Can Stick Insects Reproduce Without Males? Parthenogenesis

What is parthenogenesis?

Parthenogenesis means “virgin birth” in Greek. It is a form of reproduction in which an egg develops into a new animal without being fertilized by a male’s sperm.

In stick insects, the usual form is called thelytoky: unfertilized eggs produce only daughters. In many species the egg makes up for the missing sperm by doubling its own set of chromosomes. The daughters therefore carry only their mother’s genes. They are very similar to her, and often close to genetically identical, although this doubling can make each generation less varied over time.

Parthenogenesis is far more common in stick insects than in most insect groups. Biologists estimate that it occurs, at least occasionally, in a large share of the order Phasmatodea, which has more than 3,500 described species.

Two strategies: optional or permanent

Stick insects fall into two broad groups.

Facultative parthenogens can reproduce either way. If a male is present, the female mates and lays fertilized eggs that produce both sons and daughters. If no male appears, she lays unfertilized eggs that still hatch, but only into daughters. The giant prickly stick insect (Extatosoma tiaratum) of Australia is a well-studied example. Its fertilized eggs usually hatch in about four months, while unfertilized eggs can take up to about nine months.

Obligate parthenogens have abandoned sex entirely, or nearly so. Populations consist of females only, and every egg develops without fertilization. Several of these all-female stick insects are thought to have arisen from hybrids between two related species.

Species that live without males

• Indian stick insect (Carausius morosus). Originally from southern India, this species is reared in laboratories and schools around the world. Almost all individuals are females, and in captive stocks only about one in a thousand is a male. Females lay eggs that hatch into daughters without mating.
• Mediterranean stick insects (Bacillus). In southern Europe, Bacillus rossius has populations with males and females in some areas but all-female populations in others. Related Bacillus forms of hybrid origin in Sicily reproduce without males, and a few even show androgenesis, a very rare process in which offspring carry only the father’s genes.
• Timema walkingsticks. These small, stout stick insects live on shrubs and trees in the western United States, especially California. Five of the roughly 20 species are all-female.

The northern walkingstick, the common stick insect of eastern North America, is not on this list. As far as is known, it has normal populations of both sexes, and females mate before dropping their eggs in fall.

A million years without males

The Timema stick insects are famous among biologists. Genetic studies published in 2011 suggested that some all-female Timema lineages have reproduced without males for roughly a million years or more. That makes them among the oldest known asexual lineages of any insect.

This matters because most all-female animal lineages are thought to die out fairly quickly. Without mixing genes from two parents, harmful mutations can build up, and a population may adapt more slowly to new diseases and parasites. Long-lived asexual groups such as these Timema help scientists test how serious those costs really are.

Why reproduce without males?

Parthenogenesis has clear advantages, especially for slow-moving, often wingless insects:

• No need to find a mate. A female living alone on a bush can still produce a full family.
• Fast growth. Because every offspring is female and can lay eggs, an all-female population can, in theory, grow up to twice as fast as one with males.
• Founding new populations. A single female, or even a single egg carried by wind, water, ants or a bird, can start a new colony. Researchers have suggested this may help explain how some stick insects reached remote islands. Eggs can even survive passing through a bird’s gut, as described in how do stick insects lay eggs?.

There are costs too. Unfertilized eggs often hatch more slowly and less reliably than fertilized ones. In captivity, some lines kept for many generations without males produce smaller or less fertile females. And all-female populations have less genetic variety to draw on when conditions change.

Other animals that do it too

Stick insects are not alone. Parthenogenesis is common in aphids, water fleas and some wasps, and it occurs in whiptail lizards, some geckos and certain fish. Bees, ants and wasps use a different form in which unfertilized eggs become males.

Among large reptiles, the best-known cases come from zoos. Female Komodo dragons kept without males have laid eggs that hatched, as explained in can Komodo dragons reproduce without males?. Similar “virgin births” have been recorded in some sharks and snakes. In stick insects, however, it is not a rare surprise but an everyday way of life for many species.