REPRODUCTION

• germ plasm (obs.) : Weismann's term for the reproductive and hereditary substance of individuals which is passed on from the germ cell in which an individual originates in direct continuity to the germ cells of succeeding generations. By it new individuals are produced and hereditary characters are transmitted
• somatoplasm : the protoplasm of the body cells as distinguished from that of the germ cells

A unicellular species  may reproduce itself by ...

• agamic, asexual or vegetative reproduction (though mitosis : no meiosis => no germinal cells or gametes) produces individuals identical among them.
• binary fission along longitudinal or transverse axis (in Bacteria and Protozoa)
• endodiogeny : reproduction by the formation of 2 daughter cells within the wall of the mother cell (internal budding), the progeny being released by rupture of the mother cell, as in the protozoan Toxoplasma.

• schizogony (see below)
• budding (see below)
• when strands of Caulerpa taxifolia break off to form separate organisms, the parent heals itself with a fast-acting natural glue. This helps the algae to spread rapidly, but might also prove an Achilles heel, aiding the ecologists fighting to banish it from the Mediterranean Sea. Each alga is a single, supersized cell up to several metres in length. The cell looks like a fern, with central stems leading to finer fronds. When a frond is chopped off, by a passing power boat, for example, this leaves 2 new organisms. But it also leaves an injury a few millimetres wide in the cell. The algae survive this traumatic birth as their healing glue works a little like commercial epoxy resins, in which 2 chemicals are mixed to make a reactive adhesive that binds and sets very quickly. When C. taxifolia is wounded, a chemical inside the cell called caulerpenin is broken down by an enzyme to produce a highly reactive molecule called oxytoxin 2. This immediately binds together proteins and their components, forming a gummy gel inside the cell that can plug the reproductive rent in just 30 seconds. Within an hour, the plug hardens into protective scar tissue. Oxytoxin 2 itself is so reactive that the algae cannot keep a store of it : it's not stable enough and it's just too aggressive against proteins, explaining why a fresh batch is made every time the algae need it. But researchers don't know why doesn't C. taxifolia's healing process run rampant throughout the cell, turning its insides to jelly, but is rather localized around the wound. Algae are mostly water, but caulerpenin makes up about 10% of the rest of C. taxifolia's mass. Although not particularly toxic, the chemical seems to deter creatures such as sea urchins from eating the fronds. Until now, the algae's high concentrations of the chemical had been a mystery. The discovery could prove useful in combating a strain of the algae that invaded the Mediterranean in 1984 and has been an environmental headache ever since^ref. It smothers sea grasses, changing the local habitat and depriving fish of food. A similar plague off the California coast was tackled with aggressive chemical treatments, but these often kill all life in the area. Around Italy and France it is spreading a lot : nothing is being done to tackle the problem in much of the Mediterranean. The chemical lysine can mop up all the caulerpenin, preventing Caulerpa's wounds from healing., but to actually supply lysine in the natural habitat would be really tricky
• gamic or sexual reproduction (through meiosis => cells act as gametocytes => gametes : from a morphological point of view gametes may be identical (isogametes => isogamy) or different (anisogametocytes (microgametocyte and macrogametocyte) => anisogametes (microgamete and macrogamete) => anisogamy).
• conjugation (in Protozoa) : macronucleus degenerates while micronucleus undergoes mitosis and results in 2 a micronuclei. When 2 cells come together, a cytoplasmic bridge allows exchage of 1 micronucleus. The 2 heterogeneous micronuclei then fuse themselves in each cell.
• heterothallism : a form of sexual reproduction in which the isogamete must fuse with a gamete formed by a cell of a different mating type, as in various algae and fungi.
• syngamy (see below)
• metagenesis (see below)

According to the longest stage in life cycle, multicellular organisms may be classified as :

