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Zygosaccharomyces [SEP] Barnett, J.A., Payne, R.W., Yarrow, D., 1983. Yeasts: Characteristics and Identification. Cambridge University Press, Cambridge. K.C. Fugelsang, "Zygosaccharomyces, A Spoilage Yeast Isolated from Grape Juice" Archived September 7, 2006, at the Wayback Machine

Zygosaccharomyces bailii [SEP] Zygosaccharomyces bailii vegetative cells are usually ellipsoid, non-motile and reproduced asexually by multilateral budding, i.e. the buds can arise from various sites on the cells. During the budding process, a parent cell produces a bud on its outer surface. As the bud elongates, the parent cell's nucleus divides and one nucleus migrates into the bud. Cell wall material is filled in the gap between the bud and the parent cell; eventually the bud is separated to form a daughter cell of unequal size.

Zygosaccharomyces bailii [SEP] Z. bailii cell size varies within a range of (3.5 - 6.5) x (4.5 - 11.5) μm and the cells exist singly or in pair, rarely in short chain. It has been observed that the doubling time of this yeast is approximately 3 hours at 23 °C in yeast nitrogen base broth containing 20% (w/v) fructose (pH 4.0). In more stressful conditions, this generation time is significantly extended.

Zygosaccharomyces bailii [SEP] Besides the asexual reproduction mode, under certain conditions (e.g. nutritional stress) Z. bailii produces sexual spores (ascospores) in a sac called ascus (plural: asci). Normally, each ascus contains one to four ascospores, which are generally smooth, thin-walled, spherical or ellipsoidal.

Zygosaccharomyces bailii [SEP] It should be mentioned that the ascospores are rarely observed as it is difficult and may take a long time to induce their formation; besides many yeast strains lose the ability to produce ascospores on repeated sub-cultures in the laboratory. On various nutrient agars, Z. bailii colonies are smooth, round, convex and white to cream coloured, with a diameter of 2 – 3 mm at 3 – 7 days.

Zygosaccharomyces bailii [SEP] As the morphology properties of Zygosaccharomyces are identical to other yeast genera such as Saccharomyces, Candida and Pichia, it is impossible to differentiate Zygosaccharomyces from other yeasts or individual species within the genus based on macroscopic and microscopic morphology observations. Therefore, the yeast identification to species level is more dependent on physiological and genetic characteristics than on morphological criteria.

Zygosaccharomyces bailii [SEP] In general, any glucose-containing medium is suitable for the culture and counting of yeasts, e.g. Sabouraud medium, malt extract agar (MEA), tryptone glucose yeast extract agar (TGY), yeast glucose chloramphenicol agar (YGC). For the detection of acid-resistant yeasts like Z. bailii, acidified media are recommended, such as MEA or TGY with 0.5% (v/v) acetic acid added.

Zygosaccharomyces bailii [SEP] Plating with agar media is often used for counting of yeasts, with surface spreading technique is preferable to pour plate method because the former technique gives a better recovery of cells with lower dilution errors. The common incubation conditions are aerobic atmosphere, temperature 25 °C for a period of 5 days. Nevertheless, a higher incubation temperature (30 °C) and shorter incubation time (3 days) can be applied for Z. bailii, as the yeast grows faster at this elevated temperature.

Zygosaccharomyces bailii [SEP] Among the Zygosaccharomyces spoilage species, Z. bailii possesses the most pronounced and diversified resistance characteristics, enabling it to survive and proliferate in very stressful conditions. It appears that Z. bailii prefers ecological environments characterized by high osmotic conditions. The most frequently described natural habitats are dried or fermented fruits, tree exudates (in vineyards and orchards), and at various stages of sugar refining and syrup production.

Zygosaccharomyces bailii [SEP] Besides, it is seldom to encounter Z. bailii as a major spoilage agent in unprocessed foods; usually the yeast only attains importance in processed products when the competition with bacteria and moulds is reduced by intrinsic factors such as pH, water activity (aw), preservatives, etc. An outstanding feature of Z. bailii is its exceptional resistance to weak acid preservatives commonly used in foods and beverages, such as acetic, lactic, propionic, benzoic, sorbic acids and sulfur dioxide.

