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Haplodiploidy is a
sex-determination system A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. In some species there are hermaph ...
in which males develop from unfertilized eggs and are
haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respective ...
, and females develop from fertilized eggs and are
diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectiv ...
. Haplodiploidy is sometimes called arrhenotoky. Haplodiploidy determines the sex in all members of the insect orders
Hymenoptera Hymenoptera is a large order of insects, comprising the sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are parasitic. Females typic ...
(
bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfami ...
s, ants, and
wasp A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which look somewhat like wasps, but are in a separate suborder ...
s) and Thysanoptera ('thrips'). The system also occurs sporadically in some
spider mite Spider mites are members of the Tetranychidae family, which includes about 1,200 species. They are part of the subclass Acari (mites). Spider mites generally live on the undersides of leaves of plants, where they may spin protective silk webs, a ...
s,
Hemiptera Hemiptera (; ) is an order of insects, commonly called true bugs, comprising over 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, assassin bugs, bed bugs, and shield bugs. They range in size from to arou ...
,
Coleoptera Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describe ...
(
bark beetle A bark beetle is the common name for the subfamily of beetles Scolytinae. Previously, this was considered a distinct family (Scolytidae), but is now understood to be a specialized clade of the "true weevil" family (Curculionidae). Although the ...
s), and
rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John H ...
s. In this system, sex is determined by the number of sets of
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
s an individual receives. An offspring formed from the union of a
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, ...
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 Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respective ...
. 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 Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping gen ...
-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 . 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 . It is this point which drives the
kin selection Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution ...
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
X0 sex-determination system The XO sex-determination system (sometimes X0 sex-determination system) is a system that some species of insects, arachnids, and mammals use to determine the sex of offspring. In this system, there is only one sex chromosome, referred to as X. Male ...
. In haplodiploidy, males receive one half of the chromosomes that females receive, including
autosomes An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosomal (sex chromosome) pairs, which may have different structures. The DNA in autosomes ...
. In an X0 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 Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. ...
for a certain locus, it develops into a female, whereas
hemizygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
and
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
individuals develop into males. In other words,
diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectiv ...
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 (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
to form sperm. Therefore, the sperm would be diploid, which means that their offspring would be
triploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...
. Since hymenopteran mother and sons share the same genes, they may be especially sensitive to
inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders a ...
: Inbreeding reduces the number of different sex
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...
s present in a population, hence increasing the occurrence of diploid males. After
mating In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...
, each fertile hymenopteran female stores
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, ...
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. Social bees, wasps, and ants can modify sex ratios within colonies which maximizes relatedness among members and generates a workforce appropriate to surrounding conditions. 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 beetle Ambrosia beetles are beetles of the weevil subfamilies Scolytinae and Platypodinae ( Coleoptera, Curculionidae), which live in nutritional symbiosis with ambrosia fungi. The beetles excavate tunnels in dead, stressed, and healthy trees in wh ...
s, who lay more male eggs when the chances for males to disperse and mate with females in different sites are greater.


Sex-determination in honey bees

In
honeybee A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the current cosm ...
s, the drones (males) are entirely derived from the
queen Queen or QUEEN may refer to: Monarchy * Queen regnant, a female monarch of a Kingdom ** List of queens regnant * Queen consort, the wife of a reigning king * Queen dowager, the widow of a king * Queen mother, a queen dowager who is the mother ...
, their mother. The diploid queen has 32 chromosomes and the haploid drones have 16 chromosomes. Drones produce
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, ...
cells that contain their entire genome, so the sperm are all genetically identical except for mutations. The male bees' genetic makeup is therefore entirely derived from the mother, while the genetic makeup of the female worker bees is half derived from the mother, and half from the father. Thus, if a queen bee mates with only one drone, any two of her daughters will share, on average, 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". It is also possible for a laying worker bee to lay an unfertilised egg, which is always a male. There are rare instances of diploid drone larvae. This phenomenon usually arises when there is more than two generations of brother-sister mating. Sex determination in honey bees is initially due to a single locus, called the complementary sex determiner (''csd'') gene. In developing bees, if the conditions are that the individual is heterozygous for the ''csd'' gene, they will develop into females. If the conditions are so that the individual is
hemizygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
or homozygous for the ''csd'' gene, they will develop into males. The instances where the individual is homozygous at this gene are the instances of diploid males. Diploid males do not survive to adulthood, as the nurse worker bees will cannibalize the diploid males upon hatching. While workers can lay unfertilized eggs that become their sons, haplodiploid sex-determination system increases the individual's fitness 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 helps 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 is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution ...
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. However, many other species of bees, including bumblebees, such as ''
Bombus terrestris ''Bombus terrestris'', the buff-tailed bumblebee or large earth bumblebee, is one of the most numerous bumblebee species in Europe. It is one of the main species used in greenhouse pollination, and so can be found in many countries and areas wh ...
'', are monandrous. This means that sisters are almost always more related to one another than they would be to their own offspring, thus eliminating the conflict of variable relatedness present in honeybees.