• aplonts or aplobionts : organisms in which zygotic or initial meiosis occurs (e.g. : Protista,Fungi)
• diplonts or diplobionts : organisms in which gametic or terminal meiosis occurs (e.g. : Metazoa)
• aplodiplonts or aplodiplobionts : organisms in which a diploid micro- (female) or macro- (male) sporophytus or diplophyte undergoes sporophitic or intermediate meiosis to produce spores that then will become aploid gametophitus (e.g. : Plantae)
A multicellular species can reproduce itself by ...
• direct, nonsexual, agamic, asexual or vegetative generation or reproduction (though mitosis : no meiosis => no germinal cells or gametes => no union of sexual elements) produce individuals identical among them ...
• ... and identical to the gender :
• schizogony or merogony or sporulation or endogenesis or  or multiple fission or fragmentation : dividing cell is called schyzont while daughter cells are called merozoites.
• blastogony or budding : one or more nucleus-containing bud arises from cell surface in a exocytosis-like manner. After schizolysis, a bud scar remains on the mother cell, while a birth scar remains on the daughter cell.
• apogamy : reproduction without conjugation of gametes and usually without meiosis, as in certain seed plants
• apomixis / apomixia : asexual reproduction in a species normally reproducing sexually, as in certain seed plants. It is the process of asexual reproduction through seed, in the absence of meiosis and fertilization, generating clonal progeny of maternal origin. Major benefits to agriculture could result from harnessing apomixis in crop plants. Although >400 apomictic plant species are known, apomixis is rare among crop plants, and the transfer of apomixis to crop varieties by conventional breeding has been largely unsuccessful. Because apomictic and sexual pathways are closely related, de novo engineering of apomixis might be achieved in sexually reproducing crops. Early consideration of issues relating to biosafety and intellectual property (IP) management can facilitate the acceptance and deployment of apomixis technology in agriculture
• ... but different from the gender(s) :
• polyembryonia (monozygotic twins) : Identical twins have identical genes, because they are born of an embryo that splits at a very early stage in development. Such twins are often indistinguishable in outward appearance. But over the course of time they may experience radically dissimilar health. The expression of their genes gets more and more different with age. Most scientists have assumed that environmental and lifestyle differences cause such divergence. These things trigger chemical reactions that affect our DNA and histones. Most people had the hypothesis that changes in DNA methylation are effected by the environment : this is the first time that somebody has demonstrated that this is the case (Fraga M 2005). To assess the variability in how genes are expressed between identical twins, the researchers studied genetic material from 40 pairs, ranging in age from 3 to 74 years old, and then assessed the amount of methylation of the DNA. The resulting computer-generated images highlights areas with significant differences in methylation. The degree of chemical modification of DNA and its accompanying histones varied significantly between twins in a third of the pairs overall. But the older the twins were, the greater was the variability. Among the participants older than 28 years, chemical modification of DNA was significantly different in > 60% of twin pairs. Such changes can easily affect susceptibility to disease. The team identified one case in which a man with diabetes had an associated gene activated by a chemical change, where his healthy twin did not. The more time twins had spent apart, the more their patterns of gene activation differed. This further supports the notion that environmental factors exert a strong influence on genetic expression. Similar studies might be helpful in weeding out how much chemical influences on our genes affect risk factors for diseases. This could be the way to identify the role of the methylation of genes