Zygosaccharomyces bailii [SEP] In addition, it is reported that the yeast is able to tolerate high ethanol concentrations (≥ 15% (v/v)). The ranges of pH and aw for growth are wide, 2.0 - 7.0 and 0.80 - 0.99, respectively.

Zygosaccharomyces bailii [SEP] Besides being preservative resistant, other features that contribute to the spoilage capacity of Z. bailii are: (i) its ability to vigorously ferment hexose sugars (e.g. glucose and fructose), (ii) ability to cause spoilage from an extremely low inoculum (e.g. one viable cell per package of any size), (iii) moderate osmotolerance (in comparison to Zygosaccharomyces rouxii).

Zygosaccharomyces bailii [SEP] Therefore, foods at particular risk to spoilage by this yeast usually have low pH (2.5 to 5.0), low aw and contain sufficient amounts of fermentable sugars. The extreme acid resistance of Z. bailii has been reported by many authors.

Zygosaccharomyces bailii [SEP] On several occasions, growth of the yeast has been observed in fruit-based alcohols (pH 2.8 - 3.0, 40 - 45% (w/v) sucrose) preserved with 0.08% (w/v) benzoic acid, and in beverages (pH 3.2) containing either 0.06% (w/v) sorbic acid, 0.07% (w/v) benzoic acid, or 2% (w/v) acetic acid.

Zygosaccharomyces bailii [SEP] Notably, individual cells in any Z. bailii population differ considerably in their resistance to sorbic acid, with a small fraction able to grow in preservative levels double that of the average population. In some types of food, the yeast is even able to grow in the presence of benzoic and sorbic acids at concentrations higher than those legally permitted and at pH values below the pKₐ of the acids.

Zygosaccharomyces bailii [SEP] For example, according to the European Union (EU) legislation, sorbic acid is limited to 0.03% (w/v) in soft drinks (pH 2.5 - 3.2); however Z. bailii can grow in soft drinks containing 0.05% (w/v) of this acid (pKₐ 4.8). Particularly, there is strong evidence that the resistance of Z. bailii is stimulated by the presence of multiple preservatives. Hence, the yeast can survive and defeat synergistic preservative combinations that normally provide microbiological stability to processed foods.

Zygosaccharomyces bailii [SEP] It has been observed that the cellular acetic acid uptake was inhibited when sorbic or benzoic acid was incorporated into the culture medium. Similarly, ethanol levels up to 10% (v/v) did not adversely influence sorbic and benzoic acid resistance of the yeast at pH 4.0 - 5.0. Moreover, Sousa et al. ( 1996) have proved that in Z. bailii, ethanol plays a protective role against the negative effect of acetic acid by inhibiting the transport and accumulation of this acid intracellularly.

Zygosaccharomyces bailii [SEP] Like other microorganisms, Z. bailii has the ability to adapt to sub-inhibitory levels of a preservative, which enables the yeast to survive and grow in much higher concentrations of the preservative than before adaptation. In addition, it seems that Z. bailii resistance to acetic, benzoic and propionic acid is strongly correlated, as the cells which were adapted to benzoic acid also showed enhanced tolerances to other the preservatives.

Zygosaccharomyces bailii [SEP] Some studies have revealed the negligible effects of different sugars on preservative resistance of Z. bailii, e.g. comparable sorbic and benzoic acid resistance was observed regardless whether the cells were grown in culture medium containing glucose or fructose as fermentable substrates. However, the preservative resistance of the yeast is influenced by glucose level, with maximum resistance obtained at 10 - 20% (w/v) sugar concentrations. As Z. bailii is moderately osmotolerant, the salt and sugar levels in foods are usually insufficient to control its growth.

Zygosaccharomyces bailii [SEP] The highest tolerance to salt has been observed at low pH values, e.g. the maximum NaCl allowing growth was 12.5% (w/v) at pH 3.0 whereas this was only 5.0% (w/v) at pH 5.0.

