London zoo. Tests revealed their eggs had developed without being fertilised by sperm.
Snakes
Parthenogenesis has been reported in the Brahminy Blind snake Ramphotyphlops braminus from Africa, Asia and other regions. While this is the only known obligate parthenogenetic snake species, I personally believe there is a lot of scope for other burrowing species to exhibit this trait as well. When one thinks, these snakes live in a rather alien environment, where the chances of finding one another to mate with are rather hit-or-miss. Although pheromones must play a part in locating one another, there are strong reasons to assume parthenogenesis must be practiced more widely.
The only other snake, that I am aware of which has shown Parthenogenesis, is a Burmese python Python bivittatus from Artis Zoo in Amsterdam. Over a period of five consecutive years from 1997 up to 2002, this individual produced viable eggs containing embryos, despite having no interaction with a male. As with the Komodo dragon, because of the W and Z chromosome relationship, all offspring were male.
Amphibians
Parthenogenesis has been recorded in a number of amphibians, including, frogs, caecilians and salamanders. In fact, when we begin to trace back the origins of vertebrate parthenogenesis, using molecular analysis, it was first seen in salamanders dating to the Pliocene 3.9-5 million years ago, making them the oldest known parthenogenetic animals.
Evidence points towards parthenogenesis occurring in amphibians as a result of hybridisation between two closely related species. For example, Ambystoma jeffersonianum, Ambystoma tigrinum, and Ambystoma texanum are recognised as the hybridisation pool from which all unisexual salamanders within the genus originated. Although many amphibian species may reproduce parthenogenetically, in response to environmental cues they may then begin to produce both male and female offspring which reproduce sexually.
Spiders and scorpions
Parthenogenesis has been witnessed in a number of spider species, including Theotima, Steatoda, Heteroonops and Triaeris, and scorpion species. In those species where parthenogenesis is practiced, the presence of males may be non-existent, as is the case with Tityus serrulatus. However, this is not the case with all parthenogenetic species. What’s more, parthenogenesis can even vary depending on an individual species’ range. Throughout the Asian distribution range of Liocheles australasiae males are not evident, but through their Australian range, males can be encountered, albeit infrequently. Furthermore, it has been discovered that some populations of this scorpion produce only females when a male is not present. This may be an indication that the male is beginning to evolve or, is in the final stages of becoming totally redundant.
Although reproduction via parthenogenesis is known in a number of scorpion species, its true extent has still not been fully investigated. It may turn out to be more widespread than we first thought.
Final thoughts
Understanding parthenogenesis can give us a great insight into the adaptive laws of genetics, detailing where they may be postponed or overthrown. As insects have short life cycles, any changes in gene activity will become evident within a few generations. This is why they are great study tools for many areas of research. Given that the role parthenogenesis has played in certain island species establishing themselves, it is obvious that it has played an important role is evolution. It has also enabled species to adapt their reproductive habits to certain environmental conditions.
Further reading.
Bogart, J.P.; Licht, L.E. (1986). “Reproduction and the origins of polyploids in hybrid salamanders of the genus Ambystoma”. Canadian Journal of Genetics and Cytology. 28 (4): 605–617.
Highfield, R. 2006. No sex please, we’re lizards. Daily Telegraph. Retrieved July 28, 2007.
Judson, O. 2002. Dr. Tatiana’s Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex. New York: Metropolitan Books. ISBN 0805063315.
Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. Life: The Science of Biology, 7th edition. Sunderland, MA: Sinauer. ISBN 0716766728.
Vitt, L. J. and J. P. Caldwell. 2008. Herpetology: An Introductory Biology of Amphibians and Reptiles, 3rd Ed. Academic Press, Burlington, Massachusetts
Watts, P. C., et al. 2006. Parthenogenesis in Komodo dragons. Nature 444: 1021.
