Self-incompatibility (
SI) is a general name for several genetic mechanisms in angiosperms, which prevent self-fertilization and thus encourage
outcrossingOutcrossing is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of all individuals being subject to disease or reducing genetic abnormalities...
. In plants with SI, when a
pollenPollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the...
grain produced in a plant reaches a stigma of the same plant or another plant with a similar genotype, the process of pollen
germinationGermination is the process in which a seed or spore emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However, the growth of a sporeling from a spore, for example the growth of hyphae from fungal...
, pollen tube growth,
ovuleOvule literally means "small egg." In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integuments forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
fertilization, and
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
development is halted at one of its stages, and consequently no
seedA seed , referred to as a kernel in some plants, is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
s are produced. SI is one of the most important means to prevent
selfingInbreeding is breeding between close relatives, whether plant or animal. If practiced repeatedly, it can lead to exposure of recessive, deleterious traits. This generally leads to a decreased fitness of a population, which is called inbreeding depression. Deleterious alleles causing inbreeding...
and promote the generation of new
genotypeThe genotype is the genetic constitution of a cell, an organism, or an individual usually with reference to a specific character under consideration...
s in
plantPlants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants,...
s, and it is considered as one of the causes for the spread and success of the angiosperms on the earth.
Mechanisms of self-incompatibility
The best studied mechanisms of SI act by inhibiting the germination of pollen on stigmas, or the elongation of the pollen tube in the styles. These mechanisms are based on
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
-protein interactions, each mechanism being controlled by a single
locusIn the fields of genetics and evolutionary 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...
termed
S, which has many different
alleleAn allele is one of a series of different forms of a gene. The word is a short form of allelomorph , which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes...
s in the
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
population. Despite their similar morphological and genetic manifestations, these mechanisms have evolved independently, and are based on different cellular components; therefore, each mechanism has its own, unique S-
genesA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
.
The S-locus contains two basic protein
coding regionsCoding region, depending on context, can refer to an organism or a geneThe coding region of a gene is that portion of a gene's DNA or RNA that is composed of exons....
- one expressed in the pistil, and the other in the anther and/or pollen (referred to as the
female and
male determinants, respectively). Because of their physical proximity, these are genetically
linkedGenetic linkage occurs when particular genetic loci or alleles for genes are inherited jointly. Genetic loci on the same chromosome are physically close to one another and tend to stay together during meiosis, and are thus genetically linked. This is called autosomal linkage...
, and are inherited as a unit. The units are called S-
haplotypesThe term haplotype is a contraction of the term 'haploid genotype'. In genetics, a haplotype is a combination of alleles at multiple loci that are transmitted together on the same chromosome...
. The translation products of the two regions of the S-locus are two proteins which, by interacting with one another, lead to the arrest of pollen germination and/or pollen tube elongation, and thereby generate an SI response, preventing fertilization. However, when a female determinant interacts with a male determinant of a different haplotype, no SI is created, and fertilization ensues. This is a simplistic description of the general mechanism of SI, which is more complicated, and in some species the S-haplotype contains more than two protein coding regions.
Following is a detailed description of the different known mechanisms of SI in plants.
Gametophytic self-incompatibility (GSI)
In
gametophytic self-incompatibility (GSI), the SI
phenotypeA phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions...
of the pollen is determined by its own
gametophyticIn plants and algae that undergo alternation of generations, a gametophyte is the multicellular structure, or phase, that is haploid, containing a single set of chromosomes:...
haploidPloidy is the number of complete sets of chromosomes in a biological cell. In humans, the somatic cells that compose the body are diploid , but sex cells are haploid...
genotype. This is the more common type of SI, existing in the families:
SolanaceaeThe Solanaceae is a family of flowering plants that contains a number of important agricultural plants as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...
,
RosaceaeThe Rosaceae or rose family is a large family of plants, with about 3000 species in 100 genera . The name is derived from the genus Rosa...
,
PlantaginaceaeThe Plantaginaceae Juss. or plantain family, is a family of flowering plants in the order Lamiales. The type genus is Plantago L..In older classifications it used to be the only family of the order Plantaginales, but numerous phylogenetic studies, summarized by the Angiosperm Phylogeny Group, have...
,
FabaceaeFabaceae or Leguminosae is a large and economically important family of flowering plants, which is commonly known as the legume family, pea family, bean family or pulse family. The name 'Fabaceae' comes from the defunct genus Faba, now included into Vicia...