Zygosaccharomyces bailii [SEP] Moreover, the presence of either salt or sugar has a positive effect on the ability of Z. bailii to initiate growth at extreme pH levels, e.g. the yeast showed no growth at pH 2.0 in the absence of NaCl and sucrose, but grew at this pH in 2.5% (w/v) NaCl or 50% (w/v) sucrose. Most facultatively fermentative yeast species cannot grow in the complete absence of oxygen.

Zygosaccharomyces bailii [SEP] That means limitation of oxygen availability might be useful in controlling food spoilage caused by fermentative yeasts. However, it has been observed that Z. bailii is able to grow rapidly and ferment sugar vigorously in a complex medium under strictly anaerobic condition, indicating that the nutritional requirement for anaerobic growth was met by the complex-medium components. Therefore, restriction of oxygen entry into foods and beverages, which are rich in nutrients, is not a promising strategy to prevent the risk of spoilage by this yeast.

Zygosaccharomyces bailii [SEP] Besides, Leyva et al. ( 1999) have reported that Z. bailii cells can retain their spoilage capability by producing a significant amount of gas even in non-growing conditions (i.e. presence of sugars but absence of nitrogen source).

Zygosaccharomyces bailii [SEP] Different strategies have been suggested in accounting for Z. bailii resistance to weak acid preservatives, which include: (i) degradation of the acids, (ii) prevention of entry or removal of acids from the cells, (iii) alteration of the inhibitor target, or amelioration of the caused damage. Particularly, the intrinsic resistance mechanisms of Z. bailii are extremely adaptable and robust. Their functionality and effectiveness are unaffected or marginally suppressed by environmental conditions such as low pH, low aw and limited nutrients.

Zygosaccharomyces bailii [SEP] For a long time, it has been known that Z. bailii can maintain an acid gradient across the cell membrane, which indicates the induction of a system whereby the cells can reduce the intracellular acid accumulation. According to Warth (1977), Z. bailii uses an inducible, active transport pump to expel acid anions from the cells for counteracting the toxic effects of the acids. As the pump requires energy to function optimally, high sugar levels enhance Z. bailii preservative resistance.

Zygosaccharomyces bailii [SEP] Nevertheless, this view was disputed from an observation that the concentration of acid was exactly as predicted from the intracellular, extracellular pH's and pKₐ of the acid. Besides, it is unlikely that an active acid extrusion alone would be sufficient to achieve an unequal acid distribution across the cell membrane. Instead, Z. bailii might have developed much more efficient ways of altering its cell membrane to limit the diffusional entry of acids into the cells.

Zygosaccharomyces bailii [SEP] This, in turn, will dramatically reduce any need for active extrusion of protons and acid anions, thus saving a lot of energy. Indeed, Warth (1989) has reported that the uptake rate of propionic acid by diffusion in Z. bailii is much lower than in other acid-sensitive yeasts (e.g. Saccharomyces cerevisiae). Hence, it is conceivable that Z. bailii puts more effort on limiting the influx of acids in order to enhance its acid resistance.

Zygosaccharomyces bailii [SEP] Another mechanism of Z. bailii to deal with acid challenge is that the yeast uses a plasma membrane H⁺-adenosine triphosphatase (H⁺-ATPase) to expel proton from cells, thereby preventing intracellular acidification. In addition, Cole and Keenan (1987) have suggested that Z. bailii resistance includes an ability to tolerate chronic intracellular pH drops. Besides, the fact that the yeast is able to metabolize preservatives may also contribute to its acid tolerance.

Zygosaccharomyces bailii [SEP] Regarding the resistance of Z. bailii to SO₂, it has been proposed that the cells reduce the concentration of SO₂ by producing extracellular sulphite-binding compounds such as acetaldehyde. The fructophilic behaviour is well known in Z. bailii. Unlike most of other yeasts, Z. bailii metabolizes fructose more rapidly than glucose and grows much faster in foods containing ≥ 1% (w/w) of fructose.

Zygosaccharomyces bailii [SEP] In addition, it has been observed that the alcoholic fermentation under aerobic conditions (the Crabtree effect) in Z. bailii is influenced by the carbon source, i.e. ethanol is produced at a higher rate and with a higher yield on fructose than on glucose. This is because in Z. bailii, fructose is transported by a specific high-capacity system, while glucose is transported by a lower-capacity system, which is partially inactivated by fructose and also accepts fructose as a substrate.

