Parthenogenesis

Some stick insect species have a very interesting way to produce offspring: the female can produce young without needing a male! The eggs she produces are unfertilized, but do develop properly and grow into an adult female stick insect. The production of a new individual out of an unfertilized egg is called parthenogenesis. The female that produces these eggs is called parthenogenic. Parthenogenesis is a form of asexual reproduction.

Stick insects (Phasmatodea) are not the only insects that can produce asexually. There are also aphids, water fleas (Daphnia sp.), nematodes, plants, snakes, the whiptail lizard Cnemidophorus neomexicanus and more species that reproduce through parthenogenesis. It is possible to produce males or females through parthenogenesis, depending on the species. In stick insects only females are produced through parthenogenesis. Not all stick insect species can reproduce without fertilization, many species do need a male to fertilize the eggs.

Parthenogenesis in captivity

The occurrence of parthenogenesis has been first discovered when insects were kept in captivity without presence of a male. Female insects that were raised without males all their live did produce eggs that hatched into new females. This was a very exciting discovery and has been studied well. Both in stick insects and other species, however stick insects are the most well-known for it because many people can experience their life cycle first hand by keeping them as pets.

To have a stick insect species reproduce through parthenogenesis is very simple: you just need to keep females of a parthenogenic species in good health. Feed them well, give them enough space and humidity and they will grow into healthy adults. They will lay eggs which you can keep just as you would keep other stick insect eggs, like this. For some species it takes much longer for unfertilized eggs to hatch then for the fertilized eggs. Be patient! But be aware that not all species are capable of asexual reproduction, check the species description to find out which species is.

Are males even needed?

So if a female can produce offspring without a male, why would there be any males at all? It would even be a disadvantage to produce sons! If a female stick insect produces only daughters, and they again produce only daughters, she will have many more offspring then if she would produce sons too. This twofold cost of sexual reproduction is explained on this website. Some stick insects species seem to have indeed eliminated all males from their species. It is hard to determine if males don’t exist in the wild, because if you can’t find them, do you need to look better? But there are some species of stick insects of which no male has ever been found. Other stick insect species seem to have very few males compared to females. When catching wild specimens of these species, only a few percent of the caught specimens were male.

Other stick insect species seem to reproduce predominantly sexually in the wild, but reproduce asexually when kept in captivity. These insects have facultative asexual reproduction. It is unclear if the species evolved this method to produce offspring when a female accidentally does not find a mate, or if it only shows itself in artificial conditions in captivity.

Is a young stick insect that has been produced parthenogenetically a clone of the mother?

In most cases it is, but it does not have to be! It depends on how the egg is formed, and that depends on the species. Some species lay eggs that are genetically identical to normal (somatic) cells of the mother. These cells do not undergo meiosis, but just mitosis and this form of parthenogenesis is called apomictic parthenogenesis. These cells will start to divide and develop into an exact copy of the mother. Only if a mutation occurs in the offspring it will be genetically and maybe phenotypically different than the mother.
An other way of parthenogenesis is called automictic parthenogenesis and has more different ways. In this case the female produces normal haploid eggs through meiosis, but in one of various way diploidy is restored. This can be through e.g. doubling of the chromosomes or fusion of two haploid cells. Because recombination can occur and the genes that are present in two haploid cells can be different, the offspring is also different than the mother and different than other offspring. These stick insects are not clones of their mother or each other. However, when a species has been bred in captivity for hundreds of generations without males, the genetic diversity has become so low that even with automictic parthenogenesis the offspring and mother will be genetically completely identical.

Are asexually produced stick insects weaker then sexually produced ones?

Not always, but it can happen. That depends on the species. If the species reproduces only asexually in nature or for many decades in captivity, then the asexually produced offspring will be just as healthy as sexually produced ones. But if a species that usually reproduces sexually is bred without males, it can happen that a percentage of the eggs won’t hatch, take much longer to develop or give rise to weak offspring. This gets more noticeable the more generations pass. The species Extatosoma tiaratum has been known for not doing so well if only females are bred for multiple generations. The stick insects will become smaller and weaker every generation. It is best to introduce a male to such a group, preferably an unrelated male, to restore the health of the next generation. The cause of the drop in health is thought to be caused by inbreeding effects, like the expression of recessive alleles that are normally not expressed.

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