There are several evolutionary routes to parthenogenesis in animals  (Box 1). One of the more remarkable recent experiments on the origin of asexuality demonstrated that certain haplodiploid taxa could be 'cured' of asexuality when treated with antibiotics . Haplodiploid taxa, which include many social insects such as termites and wasps, produce males from unfertilized eggs and females from fertilized eggs. This demonstration  was explained by the fact that parthenogenesis can arise from manipulation by endosymbionts.
This system is now one in which the forces driving the establishment of asexual lineages are well known. Bacterial symbionts (such as Wolbachia, Cardinium and Rickettsia) can be transmitted only through the cytoplasm of eggs, from a female to her offspring. Males are essentially 'dead ends', and symbiont fitness is thus maximized by converting male progeny to female development. In arrhenotokous haplodiploid taxa (such as the parasitoid wasps studied by Stouthamer et al. [4, 5]), this can be achieved by the parasite taking haploid unfertilized eggs that would develop into males and diploidizing their genome, such that they develop as females. At the start of this process, symbiont-infected females do mate with males, and fertilized eggs resulting from these matings develop into normal, sexually produced daughters. It is only the unfertilized haploid eggs that are subject to genome doubling and become asexually produced daughters.
Box 1. Modes of parthenogenesis in animals
Organisms can possess lineages composed of facultative parthenogens (tychoparthenogens) or obligate parthenogens. In the case of the former, eggs infrequently and spontaneously develop without fertilisation, whilst in the latter 'cloning' is the only mode of reproduction. The forms of parthenogenesis include:
Apomictic parthenogenesis (apomixis) - meiosis is suppressed and unfertilized eggs develop into offspring by mitotic cell division. Apomixis occurs in rotifers and waterfleas.
Automictic parthenogenesis (automixis) - meiosis occurs, but diploidy is restored by duplication or fusion of gametes from the mother. Automixis is found in some stick insects and weevils.
Gynogensis - sperm from males of a related species activate egg development, but the sperm nuclei do not fuse with egg nuclei. Only maternal genes are expressed in the offspring.
Hybridogenesis - the genome is transmitted both sexually and clonally. Sperm and egg nuclei fuse and genes from the father are expressed in the offspring. When female offspring produce their own eggs, only the maternal genome is transmitted to the next generation; the father's genes are discarded. Hybridogenesis can occur in organisms that are hybrids of two species.
If unfertilized eggs develop into females, the organisms are thelytokous parthenogens. The unfertilized eggs of arrhenotokous parthenogens develop into males, as in the case of the haplodiploid insects of interest in Stouthamer et al. .