,
OnagraceaeThe Onagraceae, also known as the Willowherb family or Evening Primrose family, is a family of flowering plants. The family includes about 640-650 species of herbs, shrubs, and trees in 20-24 genera...
,
CampanulaceaeThe family Campanulaceae , of the order Asterales, contains about 70 genera and 2000 species. They are mostly herbs, shrubs, and more rarely small trees, which usually have milky non-toxic sap...
,
PapaveraceaePapaveraceae is a family of flowering plants. The family has been universally recognized by taxonomists, and is also known as the "poppy family". It is a cosmopolitan family occurring in temperate and subtropical climates. Most are herbaceous plants, but a few are shrubs and small trees.The plants...
and
PoaceaePoaceae or Gramineae is a family in the Class Liliopsida of the flowering plants. Plants of this family are usually called grasses, or, to distinguish them from other graminoids, true grasses; the shrub- or tree-like plants in this family are called bamboo...
. Two different mechanisms of GSI have been described in detail at the molecular level, and their description follows.
The RNase mechanism
The female component of GSI in the
SolanaceaeThe Solanaceae is a family of flowering plants that contains a number of important agricultural plants as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...
was found in 1989. Proteins in the same family were subsequently discovered in the
RosaceaeThe Rosaceae or rose family is a large family of plants, with about 3000 species in 100 genera . The name is derived from the genus Rosa...
and
PlantaginaceaeThe Plantaginaceae Juss. or plantain family, is a family of flowering plants in the order Lamiales. The type genus is Plantago L..In older classifications it used to be the only family of the order Plantaginales, but numerous phylogenetic studies, summarized by the Angiosperm Phylogeny Group, have...
. Despite some early doubts about the common ancestry of GSI in these distantly related families, phylogenetic studies and the finding of shared male determinants (F-box proteins) clearly established homology. Consequently, this mechanism arose approximately 90 million years ago, and is the inferred ancestral state for approximately 50% of all plants.
In this mechanism, pollen tube elongation is halted when it has proceeded approximately one third of the way through the style. The female component
ribonucleaseRibonuclease is a type of nuclease that catalyzes the degradation of RNA into smaller components. Ribonucleases can be divided into endoribonucleases and exoribonucleases, and comprise several sub-classes within the EC 2.7 and 3.1 classes of enzymes.-Function:All organisms studied contain...
, termed
S-RNase probably causes degradation of the
ribosomalRibosomes are complexes of RNA and protein that are found in all cells. The ribosome is part of the mechanism that translates the DNA sequence into the protein sequence. Ribosomes from bacteria, archaea and eukaryotes , have significantly different structure and RNA...
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
(rRNA) inside the pollen tube, in the case of identical male and female S alleles, and consequently pollen tube elongation is arrested, and the pollen grain dies.
The male component was only recently putatively identified as a member of the "F-box" protein family. The members of this group typically function as
ubiquitin ligaseA ubiquitin ligase is a protein that in combination with an E2 Ubiquitin-conjugating enzyme causes the attachment of ubiquitin to a lysine on a target protein via an isopeptide bond; the E3 ubiquitin ligase targets specific protein substrates for degradation by the proteasome...
s, and they may function by recognizing the matching S-RNase molecules and sending them to
proteasomalProteasomes are large protein complexes inside all eukaryotes and archaea, as well as in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
degradation. Despite some fairly convincing evidence that it may be the male component, several features also make it an unlikely candidate.
The S-glycoprotein mechanism
The following mechanism was described in detail in
Papaver rhoeas. In this mechanism, pollen growth is inhibited within minutes of its placement on the stigma
The female determinant is a small, extracellular molecule, expressed in the stigma; the identity of the male determinant remains elusive, but it is probably some
cell membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
receptorIn biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or cytoplasm of a cell, to which a mobile signaling molecule may attach...
. The interaction between male and female determinants transmits a cellular
signalIn biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another. Most processes of signal transduction involve ordered sequences of biochemical reactions inside the cell, which are carried out by enzymes and activated by second messengers,...
into the pollen tube, resulting in strong influx of
calciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
cations; this interferes with the intracellular
concentrationIn chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent.To...
gradient of calcium
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s which exists inside the pollen tube, essential for its elongation. The influx of calcium ions arrests tube elongation within 1-2 minutes. At this stage, pollen inhibition is still reversible, and elongation can be resumed by applying certain manipulations, resulting in ovule fertilization.
