Haplo-diploid sex-determination system

The haplodiploid sex-determination system determines the sex of the offspring of many hymenoptera

Hymenoptera

Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...

ns (bee

Bee

Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s, ant

Ant

Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

s, and wasp

Wasp

The term wasp is typically defined as any insect of the order Hymenoptera and suborder Apocrita that is neither a bee nor an ant. Almost every pest insect species has at least one wasp species that preys upon it or parasitizes it, making wasps critically important in natural control of their...

s), spider mite

Spider mite

Spider mites are members of the Acari family Tetranychidae, which includes about 1,200 species. They generally live on the under sides of leaves of plants, where they may spin protective silk webs, and they can cause damage by puncturing the plant cells to feed...

s, coleopterans (bark beetle

Bark beetle

A bark beetle is one of approximately 220 genera with 6,000 species of beetles in the subfamily Scolytinae. Traditionally, this was considered a distinct family Scolytidae, but now it is understood that bark beetles are in fact very specialized members of the "true weevil" family...

s) and rotifer

Rotifer

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s. In this system, sex is determined by the number of sets of chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s an individual receives. An offspring formed from the union of a sperm

Sperm

The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

and an egg

Egg (biology)

An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

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

Eusociality

Eusociality is a term used for the highest level of social organization in a hierarchical classification....

in the Hymenoptera and a few other taxa although this is a matter of considerable debate.

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

Locus (genetics)

In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...

, 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

Meiosis

Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

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

Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...

: Inbreeding reduces the number of different sex allele

Allele

An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

s present in a population, hence increasing the occurrence of diploid males.

After mating

Mating

In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...

, fertile Hymenopteran females store the sperm

Sperm

in an internal sac called the spermatheca

Spermatheca

The spermatheca , also called receptaculum seminis , is an organ of the female reproductive tract in insects, some molluscs, oligochaeta worms and certain other invertebrates and vertebrates...

. The mated female controls the release of stored sperm from within the organ: If she releases sperm as an egg passes down the oviduct

Oviduct

In non-mammalian vertebrates, the passageway from the ovaries to the outside of the body is known as the oviduct. The eggs travel along the oviduct. These eggs will either be fertilized by sperm to become a zygote, or will degenerate in the body...

, 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

Drone (bee)

Drones are male honey bees. They develop from eggs that have not been fertilized, and they cannot sting, since the worker bee's stinger is a modified ovipositor .-Etymology:...

(males) are entirely derived from the queen

Queen bee

The term queen bee is typically used to refer to an adult, mated female that lives in a honey bee colony or hive; she is usually the mother of most, if not all, the bees in the hive. The queens are developed from larvae selected by worker bees and specially fed in order to become sexually mature...

, their mother. The diploid queen has 32 chromosomes and the haploid drones have 16 chromosomes. Drones produce sperm

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. 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

Chromosomal crossover

Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...

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.

Batches of worker bees are short lived and are constantly being replaced by the next batch, so this kin selection

Kin selection

Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/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 more closely related than a specific batch of brood laid at a later date.

Relatedness ratios in haplodiploid

Relatedness is used to calculate the strength of kin selection

Kin selection

(via Hamilton's rule). The haplodiploidy hypothesis states that the unusual 3/4 relatedness coefficient amongst full haplodiploid sisters is responsible for the frequency of evolution of eusocial behavior in hymenoptera.

Relatedness coefficients in haplodiploid organisms are as follows.

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

Controversy

The haplodiploidy hypothesis is often given as a clear example of kin selection. Since full hymenopteran sisters share more genes than a parent shares with its offspring, it follows that helping to rear sisters should be favored over having children as an evolutionary strategy. However, this argument neglects that eusocial drones help raise their brothers in addition to their sisters, which, given the typical sex ratio of 1:1, results in raising siblings with an average relatedness coefficient of 1/2, which is no better than that of young. Ratios can be even lower in colonies where the queen has mated with multiple males, which is common in some species. This, combined with the discovery of multiple diploid eusocial organisms and at least one haplodiploid eusocial species with a male nonreproductive caste, has been referred to by one of the founding figures of sociobiology

Sociobiology

Sociobiology is a field of scientific study which is based on the assumption that social behavior has resulted from evolution and attempts to explain and examine social behavior within that context. Often considered a branch of biology and sociology, it also draws from ethology, anthropology,...

, E. O. Wilson, as the "collapse of the haplodiploid hypothesis". However, not all researchers believe the hypothesis has been disproven, and evidence continues to be collected on both sides. Statistical analysis suggests that in each of the independent evolutions of eusociality, queens mated with only one male, which suggests having highly related offspring was important. Furthermore, often the female bees give more food to the females than to the males, and in times of dearth they even kill the males.

Mechanisms

Sex-determination in honey bees

Relatedness ratios in haplodiploid

Controversy

See also