Critique of the Theory of Evolution Fig 060

The XO sex-determination system (sometimes X0 sex-determination system) is a system that some species of insects, arachnids, and mammals 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 second X. Maternal

gametes A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

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

arachnid Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegar ...

sBull, James J.; ''Evolution of sex determining mechanisms''; p. 17 with the exception of mites where a small majority are

haplodiploid Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky. Haplodiploidy determines the sex ...

, * Almost all apterygote and Paleopteran insects (e.g.,

dragonflies A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threa ...

, silverfish) * Most exopterygote insects (e.g., grasshoppers,

cricket Cricket is a bat-and-ball game played between two teams of eleven players on a field at the centre of which is a pitch with a wicket at each end, each comprising two bails balanced on three stumps. The batting side scores runs by str ...

cockroach Cockroaches (or roaches) are a Paraphyly, paraphyletic group of insects belonging to Blattodea, containing all members of the group except termites. About 30 cockroach species out of 4,600 are associated with human habitats. Some species are we ...

es) * Some nematodes,

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...

s, gastropod molluscs, and bony fish, notably in the genus ''

Ancistrus ''Ancistrus'' is a genus of nocturnal freshwater fish in the family Loricariidae of order Siluriformes, native to freshwater habitats in South America and Panama. Fish of this genus are common in the aquarium trade where they are known as bushyn ...

'' * Several mammals ** A few species of bat, including the hammer-headed bat, Buettikofer's epauletted fruit bat,

Franquet's epauletted fruit bat Franquet's epauletted fruit bat (''Epomops franqueti'') is a species of megabat in the family Pteropodidae, and is one of three different species of epauletted bats. Franquet's epauletted fruit bat has a range of habitats, varying from Subsaharan ...

, 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 In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have s ...

, 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 '' Caenorhabditis elegans''—a nematode frequently used in biological research—is one such organism. Some ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...

'' species have XO males. These are thought to arise via the loss of the

Y chromosome The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or abse ...

Evolution

XO sex determination can evolve from XY sex determination with about 2 million years.

Parthenogenesis

Parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...

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