Zygosaccharomyces bailii [SEP] The slow fermentation of sucrose is directly related to fructose metabolism. According to Pitt and Hocking (1997), Z. bailii cannot grow in foods with sucrose as the sole carbon source. As it requires time to hydrolyze sucrose into glucose and fructose (in low pH conditions), there is a long delay between manufacture and spoilage of products contaminated with this yeast when sucrose is used as the primary carbohydrate ingredient.

Zygosaccharomyces bailii [SEP] This is usually preceded by a lag of 2 – 4 weeks and apparent deterioration of product quality is only shown 2 – 3 months after manufacturing Therefore, the use of sucrose as a sweetener (instead of glucose or fructose) is highly recommended in synthetic products such as soft drinks. Fermentation of sugars (e.g. glucose, fructose and sucrose) is a key metabolic reaction of most yeasts (including Z. bailii) when cultured under facultative anaerobic conditions.

Zygosaccharomyces bailii [SEP] As sugars are common components of foods and beverages, fermentation is a typical feature of the spoilage process. Principally, these sugars are converted to ethanol and CO₂, causing the products to lose sweetness and acquire a distinctive alcoholic aroma along with gassiness. Besides, many secondary products are formed in small amounts, such as organic acids, esters, aldehydes, etc. Z. bailii is noted for its strong production of secondary metabolites, e.g. acetic acid, ethyl acetate and acetaldehyde.

Zygosaccharomyces bailii [SEP] In high enough concentrations, these substances can have a dominant effect on the sensorial quality of the products. The higher resistance of Z. bailii to weak acids than S. cerevisiae can partly be explained by its ability to metabolize preservatives. It has been demonstrated that Z. bailii is able to consume acetic acid in the presence of fermentable sugars, whereas the acetate uptake and utilization systems of S. cerevisiae are all glucose-repressed.

Zygosaccharomyces bailii [SEP] In addition, Z. bailii can also oxidatively degrade sorbate and benzoate (and use these compounds as a sole carbon source), while S. cerevisiae does not have this capability. According to Thomas and Davenport (1985), early reports of spoilage in mayonnaise and salad dressing due to Z. bailii date back to the beginning of the 20th century. More detailed investigations in the 1940s and 1950s confirmed that Z. bailii was the main spoiler in cucumber pickles, sundry pickled vegetable mixes, acidified sauces, etc.

Zygosaccharomyces bailii [SEP] Around the same time, fermentation spoilage incidents occasionally appeared in fruit syrups and beverages preserved with moderate benzoic acid levels (0.04 - 0.05% (w/w)). Again, Z. bailii was identified as the spoilage source. Nowadays, despite great improvements in formulation control, food processing equipment and sanitation technologies (e.g. automated clean-in-place), the yeast remains highly problematic in sauces, acidified foods, pickled or brined vegetables, fruit concentrates and various non-carbonated fruit drinks.

Zygosaccharomyces bailii [SEP] Z. bailii is also well recognized as one of the main spoilers in wines due to its high resistance to combinations of ethanol and organic acids at low pH. Furthermore, the spoilage by this yeast has been expanding into new food categories such as prepared mustards, fruit-flavoured carbonated soft drinks containing citrus, apple and grape juice concentrates. The ability of Z. bailii in spoiling a wide range of foods is a reflection of its high resistance to many stress factors.

Zygosaccharomyces bailii [SEP] Therefore, it has been included in the list of most dangerous spoilage yeasts by several authors. Spoilage by Z. bailii often occurs in acidic shelf-stable foods, which rely upon the combined effects of acidity (e.g. vinegar), salt and sugar to suppress microbial growth.

Zygosaccharomyces bailii [SEP] The spoiled foods usually display sensorial changes that can be easily recognized by consumers, thus resulting in significant economic losses due to consumers' complaints or product recalls Observable signs of spoilage include product leakage from containers, colour change, emission of unpleasant yeasty odours, emulsion separation (in mayonnaises, dressings), turbidity, flocculation or sediment formation (in wines, beverages) and visible colonies or brown film development on product surfaces.