Subsequently, the
cytosolThe cytosol or intracellular fluid is the liquid found inside cells. In eukaryotes this liquid is separated by cell membranes from the contents of the organelles suspended in the cytosol, such as the mitochondrial matrix inside the mitochondrion...
ic protein
p26, a
pyrophosphatasePyrophosphatase are acid anhydride hydrolases that act upon diphosphate bonds.Examples include:* Inorganic pyrophosphatase* Thiamine pyrophosphatase...
, is inhibited by
phosphorylationPhosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation turns many protein enzymes on and off, causing or preventing the mechanisms of diseases such as cancer and diabetes....
, possibly resulting in arrest of
synthesisBiosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
of molecular building blocks, required for tube elongation. There is
depolymerizationIn polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
and reorganization of
actinActin is a globular, roughly 42-kDa protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
filaments, within the pollen
cytoskeletonThe cytoskeleton is a cellular "scaffolding" or "skeleton" contained within the cytoplasm. The cytoskeleton is present in all cells; it was once thought this structure was unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
. Within 10 minutes from the placement on the stigma, the pollen is committed to a process which ends in its death. At 3-4 hours past pollination, fragmentation of pollen
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
begins, and finally (at 10-14 hours), the cell dies
apoptoticallyApoptosis is the process of programmed cell death that may occur in multicellular organisms. Programmed cell death involves a series of biochemical events leading to a characteristic cell morphology and death; in more specific terms, a series of biochemical events that lead to a variety of...
.
Sporophytic self-incompatibility (SSI)
In
sporophytic self-incompatibility (SSI), the SI phenotype of the pollen is determined by the diploid genotype of the anther (the
sporophyteAll land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or alga that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
) in which it was created. This form of SI was identified in the families:
BrassicaceaeBrassicaceae or Cruciferae, also known as the crucifers, the mustard family or cabbage family is a family of flowering plants . The name Brassicaceae is derived from the included genus Brassica. Cruciferae is an older name, meaning "cross-bearing", because the four petals of their flowers are...
,
AsteraceaeThe family Asteraceae or Compositae is the second largest family of flowering plants, in terms of number of species....
,
ConvolvulaceaeThe Convolvulaceae, known commonly as the bindweed or morning glory family, is a group of about 60 genera and more than 1,650 species of mostly herbaceous vines, but also trees, shrubs and herbs.- Description :...
,
BetulaceaeBetulaceae, or the Birch Family, includes six genera of deciduous nut-bearing trees and shrubs, including the birches, alders, hazels, hornbeams and hop-hornbeams, numbering about 130 species...
,
CaryophyllaceaeThe Caryophyllaceae, commonly called the pink family or carnation family, is a family of flowering plants. The species are dicotyledons included in the order Caryophyllales. It is a large family, with 88 genera and some 2,000 species....
,
SterculiaceaeSterculiaceae is a botanical name for a group of flowering plants at the rank of family, which is now widely considered obsolete. As is true for any botanical name, the circumscription, status and placement of the taxon has varied with taxonomic point of view...
and
PolemoniaceaeThe Polemoniaceae comprises 18-25 genera with between 270-400 species of mostly annual plants, native to the Northern Hemisphere and also South America, with the center of diversity in western North America, especially in California...
. Up to this day, only one mechanism of SSI has been described in detail at the molecular level, in
BrassicaBrassica is a genus of plants in the mustard family . The members of the genus may be collectively known either as cabbages, or as mustards...
(Brassicaceae).
Since SSI is determined by a diploid genotype, the pollen and pistil each express the translation products of two different alleles, i.e. two male and two female determinants. Dominance relationships often exist between pairs of alleles, resulting in complicated patterns of compatibility/self-incompatibility. These dominance relationships also allow the generation of individuals
homozygousZygosity is the similarity of genes for a trait in an organism. If both genes are the same, the organism is homozygous for the trait. If both genes are different, the organism is heterozygous for that trait...
for a recessive S allele.
Compared to a population in which all S alleles are co-dominant, the presence of dominance relationships in the population, raises the chances of compatible mating between individuals. The frequency ratio between recessive and dominant S alleles, reflects a dynamic balance between reproduction assurance (favoured by recessive alleles) and avoidance of selfing (favoured by dominant alleles).
