Males Not Required!
Practical Reptile Keeping|April 2017

In most cases, mating is required to produce off spring, but in certain instances, particularly in the case of insects as well as some reptiles and amphibians, a single female may be able to create the next generation on her own. Paul Donovan investigates.

Karen Arnold

I am sure that at some stage or other, most of us have bought a particular species of insect, which then produces off spring. Perhaps we had no idea that the individual which we were acquiring was a female, and assumed that because it had young, it must have been mated before we bought it. This may not necessarily be correct though.

If we delve a bit deeper, into what is described as asexual reproduction or parthenogenesis, we come across a fascinating mode of reproduction, whereby a female can give birth to off spring which involved no intervention or indeed a genetic contribution by a male. This phenomenon was first discovered by the naturalist and philosopher Charles Bonnet when he was studying aphids in the 1740s.

Over the next century, parthenogenesis was observed in a number of insect species, including bagworm and silkworm moths, as well as drone bees. However, it was not until the 1840s that it was actually named ‘parthenogenesis’ which literally means ‘virgin creation’, often described today also as ‘virgin birth’.

What is it?

In the broadest biological sense of the term, parthenogenesis simply means an unfertilised ovum that ultimately produces a fully functional adult. In other words, at cellular level, the difference between species that reproduce parthenogenetically, as distinct from sexually, is that in the latter case, meiosis - the name for the process that divides a cell so that only half the chromosomes are present - is followed by fusion of a male and female gametes. This takes place as a result of fertilisation of the egg by a sperm. In parthenogenesis, however, meiosis is changed so that only one particular set of chromosomes is transferred in a non-random fashion.

This can lead to the suggestion that unisex populations are literally producing virgin clones of themselves, whereas this is not the case, as I will explain later. A species can be either obligately parthenogenetic (meaning that it reproduces exclusively through asexual reproduction) or it may do so facultatively, meaning that it can switch between asexual and sexual reproduction, with the latter method requiring the presence of a male for a successful outcome.

As far as vertebrates are concerned, parthenogenesis as a mode of reproduction has been reported in several species of fish, various amphibians and a few reptiles. Nevertheless, it is common in a number of invertebrates, such as aphids, thrips, stick insects, mites, and amongst members of the Order Hymenoptera, which includes ants, bees, and wasps. Parthenogenesis in members of this latter group is called arrhenotoky (literally meaning ‘birth of a male person’), because only males come from unfertilised eggs; those that give rise to females are fertilised.

As far as those invertebrates which are popularly kept in the home are concerned, the group where parthenogenesis is most commonly encountered is in the case of stick insects (Phasmatodea). Phasmids are sometimes regarded as being obligately parthenogenetic, as in the case of the Indian or laboratory stick insect (Carausius morosus) which consists of an almost exclusively female population and the occasional male that does arise will not mate.

Nevertheless, the majority of species show high levels of facultative parthenogenesis. In certain cases, as with the pink-winged stick insect (Sipyloidea sipylus), there are differences between different populations, with only some being parthenogenetic.

How did parthenogenesis evolve?

Although little is known of the origins of parthenogenesis, the ancestors of today’s parthenogenetic species unquestionably used to reproduce sexually. It is suspected that this shift in their reproductive biology must have come about as a result of genetic instabilities which accompanied the interference of foreign chromosomes. According to one study, it is believed that all parthenogenetic vertebrate species seem to have arisen from interspecific hybridisation, based on studies of their chromosomes, protein variations, and DNA sequences.

Nevertheless, there are also cases where parthenogenesis can occur without hybridisation. One of these is the condition known as induced thelytoky, when unfertilised eggs develop into females. This can be triggered in a number of insect species by certain types of bacteria belonging to the genera Rickettsia, Wolbachia and Cardinium. In other words, parthenogenesis can sometimes result from a bacterial infection, and every year, more strains of bacteria capable of having this effect are being found.

Geographical distribution

A single female from a parthenogenetic species, or even an egg from a female of this type which finds its way onto an island may then give rise to an entirely new colony. This in part explains why many such species are island forms. A good example of this phenomenon is the smooth stick insect (Clitarchus hookeri) from New Zealand which reached the Isles of Scilly in 1949, along with some imported plants. These phasmids are now thriving there as an all-female colony.

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