Sex determination in chalcidoid wasps

In wasps of the genus ''
Nasonia ''Nasonia'' are a genus of small pteromalid parasitoid wasps that sting and lay eggs in the pupae of various flies. The fly species that ''Nasonia'' usually parasitize are primarily blow flies and flesh flies, making ''Nasonia'' a useful tool f ...
'', a non-CSD method of sex determination has been documented. The most recent accepted model for this non-CSD system is called Maternal Effect Genomic Imprinting Sex Determination (MEGISD). This model involves a masculinizing/virilizing maternal effect gene that “imprints upon” the cytoplasmic component of oocytes, and an “unimprinted” paternal contribution (in female offspring) that provides a counter effect to
virilization Virilization or masculinization is the biological development of adult male characteristics in young males or females. Most of the changes of virilization are produced by androgens. Virilization is most commonly used in three medical and biology ...
and allows for female development to occur. Since all diploid eggs become female (due to the factor originating in the male genetic contribution that prevents masculinization), this differs from CSD in that under CSD, diploid eggs can become males if they are homozygous or hemizygous.Zou, Yuan et al. “A chimeric gene paternally instructs female sex determination in the haplodiploid wasp Nasonia.” Science 370 (2020): 1115 - 1118.


Relatedness ratios in haplodiploidy

Relatedness is used to calculate the strength of
kin selection Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution ...
(via Hamilton's rule). The haplodiploidy hypothesis proposes that the unusual relatedness coefficient amongst full haplodiploid sisters is responsible for the frequency of evolution of
eusocial Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping gen ...
behavior in hymenopterans. A eusocial worker helping her mother birth more sisters propagates more of her own genes than had she reproduced herself. In normal
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
, the father has two sets of chromosomes, and crossing over takes place between the
chromatid A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chr ...
s of each pair during the
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
which produces the
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, ...
. Therefore, ''the sperms are not identical'', because in each chromosome of a pair there will be different
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...
s at many of the loci. But when the father is haploid ''all the sperms are identical'' (except for a small number where
gene mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosi ...
s have taken place in the germ line). So, all female offspring inherit the male's chromosomes 100% intact. ''As long as a female has mated with only one male'', all her daughters share a complete set of chromosomes from that male. In Hymenoptera, the males generally produce enough sperm to last the female for her whole lifetime after a single mating event with that male. Relatedness coefficients in haplodiploid organisms are as follows, assuming that a female has only mated once. These ratios apply, for example, throughout a bee hive, unless some laying workers produce offspring, which will all be males from unfertilised eggs: in that case, average relatedness will be lower than shown. : Under this assumption that mothers only mate once, sisters are more strongly related to each other than to their own daughters. This fact has been used to explain the evolution of eusociality in many hymenopterans. However, colonies which have workers from multiple queens or queens which have mated multiple times will have worker-to-worker relatedness which is less than worker-to-daughter relatedness, such as in '' Melipona scutellaris''.


See also

*
Green-beard effect The green-beard effect is a thought experiment used in evolutionary biology to explain selective altruism among individuals of a species. The idea of a green-beard gene was proposed by William D. Hamilton in his articles of 1964, and got th ...
*
Ploidy Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectiv ...
*
Pseudo-arrhenotoky Pseudo-arrhenotoky or paternal genome elimination is the phenomenon where males develop from fertilized eggs but the paternal genome is heterochromatinized or lost in the somatic cells and not passed on to their offspring. This phenomenon occurs i ...
*
Sexual differentiation Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. Sex determination is often distinct from sex differentiation; sex determination is the designation for the dev ...
*
Worker policing Worker policing is a behavior seen in colonies of social hymenopterans (ants, bees, and wasps) whereby worker females eat or remove eggs that have been laid by other workers rather than those laid by a queen. Worker policing ensures that the o ...
*
X chromosome The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex ...
*
Y chromosome The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or abs ...


References


Bibliography

* * * {{DEFAULTSORT:Haplodiploid Sex-Determination System Sex-determination systems Beekeeping Hymenoptera Insect genetics