The SI mechanism in Brassica
As previously mentioned, the SI phenotype of the pollen is determined by the diploid genotype of the anther. In
BrassicaBrassica is a genus of plants in the mustard family . The members of the genus may be collectively known either as cabbages, or as mustards...
, the pollen coat, derived from the anther's
tapetumTapetum can refer to:* Tapetum * Tapetum lucidum* Tapetum of corpus callosum...
tissueTissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function...
, carries the translation products of the two S alleles. These are small,
cysteineCysteine is an α-amino acid with the chemical formula HO
2CCHCH
2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is nonpolar and thus cysteine is usually classified as...
-rich proteins. The male determinant is termed
SCR or
SP11, and is expressed in the anther tapetum (i.e. sporophytically), as well as in the
microsporeIn biology, a microspore is a small spore as contrasted to the larger megaspore. This combination is found only in heterosporous organisms. Most plants that reproduce by spore without seed only produce one class of spore. In the ferns, the only megasporous plants are aquatic or semi-aquatic,...
and pollen (i.e. gametophytically). There are possibly up to 100 polymorphs of the S-haplotype in Brassica, and within these there is a dominance hierarchy.
The female determinant of the SI response in
Brassica, is a transmembrane protein termed
SRK, which has an intracellular
kinaseIn chemistry and biochemistry, a kinase, alternatively known as a phosphotransferase, is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates. The process is referred to as phosphorylation...
domain, and a variable extracellular domain. SRK is expressed in the stigma, and probably functions as a receptor for the SCR/SP11 protein in the pollen coat. Another stigmatic protein, termed
SLG, is highly similar in
sequenceA sequence in biology is the one-dimensional ordering of monomers, covalently linked within in a biopolymer; it is also referred to as the primary structure of the biological macromolecule.-See also:* Protein sequence* DNA sequence...
to the SRK protein, and seems to function as a
co-receptorA co-receptor is a cell surface receptor that binds a signalling molecule in addition to a primary receptor in order to facilitate ligand recognition and initiate a biological process, such as entry of a pathogen into a host cell....
for the male determinant, amplifying the SI response.
The interaction between the SRK and SCR/SP11 proteins results in autophosphorylation of the intracellular kinase domain of SRK, and a signal is transmitted into the papilla cell of the stigma. Another protein essential for the SI response is
MLPK, a
serineSerine is an organic compound with the formula HO
2CCHCH
2OH.-Occurrence:...
-
threonineThreonine is an α-amino acid with the chemical formula HO
2CCHCHCH
3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
kinase, which is anchored to the plasma membrane from its intracellular side. The downstream cellular and molecular events, leading eventually to pollen inhibition, are poorly described.
Other mechanisms of self-incompatibility
These mechanisms are less abundant and have received only limited attention in scientific research. Therefore, they are still poorly understood.
Heteromorphic self-incompatibility
A distinct SI mechanism exists in
heterostylousHeterostyly is a unique form of polymorphism and herkogamy in flowers. In a heterostylous species, two or three different morphological types of flowers, termed morphs, exist in the population. On each individual plant, all flowers share the same morph. The flower morphs differ in the lengths of...
flowers, termed
heteromorphic self-incompatibility. This mechanism is probably not
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
arily related to the more familiar mechanisms, which are differentially defined as
homomorphic self-incompatibility.
Almost all heterostylous
taxa|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a...
feature SI to some extent. The loci responsible for SI in heterostylous flowers, are strongly linked to the loci responsible for flower
polymorphismPolymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
, and these traits are inherited together. Distyly is determined by a single locus, which has two alleles; tristyly is determined by two loci, each with two alleles. Heteromorphic SI is sporophytic, i.e. both alleles in the male plant, determine the SI response in the pollen. SI loci always contain only two alleles in the population, one of which is dominant over the other, in both pollen and pistil. Variance in SI alleles parallels the variance in flower morphs, thus pollen from one morph can fertilize only pistils from the other morph. In tristylous flowers, each flower contains two types of
stamenThe stamen is the male organ of a flower. Each stamen generally has a stalk called the filament , and, on top of the filament, an anther , and pollen sacs,...
s; each stamen produces pollen capable of fertilizing only one flower morph, out of the three existing morphs.