Web resources :
• Epigenetics
• Twin Research
• gamic or sexual generation or reproduction through meiosis => more than one kind of aploid germinal cells or gametes separated by the remaining diploid somatic cells (i.e. : only in multicellular Eukarya) => copulation : sexual union between male and female; the act transferring the sperm from male to female; used particularly for nonhuman animals => production of a new individual (organism) by the union of male and female gametes. Teleomorph : sexual stage in reproduction in which cells are formed by the process of meiosis and genetic recombination. From a morphological point of view gametes may be identical (isogametes => isogamy) or different (anisogametes : microgamete and macrogamete => anisogamy). It allows a further mechanism of intraspecies variability (apolegamy : selection, especially sexual selection in breeding). If the 2 sexes have a different phenotype they are called diecious, otherwise monoecious.
• gonochorism : individuals different in form (sexual dimorphism) produce the 2 types of gametes (spermatocyte in the male sex, oocyte or egg in the female sex) (e.g. dioecious Plantae, Homo sapiens), whose one is more mobile and more represented and the other less mobile and less represented.
• monovulatory species : a species of animal, the females of which usually discharge only one ovum in any one ovulatory cycle.
• diovulatory species : a species of animal, the females of which ordinarily discharge 2 ova in one ovulatory cycle
• polyovulatory species : a species of animal, the females of which normally discharge several (3–16) ova at each ovulatory cycle.
From a genetic point of view chromosomes may be divided into :
• autosomes (A)
• hetero(chromo)somes or sex chromosomes (indicated with X and Y in Homo, or with W and Y in birds). Full-text original papers on sex chromosomes.
• Ornithorhynchus anatinus has 5 pairs of sex chromosomes, the largest number found in mammals so far, and also hints that the sex determination systems of birds and mammals may be linked. The platypus is native to Australia, and belongs to a primitive group of mammals called the monotremes, along with only two other surviving species: the long-beaked and short-beaked echidnas. Monotremes were the first group to branch off after mammals evolved 210 million years ago. Their egg-laying shares a common origin with birds and reptiles, although the bill is thought to have evolved independently. The platypus has 26 pairs of chromosomes in total, compared with the 23 pairs present in humans. But researchers had long been confused about which ones are autosomal (inherited equally by males and females), and which ones determine sex. One of the platypus's sex chromosome pairs contains units similar to X and Y in mammals, but another resembles the ZZ/ZW sex chromosome system found in birds. Bird and mammal sex chromosomes both evolved from autosomal chromosomes. But they evolved from different pairs in the two classes, so scientists had believed the systems arose separately. But because the platypus system contains elements of both, it is possible that the two systems are related. The result challenges the accepted view that mammal and bird sex chromosomes evolved independently^ref. They now plan to study the two echidna species to see if they use the same system. So what is the advantage of having so many sex chromosomes? It's hard to speculate on how that could have evolved.
• the Y chromosome plays a crucial role in determining male sex in humans. The DNA that makes up the chromosome is highly repetitive, making it very difficult to sequence. But in June 2003, researchers in the USA announced that they had done so, and had found 78 genes, including several involved in sperm production. In 2005 a region of the chromosome that originally went undetected was found. Then they initially compared the physical map of the chromosome with the cloned sequence, the sequence didn't seem to be long enough. The new section spans just 0.5 million bp of DNA (interchromosomal segmental duplications of the pericentromeric region) : that's about 2% of the chromosome that was not detected before. The region contains 8 sequences that appear to be new genes to check for functionality. But after comparing these sequences with others, they may be involved in determining men's height, and may also provide clues about cancers of male sex organs^ref. The human Y appeared 300 million years ago and has since eroded into a dinky chromosome, because it lacks the mechanism other chromosomes have to get rid of damaged DNA. So mutations have disabled hundreds of its original genes, causing them to be shed as useless. The Y now contains only 27 genes or families of virtually identical genes. In 2003, Page reported that the modern-day Y has an unusual mechanism to fix about half of its genes and protect them from disappearing. But some scientists disagreed with his conclusion. The new paper focuses on a region of the Y chromosome where genes cannot be fixed that way. The only real question is when, not if, the Y chromosome disappears. The Y chromosome has already disappeared in some other animals and there's no reason to expect it can't happen to humans. If it happened in people, some other chromosome would probably take over the sex-determining role of the Y. The human Y chromosome, transmitted clonally through males, contains far fewer genes than the sexually recombining autosome from which it evolved. The enormity of this evolutionary decline has led to predictions that the Y chromosome will be completely bereft of functional genes within 10 million years Although recent evidence of gene conversion within massive Y-linked palindromes runs counter to this hypothesis, most unique Y-linked genes are not situated in palindromes and have no gene conversion partners. The 'impending demise' hypothesis thus rests on understanding the degree of conservation of these genes. By systematically comparing the DNA sequences of unique, Y-linked genes in chimpanzee and human, which diverged about 6 million years ago, evidence was found that in the human lineage, all such genes were conserved through purifying selection. In the chimpanzee lineage, by contrast, several genes have sustained inactivating mutations. Gene decay in the chimpanzee lineage might be a consequence of positive selection focused elsewhere on the Y chromosome and driven by sperm competition^ref