Zygosaccharomyces bailii [SEP] The specific off-flavour that has been attributed to Z. bailii is related to H₂S. In addition, the taste of spoiled foods can be modified by the production of acetic acid and fruity esters. It has been reported that growth of Z. bailii also results in significant gas and ethanol formation, causing a typical alcoholic taste. The excessive gas production is a direct consequence of high fermentable ability of this yeast and in more solid food, gas bubbles can appear within the product.

Zygosaccharomyces bailii [SEP] Under extreme circumstances, the produced gas pressure inside glass jars or bottles can reach such a level that explosions may take place, creating an additional hazard of injuries from broken glass. It should be mentioned that in general, detectable spoilage by yeasts requires the presence of a high number of cells, approximately 5 - 6 log CFU/ml. Apart from spoiling foods, as a direct consequent of growth, Z. bailii can modify the product texture and composition such that it may be more readily colonized by other spoilage microorganisms.

Zygosaccharomyces bailii [SEP] For example, by utilizing acetic acid, the yeast can raise the pH of pickles sufficiently to allow the growth of less acid-tolerant bacteria. Besides, as with other yeasts, the concentration of fermentable sugar in a product affects the rate of spoilage by Z. bailii, e.g. the yeast grows faster in the presence of 10% (w/w) than 1% (w/w) glucose.

Zygosaccharomyces bailii [SEP] Particularly, Z. bailii can grow and cause spoilage from extremely low inocula, as few as one viable cell in ≥ 10 liters of beverages. That means detection of low numbers of yeast cells in a product does not guarantee its stability. No sanitation or microbiological quality control program can cope with this degree of risk. Hence, the only alternatives would be reformulation of food to increase the stability and/or application of high-lethality thermal-processing parameters. Yeast in winemaking Zygosaccharomyces Lindner P (1895).

Zygosaccharomyces bailii [SEP] Mikroskopische Betriebskontrolle in den Gärungsgewerben mit einer Einführung in die Hefenreinkultur, Infektionslehre und Hefenkunde. Berlin: P. Parey. Barnett JA, Payne RW, Yarrow D (1983). Yeasts: Characteristics and Identification. Cambridge: Cambridge University Press. James SA, Stratford M (2003). " Spoilage yeasts with emphasis on the genus Zygosaccharomyces". In Boekhout T, Robert V (eds.). Yeasts in food - Beneficial and detrimental aspects. Cambridge: Woodhead Publishing Ltd. and CRC Press. pp.

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Zygosepalum [SEP] Species accepted by the Plants of the World Online as of 2022: Zygosepalum angustilabium (C.Schweinf.) Garay Zygosepalum ballii (Rolfe) Garay Zygosepalum kegelii (Rchb.f.) Rchb.f. Zygosepalum labiosum (Rich.) C.Schweinf. Zygosepalum lindeniae (Rolfe) Garay & Dunst. Zygosepalum marginatum Garay Zygosepalum revolutum Garay & G.A.Romero Zygosepalum tatei (Ames & C.Schweinf.) Garay & Dunst. List of Orchidaceae genera "Zygosepalum". Plants of the World Online. Royal Botanic Gardens Kews. 2022.

Zygosepalum [SEP] Retrieved 7 January 2022. Pridgeon, A.M., Cribb, P.J., Chase, M.A. & Rasmussen, F. eds. ( 1999). Genera Orchidacearum 1. Oxford Univ. Press. Pridgeon, A.M., Cribb, P.J., Chase, M.A. & Rasmussen, F. eds. ( 2001). Genera Orchidacearum 2. Oxford Univ. Press. Pridgeon, A.M., Cribb, P.J., Chase, M.A. & Rasmussen, F. eds. ( 2003). Genera Orchidacearum 3. Oxford Univ.

