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The haplodiploid sex-determination system is typical of bees and wasps.

The haplodiploid sex-determination system determines the sex of the offspring of many hymenopterans (bees, ants, and wasps), spider mites, coleopterans (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. This haplodiploid sex-determination system produces a number of peculiarities; chief among these is that a male has no father and cannot have sons, but he has a grandfather and can have grandsons. Haplodiploidy is postulated as having paved the way for the evolution of eusociality in the Hymenoptera and a few other taxa although this is a matter of considerable debate.[1] [2]

Contents

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 allele, 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, fertile Hymenopteran females store the 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 the oviduct, the egg is fertilized. [3] Social bees, wasps, and ants can modify sex ratios within colonies to maximize relatedness among members, and to generate a workforce appropriate to surrounding conditions. [4] good information

Sex-determination in honey bees

In honeybees the drones (males) are entirely derived from the queen, their mother. The diploid queen has 32 chromosomes and the haploid drones have 16 chromosomes. Drones produce sperm cells that contain their entire genome, so the sperm are all genetically identical except for mutations. The genetic makeup of the female worker bees is half derived from the mother, and half from the father, but the male bees' genetic makeup is entirely derived from the mother.[5] Thus, if a queen bee mates with only one drone, any two of her daughters will share, on average, 3/4 of their genes. The diploid queen's genome is recombined for her daughters, but the haploid father's genome is inherited by his daughters "as is".

While workers can lay unfertilized eggs that become their sons, haplodiploid sex-determination system is beneficial to the individual due to indirect selection. Since the worker is more related to the queen's daughters (her sisters) than to her own offspring, helping the queen's offspring to survive aids the spread of the same genes that the worker possesses more efficiently than direct reproduction. [6] Batches of worker bees are short lived and are constantly being replaced by the next batch, so this kin selection is possibly a strategy to ensure the proper working of the hive. However, since queens usually mate with a dozen drones or more, not all workers are full sisters. Due to the separate storage of drone sperm, a specific batch of brood may be closer related than a specific batch of brood laid at a later date.

Shared gene proportions in haplo-diploid sex-determination system relationships
Sex Daughter Son Mother Father Full Sister Full Brother
Female 1/2 1/2 1/2 1/2 3/4 1/4
Male 1 N/A 1 N/A 1/2 1/2

References

  1. ^ William O. H. Hughes, Benjamin P. Oldroyd, Madeleine Beekman, Francis L. W. Ratnieks (2008-05-30). "Ancestral Monogamy Shows Kin Selection Is Key to the Evolution of Eusociality" (html). Science (American Association for the Advancement of Science) 320 (5880): 1213–1216. doi:10.1126/science.1156108. http://www.sciencemag.org/cgi/content/abstract/320/5880/1213. Retrieved 2008-08-04. 
  2. ^ Edward O. Wilson and Bert Hölldobler (2005-09-20). "Eusociality: Origin and consequences" (PDF). Proceedings of the National Academy of Sciences (United States National Academy of Sciences) 102 (38): 13367–13371. doi:10.1073/pnas.0505858102. http://www.pnas.org/content/102/38/13367.full.pdf+html. Retrieved 2008-08-04. 
  3. ^ van Wilgenburg, Ellen; Driessen, Gerard & Beukeboom, Leo W. Single locus complementary sex determination in Hymenoptera: an "unintelligent" design? Frontiers in Zoology 2006, 3:1
  4. ^ Mahowald, Michael; von Wettberg, Eric Sex determination in the Hymenoptera Swarthmore College (1999)
  5. ^ Sinervo, Barry Kin Selection and Haplodiploidy in Social Hymenoptera 1997
  6. ^ Foster, Kevin R.; Ratnieks, Francis L. W. The Effect of Sex-Allocation Biasing on the Evolution of Worker Policing in Hymenopteran Societies The American Naturalist, volume 158 (2001), pages 615–623

See also

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