X-linked heredity is called diaginic, while Y-linked heredity is called olandric. Regions of the genome that determine the sexual identity of the infectious fungus Cryptococcus neoformansbear striking similarities to the human Y chromosome. Evidence suggests a close tie between the genes involved in sexual identity and virulence. Despite the lack of recombination, some fungal mating type gene repair might occur through the exchange of gene segments within chromosomes
The heterogametic or digametic sex (able to produce gametes with different sex chromosomes) may be the female one (Abraxas type) or the male one (Drosophila type) : in the latter case male individuals may have a 2A + XY genome (Lygaeus model, Homo sapiens included) or a 2A + X0 genome (Protenor berfagei model). Sex determination in the descendant may be :
• progamic : e.g. determined from oocyte size in Dinophilus apatris
• syngamic : e.g. determined by a stochastic choice of the spermatocyte which enters the oocyte in Homo sapiens
• epigamic : e.g. determined by the stochastic choice of the zygote implantation place in Bonellia viridis
• fertilization or amphigony (in Homo sapiens : amphimixi or karyogamy) => fertilized egg or zygote.
• autonomous development of one of the 2 gametes
• parthenogenesis / apogamy : a modified form of sexual reproduction by the development of a gamete without fertilization from the gamete of the other sex, as occurs in some plants and invertebrates, especially arthropods, e.g., honey bees and wasps, and in certain lizards. It may occur as a natural phenomenon or be induced by chemical, thermal, or mechanical stimulation (artificial parthenogenesis)
• arrhenotoca (from a male gamete)
• telitoca (from a female gamete) : in higher crustaceans (Crustacea) and "Marmorkrebs" crayfish (Cambaridae)
Only mammals have relinquished parthenogenesis, a means of producing descendants solely from maternal germ cells, probably due to requirement for sperm mRNAs. Mouse parthenogenetic embryos die by day 10 of gestation. Bi-parental reproduction is necessary because of parent-specific epigenetic modification of the genome during gametogenesis. This leads to unequal expression of imprinted genes from the maternal and paternal alleles. However, there is no direct evidence that genomic imprinting is the only barrier to parthenogenetic development. Knocking out a 13-kilobase region in the H19 gene —a maternally expressed gene thought to function as a noncoding mRNA that blocks in cis the expression of Igf2— of non-growing oocyte in a reconstructed oocyte containing 2 haploid sets of solely maternal genome, derived from non-growing and fully grown oocytes results in a normally developed and viable parthenote, suggesting a pivotal role for the paternally imprinted H19 gene in allowing Igf2 expression from the paternal allele and controlling the requirement for a paternal genome^ref. Using a maternally imprinted genome and a neutral genome still proves to be lethal. Anyway Igf2 mutations—total knockout, no Igf2 whatsoever—gives you live mice and every mouse survives: they're all small. This suggests that expression of Igf2 is not an absolute requirement for normal embryonic development. Researchers took almost 500 oocyte H19-deletion manipulations to produce 2 normal live parthenotes, yet only 12 to 15 manipulations with normal oocytes were attempted as controls. That could be a stochastic event that sometimes makes mice survive, and it might help but Igf2 is not a key gene. They are fertile but the presence of the mutation that rendered the maternal allele paternal-like in 50% of offspring would be lethal in combination with a paternal allele as overexpression of Igf2 would occur—a condition that he said was previously described as lethal. But the genotype of these pups was not reported in the paper and authors of the study say the pups were normal, suggesting that overexpression of Igf2 is not lethal
• ginogenesis or pseudogony (the male gamete activates the female gamete but doesn't introduce its genome)
• haplodiploidy (bees, wasps, and ants) : unfertilized eggs remain haploid and develop as males, while fertilized eggs become diploid and female. Femaleness requires 2 different alleles of complementary sex determiner (csd), which has 19 alleles. Although haplodiploid sex determination has stood the test of evolution, it has stymied beekeepers, because inbreeding spawns doomed homozygotes : these diploid males are recognized as such by workers and are eaten. The system prevents breeding to fix desired genes, because colonies of inbred bees are subviable.
According to partner number, a species can be considered ...
• monogamy : the animal mating system in which each individual mates with just one partner for the entire breeding season.
• polygamy : animal mating in which the individual mates with more than one partner
• polygyny : 1. polygamy (q.v.) in which a man is married concurrently to more than one woman.  2. an animal mating system seen in polygamous species, in which the male mates with more than one female.  3. union of two or more female pronuclei with a male pronucleus, resulting in polyploidy of the zygote
• polyandry : 1. polygamy (q.v.) in which a woman is concurrently married to more than one man.  2. an animal mating system seen in polygamous species, in which the female mates with more than one male.  3. union of two or more male pronuclei with a female pronucleus, resulting in polyploidy of the zygote
• postcopulatory sperm competition is a key aspect of sexual selection and is believed to drive the rapid evolution of both reproductive physiology and reproduction-related genes. It is well-established that mating behavior determines the intensity of sperm competition, with polyandry (i.e., female promiscuity) leading to fiercer sperm competition than monandry. Studies in mammals, particularly primates, showed that, owing to greater sperm competition, polyandrous taxa generally have physiological traits that make them better adapted for fertilization than monandrous species, including bigger testes, larger seminal vesicles, higher sperm counts, richer mitochondrial loading in sperm and more prominent semen coagulation. The degree of polyandry can also impact the dynamics of molecular evolution : the evolution of semenogelin II (SEMG2) gene, a main structural component of semen coagulum, is accelerated in polyandrous primates relative to monandrous primates^ref
