Critique of the Theory of Evolution Fig 060

The XO sex-determination system (sometimes referred to as X0 sex-determination system) is a system that some species of insects, arachnids, and mammals (not including humans) use to determine the sex of offspring. In this system, there is only one sex chromosome, referred to as X. Males only have one X chromosome (XO), while females have two (XX). The letter O (sometimes a zero) signifies the lack of a

Y chromosome The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining chromosome because the presence of the ...

. Maternal gametes always contain an X chromosome, so the sex of the animals' offspring depends on whether a sex chromosome is present in the male gamete. Its

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...

normally contains either one X chromosome or no sex chromosomes at all. This system determines the sex of offspring among: * Most

arachnid Arachnids are arthropods in the Class (biology), class Arachnida () of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, camel spiders, Amblypygi, wh ...

sBull, James J.; ''Evolution of sex determining mechanisms''; p. 17 with the exception of mites where a small majority are haplodiploid * Almost all apterygote and Paleopteran insects (e.g., dragonflies, silverfish) * Most exopterygote insects (e.g., grasshoppers,

cricket Cricket is a Bat-and-ball games, bat-and-ball game played between two Sports team, teams of eleven players on a cricket field, field, at the centre of which is a cricket pitch, pitch with a wicket at each end, each comprising two Bail (cr ...

cockroach Cockroaches (or roaches) are insects belonging to the Order (biology), order Blattodea (Blattaria). About 30 cockroach species out of 4,600 are associated with human habitats. Some species are well-known Pest (organism), pests. Modern cockro ...

es) * Some

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

s, crustaceans, gastropod molluscs, and

bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...

, notably in the genus '' Ancistrus'' * Several mammals, including: ** A few species of bat, including the hammer-headed bat, Buettikofer's epauletted fruit bat, Franquet's epauletted fruit bat, Peters's epauletted fruit bat, and Gambian epauletted fruit bat ** The Ryukyu spiny rat and Tokunoshima spiny rat In a variant of this system, most individuals have two sex chromosomes (XX) and are hermaphroditic, producing both eggs and sperm with which they can fertilize themselves, while rare individuals are male and have only one sex chromosome (XO). The

model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...

'' Caenorhabditis elegans''—a

frequently used in biological research—is one such organism. Most spiders have a variation of the XO system in which males have two different X chromosomes (XXO), while females have a pair of X chromosomes and a pair of X chromosomes (XXXX). Some spiders have more complex systems involving as many as 13 different X chromosomes. Some '' Drosophila'' species have XO males. These are thought to arise via the loss of the

. In humans the XO designation attaches to individuals with Turner syndrome.

Evolution

XO sex determination can evolve from XY sex determination within about 2 million years. It typically evolves due to Y-chromosome degeneration. As the Y-chromosome is not paired (though see pseudoautosomal region), it is susceptible to decay by Muller's ratchet. Similarly, the W chromosome in a ZW sex-determination system is susceptible to decay, resulting in a ZZ/ ZO system.

Parthenogenesis

Parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

with XO sex-determination can occur by different mechanisms to produce either male or female offspring.

References

{{Sex determination and differentiation Sex-determination systems Insect genetics Arachnid anatomy

Evolution

Parthenogenesis

See also

References