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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.〔King R.C; Stansfield W.D. and Mulligan P.K. 2006. ''A dictionary of genetics''. 7th ed, Oxford University Press, p194. ISBN 0-19-530761-5〕 Haplodiploidy is sometimes called arrhenotoky. Haplodiploidy determines the sex in all members of the insect orders Hymenoptera (bees, ants, and wasps)〔Grimaldi D. and Engel M.S. 2005. ''The evolution of the insects''. Cambridge University Press. ISBN 0-521-82149-5〕p408 and Thysanoptera ('thrips').〔 The system also occurs sporadically in some spider mites, Hemiptera, Coleoptera (bark beetles), and rotifers. In this system, sex is determined by the number of sets of chromosomes an individual receives. An offspring formed from the union of a sperm and an egg develops as a female, and an unfertilized egg develops as a male. This means that the males have half the number of chromosomes that a female has, and are haploid. The haplodiploid sex-determination system has a number of peculiarities. For example, a male has no father and cannot have sons, but he has a grandfather and can have grandsons. Additionally, if a eusocial-insect colony has only one queen, and she has only mated once, then the relatedness between workers (diploid females) in a hive or nest is 0.75. This means the workers in such monogamous single-queen colonies are significantly more closely related than in other sex determination systems where the relatedness of siblings is usually no more than 0.5. It is this point which drives the kin selection theory of how eusociality evolved.〔 Whether haplodiploidy did in fact pave the way for the evolution of eusociality is still a matter of debate.〔 〕〔 〕 Another feature of the haplodiploidy system is that recessive lethal and deleterious alleles will be removed from the population rapidly because they will automatically be expressed in the males (dominant lethal and deleterious alleles are removed from the population every time they arise, as they kill any individual they arise in). Haplodiploidy is not the same thing as an XO sex-determination system. In Haplodiploidy, males receive one half of the chromosomes that female receive, including autosomes. In an XO sex-determination system, males and females receive an equal number of autosomes, but when it comes to sex chromosomes, females will receive two X chromosomes while males will receive only a single X chromosome. == Mechanisms == Several models have been proposed for the genetic mechanisms of haplodiploid sex-determination. The model most commonly referred to is the ''complementary allele model''. According to this model, if an individual is heterozygous for a certain locus, it develops into a female, whereas hemizygous and homozygous individuals develop into males. In other words, diploid offspring develop from fertilized eggs, and are normally female, while haploid offspring develop into males from unfertilized eggs. Diploid males would be infertile, as their cells would not undergo meiosis to form sperm. Therefore, the sperm would be diploid, which means that their offspring would be triploid. Since hymenopteran mother and sons share the same genes, they may be especially sensitive to inbreeding: Inbreeding reduces the number of different sex alleles present in a population, hence increasing the occurrence of diploid males. After mating, each fertile hymenopteran female stores sperm in an internal sac called the spermatheca. The mated female controls the release of stored sperm from within the organ: If she releases sperm as an egg passes down her oviduct, the egg is fertilized.〔van Wilgenburg, Ellen; Driessen, Gerard & Beukeboom, Leo W. (Single locus complementary sex determination in Hymenoptera: an "unintelligent" design? ) Frontiers in Zoology 2006, 3:1〕 Social bees, wasps, and ants can modify sex ratios within colonies which maximizes relatedness among members and generates a workforce appropriate to surrounding conditions.〔Mahowald, Michael; von Wettberg, Eric (Sex determination in the Hymenoptera ) Swarthmore College (1999)〕 In other solitary hymenopterans, the females lay unfertilized male eggs on poorer food sources while laying the fertilized female eggs on better food sources, possibly because the fitness of females will be more adversely affected by shortages in their early life. Sex ratio manipulation is also practiced by haplodiploid ambrosia beetles, who lay more male eggs when the chances for males to disperse and mate with females in different sites are greater. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「haplodiploidy」の詳細全文を読む スポンサード リンク
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