Zygosepalum [SEP] Press Berg Pana, H. 2005. Handbuch der Orchideen-Namen. Dictionary of Orchid Names. Dizionario dei nomi delle orchidee. Ulmer, Stuttgart Data related to Zygosepalum at Wikispecies Media related to Zygosepalum at Wikimedia Commons

Zygosepalum labiosum [SEP] Zygosepalum labiosum has scandent rhizomes with ovoid pseudobulbs. Its leaves are 25 cm (9.8 in) long. The orchid's inflorescence is up to 20 cm (7.9 in) long with one to three flowers. The flowers are up to 10 cm (3.9 in) in width, with greenish sepals and petals with red markings at their base. The lip is white with a violet callus and violet veins. Peter Rivière, ed. ( 2006).

Zygosepalum labiosum [SEP] The Guiana Travels of Robert Schomburgk, 1835-1844: Explorations on behalf of the Royal Geographical Society, 1835-1839. Hakluyt Society Series. 16 (abridged, illustrated ed.). Ashgate Publishing, Ltd. p. 30. ISBN 9780904180862. I. F. La Croix (2008). The New Encyclopedia of Orchids: 1500 Species in Cultivation (illustrated ed.). Timber Press. p. 498. ISBN 9780881928761.

Zygosity [SEP] The words homozygous, heterozygous, and hemizygous are used to describe the genotype of a diploid organism at a single locus on the DNA. Homozygous describes a genotype consisting of two identical alleles at a given locus, heterozygous describes a genotype consisting of two different alleles at a locus, hemizygous describes a genotype consisting of only a single copy of a particular gene in an otherwise diploid organism, and nullizygous refers to an otherwise-diploid organism in which both copies of the gene are missing.

Zygosity [SEP] A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. An individual that is homozygous-dominant for a particular trait carries two copies of the allele that codes for the dominant trait. This allele, often called the "dominant allele", is normally represented by the uppercase form of the letter used for the corresponding recessive trait (such as "P" for the dominant allele producing purple flowers in pea plants).

Zygosity [SEP] When an organism is homozygous-dominant for a particular trait, its genotype is represented by a doubling of the symbol for that trait, such as "PP". An individual that is homozygous-recessive for a particular trait carries two copies of the allele that codes for the recessive trait.

Zygosity [SEP] This allele, often called the "recessive allele", is usually represented by the lowercase form of the letter used for the corresponding dominant trait (such as, with reference to the example above, "p" for the recessive allele producing white flowers in pea plants). The genotype of an organism that is homozygous-recessive for a particular trait is represented by a doubling of the appropriate letter, such as "pp".

Zygosity [SEP] A diploid organism is heterozygous at a gene locus when its cells contain two different alleles (one wild-type allele and one mutant allele) of a gene. The cell or organism is called a heterozygote specifically for the allele in question, and therefore, heterozygosity refers to a specific genotype. Heterozygous genotypes are represented by an uppercase letter (representing the dominant/wild-type allele) and a lowercase letter (representing the recessive/mutant allele), as in "Rr" or "Ss".

Zygosity [SEP] Alternatively, a heterozygote for gene "R" is assumed to be "Rr". The uppercase letter is usually written first. If the trait in question is determined by simple (complete) dominance, a heterozygote will express only the trait coded by the dominant allele, and the trait coded by the recessive allele will not be present. In more complex dominance schemes the results of heterozygosity can be more complex.

Zygosity [SEP] A heterozygous genotype can have a higher relative fitness than either the homozygous dominant or homozygous recessive genotype – this is called a heterozygote advantage. A chromosome in a diploid organism is hemizygous when only one copy is present. The cell or organism is called a hemizygote. Hemizygosity is also observed when one copy of a gene is deleted, or, in the heterogametic sex, when a gene is located on a sex chromosome. Hemizygosity is not the same as haploinsufficiency, which describes a mechanism for producing a phenotype.

Zygosity [SEP] For organisms in which the male is heterogametic, such as humans, almost all X-linked genes are hemizygous in males with normal chromosomes, because they have only one X chromosome and few of the same genes are on the Y chromosome. Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are also considered to be hemizygous, because the introduced allele is expected to be incorporated into only one copy of any locus.

