Everything about Haplodiploidy totally explained
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 can't have sons, but he's 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.
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 don't 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. 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.
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. 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
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 |
1/2 |
1/2 |
1/4
|
Further Information
Get more info on 'Haplodiploidy'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://haplodiploid_sex-determination_system.totallyexplained.com">Haplodiploid sex-determination system Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
