What is parthenogenesis?: No-mate, no-problem

In the world of exotic invertebrates, some species possess an extraordinary ability to reproduce without the need for a male. This process, known as parthenogenesis, allows females to produce offspring from unfertilised eggs and is commonly observed in phasmid species (stick and leaf insects).

Parthenogenesis is a form of asexual reproduction in which an unfertilised egg develops into a viable offspring without male involvement. 'Viable' means the offspring develop properly and grow normally. The term derives from the Greek words 'parthenos' (virgin) and 'genesis' (creation). This shift from sexual reproduction to parthenogenesis has occurred repeatedly in animals, including invertebrates, aphids, reptiles, and fish. It is most extensively studied in invertebrates due to the ease of observing their life cycles in captivity. For some species, it's a last resort when mating options are limited; for others, it's their sole method of reproduction.

Parthenogenesis offers significant short-term advantages for species. It enables rapid population growth as females can produce offspring without the need to find a mate. This reproductive method also allows for genetic preservation, as offspring are typically genetically identical to their mother, which can help maintain successful genetic traits. However, this same genetic preservation can also be a species' downfall. The lack of genetic diversity can make populations more susceptible to diseases, harmful mutations, and environmental changes. This vulnerability highlights the double-edged nature of parthenogenetic reproduction in the long term.

The impact of continued parthenogenetic reproduction varies among species. Some have reproduced this way for millions of years without issues. Others, like the giant prickly stick insect (Extatosoma tiaratum), experience health deterioration after multiple generations of female-only breeding, resulting in smaller and weaker insects. For these species, introducing males for sexual reproduction is beneficial, creating a stronger gene pool and producing offspring less prone to spontaneous death.

Research has also shown that eggs produced via parthenogenesis often take longer to hatch than those from sexual reproduction, an important consideration when observing these species' life cycles.

Overall, when considering captive invertebrate cultures, the worries of environmental change and pathogenic concerns are minimal. Many cultures of phasmid and phyllium can be bred from the same stock year after year with no problem.

While some species appear to have eliminated males entirely, others maintain a low male population in the wild, often comprising only 1-2% of individuals. This is reflected when catching wild specimens of these species, a great example being the collection of male phyllium giganteum. The males do exist, they are just very rare.

Interestingly, some species that reproduce sexually in the wild switch to parthenogenesis in captivity. This is known as facultative parthenogenesis and could be an evolutionary strategy for times when mates are scarce or a response to artificial conditions. The triggers behind this reproductive flexibility remain a subject of ongoing study, highlighting the complex relationship between genetics, environment, and reproduction in invertebrates.

The Indian stick insect (Carausius morosus) provides a great case study of parthenogenesis in invertebrates. Native to southern India, this species is a staple of phasmid keeping. In captivity, C. morosus populations are almost entirely female. The probability of a true male Indian stick insect occurring is very rare, at 0.01%, which is 1 in 10000. These insects reproduce exclusively through parthenogenesis.

Adult females lay eggs regardless of mating, and these eggs develop through a process called automixis (the fusion of nuclei or gametes derived from the same individual). After 3-4 months of incubation, nymphs emerge and go through six instars over 3-4 months before reaching adulthood. The cycle then continues as new adult females begin laying eggs within 1-2 weeks.

This reproductive strategy results in offspring genetically identical to their mother, barring mutations, and allows for this rapid population growth. A single female can produce up to 2-3 eggs per day, potentially resulting in hundreds of offspring over her lifetime. There have been numerous cases of people keeping just a couple of Indian stick insects, only to end up with hundreds of offspring scattered throughout their homes! And this is due to the beauty of parthenogenesis.