Zygosity [SEP] A transgenic individual can later be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotype of each individual. In cultured mammalian cells, such as the Chinese hamster ovary cell line, a number of genetic loci are present in a functional hemizygous state, due to mutations or deletions in the other alleles. A nullizygous organism carries two mutant alleles for the same gene.

Zygosity [SEP] The mutant alleles are both complete loss-of-function or 'null' alleles, so homozygous null and nullizygous are synonymous. The mutant cell or organism is called a nullizygote. Zygosity may also refer to the origin(s) of the alleles in a genotype. When the two alleles at a locus originate from a common ancestor by way of nonrandom mating (inbreeding), the genotype is said to be autozygous. This is also known as being "identical by descent", or IBD.

Zygosity [SEP] When the two alleles come from different sources (at least to the extent that the descent can be traced), the genotype is called allozygous. This is known as being "identical by state", or IBS. Because the alleles of autozygous genotypes come from the same source, they are always homozygous, but allozygous genotypes may be homozygous too. Heterozygous genotypes are often, but not necessarily, allozygous because different alleles may have arisen by mutation some time after a common origin.

Zygosity [SEP] Hemizygous and nullizygous genotypes do not contain enough alleles to allow for comparison of sources, so this classification is irrelevant for them. As discussed above, "zygosity" can be used in the context of a specific genetic locus (example). The word zygosity may also be used to describe the genetic similarity or dissimilarity of twins. Identical twins are monozygotic, meaning that they develop from one zygote that splits and forms two embryos.

Zygosity [SEP] Fraternal twins are dizygotic because they develop from two separate Oocytes (egg cells) that are fertilized by two separate sperm. Sesquizygotic twins are halfway between monozygotic and dizygotic and are believed to arise after two sperm fertilize a single oocyte which subsequently splits into two morula. In population genetics, the concept of heterozygosity is commonly extended to refer to the population as a whole, i.e., the fraction of individuals in a population that are heterozygous for a particular locus.

Zygosity [SEP] It can also refer to the fraction of loci within an individual that are heterozygous. Typically, the observed (H_{o}) and expected (H_{e}) heterozygosities are compared, defined as follows for diploid individuals in a population: Observed H_{o}={\frac {\sum \limits _{{i=1}}^{{n}}{(1\ {\textrm {if}}\ a_{{i1}}\neq a_{{i2}})}}{n}} where n is the number of individuals in the population, and a_{{i1}},a_{{i2}} are the alleles of individual i at the target locus.

Zygosity [SEP] Expected H_{e}=1-\sum \limits _{{i=1}}^{{m}}{(f_{i})^{2}} where m is the number of alleles at the target locus, and f_{i} is the allele frequency of the i^{th} allele at the target locus. Heterosis Heterozygote advantage Loss of heterozygosity Nucleotide diversity measures polymorphisms on the level of nucleotides rather than on level of loci.

Zygosity [SEP] Pseudolinkage Runs of Homozygosity (ROH) Carr, Martin; Cotton, Samuel; Rogers, David W; Pomiankowski, Andrew; Smith, Hazel; Fowler, Kevin (2006). " Assigning sex to pre-adult stalk-eyed flies using genital disc morphology and X chromosome zygosity". BMC Developmental Biology. Springer Nature. 6 (1): 29. doi:10.1186/1471-213x-6-29. ISSN 1471-213X. PMC 1524940. PMID 16780578. Lawrence, Eleanor (2008).

Zygosity [SEP] Henderson's Dictionary of Biology (14th ed.). Lodish, Harvey; et al. ( 2000). " Chapter 8: Mutations: Types and Causes". Molecular Cell Biology (4th ed.). W. H. Freeman. ISBN 9780716731368. Gupta, Radhey S.; Chan, David Y.H.; Siminovitch, Louis (1978). " Evidence for functional hemizygosity at the Emtr locus in CHO cells through segregation analysis". Cell. Elsevier BV.

Zygosity [SEP] 14 (4): 1007–1013. doi:10.1016/0092-8674(78)90354-9. ISSN 0092-8674. PMID 688393. S2CID 46331900. Pujol, C.; Messer, S. A.; Pfaller, M.; Soll, D. R. (2003-04-01). " Drug Resistance Is Not Directly Affected by Mating Type Locus Zygosity in Candida albicans". Antimicrobial Agents and Chemotherapy. American Society for Microbiology. 47 (4): 1207–1212. doi:10.1128/aac.47.4.1207-1212.2003. ISSN 0066-4804.

