Haplodiploidy
The haplodiploid sex-determination system determines the sex of the offspring of many Hymenopterans (bees, ants, and wasps), and coleopterans (bark beetles). 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 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 may have paved the way for the evolution of eusociality in the Hymenoptera and a few other taxa although this is a matter of considerable debate and ongoing research.[1] [2]
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 are infertile, as their sperm do not undergo meiosis which means that their offspring would be triploid. This also means that Hymenopterans 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]
Sex-determination in honey bees
In honeybees the drones (males) are entirely derived from the queen, their mother. The queen has 32 chromosomes and the drones have 16 chromosomes. Drones produce sperm cells that contain their entire genome, so the sperm are all genetically identical barring 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. The worker is more related to the queen's daughters (her sisters) than to the workers' sons (her nephews). Helping the queen's offspring to survive is aiding the spread of the same genes that the worker possesses [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. Kin selection may explain the evolution of these eusocial colonies.
Shared gene proportions in haplo-diploid sex-determination system relationships
| Sex | Daughter | Son | Mother | Father | Full Sister | Full Brother | Niece/Nephew |
|---|---|---|---|---|---|---|---|
| Female | 1/2 | 1/2 | 1/2 | 1/2 | 3/4 | 1/4 | 3/8 |
| Male | 1 | 0 | 1 | 0 | 1/4 | 1/2 | 1/4 |
References
- ^ 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. 320 (5880). American Association for the Advancement of Science: 1213–1216. Retrieved 2008-08-04.
{{cite journal}}: Check date values in:|date=(help)CS1 maint: multiple names: authors list (link) - ^ Edward O. Wilson and Bert Hölldobler (2005-09-20). "Eusociality: Origin and consequences" (PDF). Proceedings of the National Academy of Sciences. 102 (38). United States National Academy of Sciences: 13367–13371. Retrieved 2008-08-04.
{{cite journal}}: Check date values in:|date=(help) - ^ van Wilgenburg, Ellen; Driessen, Gerard & Beukeboom, Leo W. Single locus complementary sex determination in Hymenoptera: an "unintelligent" design? Frontiers in Zoology 2006, 3:1
- ^ Mahowald, Michael; von Wettberg, Eric Sex determination in the Hymenoptera Swarthmore College (1999)
- ^ Sinervo, Barry Kin Selection and Haplodiploidy in Social Hymenoptera 1997
- ^ 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
- Beye, Martin; Hunt, Greg J. ; Page, Robert E. ; Fondrk, M. Kim; Grohmann, Lore and Moritz, R. F. A. Unusually High Recombination Rate Detected in the Sex Locus Region of the Honey Bee (Apis mellifera) Genetics (journal), Vol. 153, 1701-1708, December 1999
- Wu, Z.; Hopper, K. R.; Ode, P. J. ; Fuester, R. W.; Tuda, M. and Heimpel, G. E. Single-locus complementary sex determination absent in Heterospilus prosopidis (Hymenoptera: Braconidae) Heredity (2005) 95, 228–234
- Ratnieks, Francis Reproductive harmony via mutual policing by workers in eusocial hymenoptera American Naturalist 132(2) 217-236 ; 1988