A population of a distylous plant contains only two SI genotypes: ss and Ss. Fertilization is possible only between genotypes; each genotype cannot fertilize itself. This restriction maintains a 1:1 ratio between the two genotypes in the population; genotypes are usually randomly scattered in space. Tristylous plants contain, in addition to the S locus, the M locus, also with two alleles. The number of possible genotypes is greater here, but a 1:1 ratio exists between individuals of each SI type.
Cryptic self-incompatibility (CSI)
Cryptic self-incompatibility (CSI) exists in a limited number of taxa (for example, there is evidence for CSI in
Silene vulgarisSilene vulgaris or Bladder Campion is a plant species of the genus Silene.The young shoots are edible, but saponin gives them a bitter flavor....
,
CaryophyllaceaeThe Caryophyllaceae, commonly called the pink family or carnation family, is a family of flowering plants. The species are dicotyledons included in the order Caryophyllales. It is a large family, with 88 genera and some 2,000 species....
). In this mechanism, the simultaneous presence of cross and self pollen on the same stigma, results in higher seed set from cross pollen, relative to self pollen. However, as opposed to 'complete' or 'absolute' SI, in CSI, self-pollination without the presence of competing cross pollen, results in successive fertilization and seed set; in this way, reproduction is assured, even in the absence of cross-pollination. CSI acts, at least in some species, at the stage of pollen tube elongation, and leads to faster elongation of cross pollen tubes, relative to self pollen tubes. The cellular and molecular mechanisms of CSI have not been described.
The strength of a CSI response can be defined, as the ratio of crossed to selfed ovules, formed when equal amounts of cross and self pollen, are placed upon the stigma; in the taxa described up to this day, this ratio ranges between 3.2 and 11.5.
Late-acting self-incompatibility (LSI)
Late-acting self-incompatibility (LSI) is also termed
ovarianThe ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in females are homologous to testes in males, in that they are both gonads and endocrine glands.-Human anatomy:...
self-incompatibility (OSI). In this mechanism, self pollen germinates and reaches the ovules, but no
fruitThe term fruit has different meanings dependent on context, and the term is not synonymous in food preparation and biology. Fruits are the means by which flowering plants disseminate seeds, and the presence of seeds indicates that a structure is most likely a fruit, though not all seeds come from...
is set. LSI can be pre-
zygoticA zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development...
(e.g. deterioration of the embryo sac prior to pollen tube entry, as in
Narcissus triandrusNarcissus triandrus is a species of plant in the Amaryllis family native to southwestern Europe. The common name is "Angel's Tears", and the plant has pendulant cream to yellowish flowers.-Source:...
) or post-zygotic (malformation of the
zygoteA zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development...
or
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, as in certain species of
AsclepiasAsclepias L. , the milkweeds, is a genus of herbaceous perennial, dicotyledonous plants that contains over 140 known species...
and in
Spathodea campanulata).
The existence of the LSI mechanism among different taxa and in general, is subject for scientific debate. Criticizers claim, that absence of fruit set is due to genetic defects (homozygosity for lethal recessive alleles), which are the direct result of self-fertilization (
inbreeding depressionInbreeding depression is reduced fitness in a given population as a result of breeding of related individuals. Breeding between closely related individuals, called inbreeding, results in more recessive deleterious traits manifesting themselves. The more closely related the breeding pair is, the...
). Supporters, on the other hand, argue for the existence of several basic criteria, which differentiate certain cases of LSI from the inbreeding depression phenomenon.
Self-compatibility (SC)
SI is not universal in flowering plants. Indeed, a great many species are
self-compatible (
SC). The best estimates indicate that approximately one half of angiosperm species are SI.
Pollinator declineThe term pollinator decline refers to the reduction in abundance of pollinators in many ecosystems worldwide during the end of the twentieth century....
, variability in pollinator service, and life history traits that are associated with weediness, among other factors, may favor the loss of SI. As a result, mutations that break down SI (result in SC) may become common or entirely dominate in natural populations. Similarly, human-mediated artificial selection through
selective breedingSelective breeding is the process of breeding plants and animals for particular genetic traits. Typically, strains which are selectively bred are domesticated, and the breeding is sometimes done by a professional breeder. Bred animals are known as breeds, while bred plants are known as varieties,...
may be responsible for the commonly observed SC in cultivated plants. SC enables more efficient breeding techniques to be employed for crop improvement.
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