Zygosity [SEP] PMC 152535. PMID 12654648. Strachan, Tom; Read, Andrew P. (1999). " Chapter 17". Human Molecular Genetics (2nd ed.). Gabbett MT, Laporte J, Sekar R, et al. Molecular support for heterogonesis resulting in sesquizygotic twinning. N Engl J Med. 2019;380(9):842‐849. https://www.nejm.org/doi/full/10.1056/NEJMoa1701313 López Herráez, David; Bauchet, Marc; Tang, Kun; Theunert, Christoph; Pugach, Irina; Li, Jing; et al. (

Zygosity [SEP] 2009-11-18). Hawks, John (ed.). " Genetic Variation and Recent Positive Selection in Worldwide Human Populations: Evidence from Nearly 1 Million SNPs". PLOS ONE. Public Library of Science (PLoS). 4 (11): e7888. Bibcode:2009PLoSO...4.7888L. doi:10.1371/journal.pone.0007888. ISSN 1932-6203. PMC 2775638. PMID 19924308. Media related to Zygosity at Wikimedia Commons

Zygostates [SEP] Kew World Checklist of Selected Plant Families

Zygote [SEP] In fungi, the sexual fusion of haploid cells is called karyogamy. The result of karyogamy is the formation of a diploid cell called the zygote or zygospore. This cell may then enter meiosis or mitosis depending on the life cycle of the species. In plants, the zygote may be polyploid if fertilization occurs between meiotically unreduced gametes. In land plants, the zygote is formed within a chamber called the archegonium.

Zygote [SEP] In seedless plants, the archegonium is usually flask-shaped, with a long hollow neck through which the sperm cell enters. As the zygote divides and grows, it does so inside the archegonium. In human fertilization, a released ovum (a haploid secondary oocyte with replicate chromosome copies) and a haploid sperm cell (male gamete)—combine to form a single 2n diploid cell called the zygote.

Zygote [SEP] Once the single sperm fuses with the oocyte, the latter completes the division of the second meiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the male pronucleus. The other product of meiosis is the second polar body with only chromosomes but no ability to replicate or survive. In the fertilized daughter, DNA is then replicated in the two separate pronuclei derived from the sperm and ovum, making the zygote's chromosome number temporarily 4n diploid.

Zygote [SEP] After approximately 30 hours from the time of fertilization, a fusion of the pronuclei and immediate mitotic division produce two 2n diploid daughter cells called blastomeres. Between the stages of fertilization and implantation, the developing embryo is sometimes termed as a preimplantation-conceptus. This stage has also been referred to as the pre-embryo in legal discourses including relevance to the use of embryonic stem cells. In the US the National Institutes of Health has determined that the traditional classification of pre-implantation embryo is still correct.

Zygote [SEP] After fertilization, the conceptus travels down the fallopian tube towards the uterus while continuing to divide without actually increasing in size, in a process called cleavage. After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula. Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation.

Zygote [SEP] When the blastocyst hatches from the zona pellucida, it can implant in the endometrial lining of the uterus and begin the gastrulation stage of embryonic development. The human zygote has been genetically edited in experiments designed to cure inherited diseases. The formation of a totipotent zygote with the potential to produce a whole organism depends on epigenetic reprogramming. DNA demethylation of the paternal genome in the zygote appears to be an important part of epigenetic reprogramming.

Zygote [SEP] In the paternal genome of the mouse, demethylation of DNA, particularly at sites of methylated cytosines, is likely a key process in establishing totipotency. Demethylation involves the processes of base excision repair and possibly other DNA- repair- based mechanisms. A Chlamydomonas zygote contains chloroplast DNA (cpDNA) from both parents; such cells are generally rare, since normally cpDNA is inherited uniparentally from the mt+ mating type parent. These rare biparental zygotes allowed mapping of chloroplast genes by recombination.