Thelytoky

Thelytoky or thelytokous parthenogenesis is a type of parthenogenesis in which females are produced from unfertilized eggs. It is rare in the animal kingdom and has only been reported in about 1500 species. [White M (1984). Chromosomal mechanisms in animal reproduction. Bull Zool 51: 1–23.] It is more common in invertebrates, like arthropods, but can also occur in vertebrates, like some whiptail lizards. Hymenoptera (ants, bees and wasps) for example, are haplodiploid and usually reproduce by arrhenotokous parthenogenesis, in which unfertilized eggs develop into haploid males, and fertilized eggs develop into diploid females but thelytoky has been described in several taxa, including Cynipidae, Tenthredinidae, Aphelinidae, Ichneumonidae, Apidae and Formicidae. [Suomalainen E, Saura A, Lokki J (1987). Cytology and Evolution in Parthenogenesis. CRC Press Inc.: Boca Raton, FL.] It can also be induced in Hymenoptera by the bacteria Wolbachia and Cardinium. [G. Jeong and Stouthamer, R. (2005) Genetics of female functional virginity in the Parthenogenesis-"Wolbachia" infected parasitoid wasp Telenomus nawai (Hymenoptera: Scelionidae). "Heredity" 94:402-407]

Thelytoky can occur by a number of different mechanisms each of which has a different impact on the level of inbreeding.

An example of thelytoky is the reproduction of female workers or queens by laying worker bees. It occurs in the Cape bee, "Apis mellifera capensis" and has been found in other strains at very low frequency. In honey bees, thelytoky occurs when diploidy is restored by the fusion of two meiotic products. [E. Baudry et al. (2004) Whole-Genome Scan in Thelytokous-Laying Workers of the Cape Honey bee (Apis mellifera capensis): Central Fusion, Reduced Recombination Rates and Centromere Mapping Using Half-Tetrad Analysis. "Genetics" 167:243-252] Usually, unfertilized eggs are haploid containing only a single set of chromosomes (16) from the mother. Cape bee laying workers are capable of laying unfertilized diploid (32 chromosomes) eggs. These eggs undergo an unusual biological life cycle. One stage during meiosis is anaphase when the chromosomes separate. In parthenogenesis (the reproduction without male fertilization), anaphase is followed by fusion of two meiotic products to restore egg diploidy (the egg pronucleus and the central descendant of the first polar body fuse to form a diploid nucleus called zygote). Depending on how the zygote (diploid egg) is fed it can develop into a worker bee or a queen bee.

References

ee also

*Arrhenotoky
*Epitoky