According to partner choice way, a species can be considered ...
• inbreed : partner is chosen exclusively among relatives. Inbreeding is often used in laboratory practice to create syngeneic experiment animals
• outbreed : partner is chosen exclusively out of relatives.
• random breed : partner may belong or not to relatives.
According to the modality in which embryo develops, a species can be considered ...
• vivipation : the form of reproduction in which the embryo develops within and derives nutrition directly from the maternal organism by viviparous species
• oviparity : the quality of being oviparous (producing, and then laying or depositing eggs (oviposition) from which the young are hatched outside the body of the maternal organism)
• ovoviviparity : the quality of being ovoviviparous (bearing living young that hatch from large, yolk-filled eggs inside the body of the maternal organism, the embryo being nourished by food stored in the egg; said of lizards, etc)
• hermaphroditism or androginia : both types of gametes produced by the same individual (i.e. : no sex exists) => autofertilization (e.g.: moniecious Plantae)
The mutational deterministic hypothesis for the origin and maintenance of sexual reproduction posits that sex enhances the ability of natural selection to purge deleterious mutations after recombination brings them together into single genomes. This explanation requires negative epistasis, a type of genetic interaction where mutations are more harmful in combination than expected from their separate effects. The conceptual appeal of the mutational deterministic hypothesis has been offset by our inability to identify the mechanistic and evolutionary bases of negative epistasis. Negative epistasis can evolve as a consequence of sexual reproduction itself. Using an artificial gene network model, it was shown that recombination between gene networks imposes selection for genetic robustness, and that negative epistasis evolves as a by-product of this selection. Sexual reproduction selects for conditions that favour its own maintenance, a case of evolution forging its own path^ref
Those who have been keeping a mental tally of the differences between males and females can now add another 25,281 items to the list. That's the number of differences that researchers have found in gene expression between male and female mice^ref. With the exception of sex chromosomes, males and females have all the same genes; although variations in these genes and the genetic material between them produce the different types and amounts of proteins that make one person different from another. Researchers have found that other factors, such as sex hormones, can also affect the amount of protein produced, making gene expression broadly different in males and females. That in turn could affect, for example, the way that drugs are metabolized by men versus women. A few thousand such differences have been identified before. But now researchers have taken a huge step up in cataloguing the number of genes that behave differently from one sex to the next. The results have particular importance for understanding common diseases, almost all of which exhibit some sex bias. For example, women are much more susceptible to most autoimmune diseases, such as lupus and multiple sclerosis, whereas men are more susceptible to heart disease and Alzheimer's disease. The study revealed the huge extent of sex differences in the genes by surveying > 1,200 samples of brain, muscle, fat and liver from 334 mice. The team used microarrays to simultaneously assess the expression of > 23,000 genes in all of the 4 tissues, looking for differences between the male and female mice. For most genes the expression difference was less than 20%, but for some it was > 300%. > 25,000 examples of different gene expression. In the liver, where drugs are metabolised, about 70% of expressed genes were different between the sexes. Only 14% of the genes expressed in the brain were found to be different, although the researchers caution that this preliminary result might not mean anything significant about the difference between male and female brains. The striking observation is that the number of genes is much larger than anyone thought. 1,500 examples of sexually biased gene expression in the mouse liver have been previously identified. Other experiments show that these differences are mostly due to the influence of hormones on genes. The team doesn't yet know what overall difference it makes to the mice for all these genes to be expressed differently. But the data have already been used to identify regions of the genome that might contribute to mouse obesity, a condition that hits more females than males^ref. A greater understanding of these differences could help to tailor-make drugs for the different sexes.
• metagenesis / alternate generation : the alternate reproduction by asexual (agamic) and sexual (gamic) means in subsequent generations in an animal or plant species.

---

Copyright © 2001-2005 Daniele Focosi. All rights reserved  |  Terms of use  | Legal notices
About this site  |  Site map  |  Acknowledgements | Open forum  |  Tell a friend about this site  |  Current link partners
 Abbreviations and acronyms  |  Medical terminology  |  Add a link  |  Translate this site into your favourite language  |  Softwares

---

---

Search EntrezNCBI Web Site-------------PubMedGeneLocusLinkTaxonomyProteinNucleotideStructureGenomeBooksCancerChromosomesConserved Domains3D DomainsGEO ProfilesGEO DatasetsHomoloGeneJournalsMeSHNLM CatalogOMIMPMCPopSetPubChem BioAssayPubChem CompoundPubChem SubstanceSNPUniGeneUniSTS for

Search AboutAltaVistaAsk JeevesDictionaryDirectHitExciteFindWhatGoogleGoogle GroupsHotBotLookSmartLycosMetacrawlerMetaLocateNorthern LightOpen DirectorySnapThesaurusWebcrawlerYahoo for

Search Medical Dictionary  for