Research over the last two decades has identified a group of meiosis‐specific proteins, consisting of budding yeast Spo13, fission yeast Moa1, mouse MEIKIN, and Drosophila Mtrm, with essential functions in meiotic chromosome segregation. These proteins, which we call meiosis I kinase regulators (MOKIRs), mediate two major adaptations to the meiotic cell cycle to allow the generation of haploid gametes from diploid mother cells. Firstly, they promote the segregation of homologous chromosomes in meiosis I (reductional division) by ensuring (...) that sister kinetochores face towards the same pole (mono‐orientation). Secondly, they safeguard the timely separation of sister chromatids in meiosis II (equational division) by counteracting the premature removal of pericentromeric cohesin, and thus prevent the formation of aneuploid gametes. Although MOKIRs bear no obvious sequence similarity, they appear to play functionally conserved roles in regulating meiotic kinases. Here, the known functions of MOKIRs are reviewed and their possible mechanisms of action are discussed. Also see the video abstract here https://youtu.be/tLE9KL89bwk. (shrink)

Biologists have long theorized about the evolution of life cycles, meiosis, and sexual reproduction. We revisit these topics and propose that the fundamental difference between life cycles is where and when multicellularity is expressed. We develop a scenario to explain the evolutionary transition from the life cycle of a unicellular organism to one in which multicellularity is expressed in either the haploid or diploid phase, or both. We propose further that meiosis might have evolved as a mechanism to (...) correct for spontaneous whole‐genome duplication (auto‐polyploidy) and thus before the evolution of sexual reproduction sensu stricto (i.e. the formation of a diploid zygote via the fusion of haploid gametes) in the major eukaryotic clades. In addition, we propose, as others have, that sexual reproduction, which predominates in all eukaryotic clades, has many different advantages among which is that it produces variability among offspring and thus reduces sibling competition. (shrink)

It is tempting to assume that understatement and overstatement, meiosis and hyperbole, are analogous figures of speech, differing only in whether the speaker represents a quantity as larger, or as smaller, than she means to claim that it is. But these tropes have hugely different roles in conversation. Understatement is akin to irony, perhaps a species of it. Overstatement is an entirely different kettle of fish. Things get interestingly messy when we notice that to overstate how large or expensive (...) or distant something is, is to understate how small or inexpensive or close it is, and vice versa. So it may seem, anyway. I propose an account of the two tropes that counts some utterances, in their conversational contexts, as overstatements of a quantity but not understatements of the opposite quantity, and other utterances as understatements only. This account shows why understatement is closely related to irony and overstatement is not. Although overstatement and understatement (or irony) are very different, they are sometimes combined. An understatement of one quantity may be an overstatement of a different one. (shrink)

A framework for understanding the complex movements of mitosis and meiosis has been provided by the recent discovery of microtubule motor proteins, required for the proper distribution of chromosomes or the structural integrity of the mitotic or meiotic spindle. Although overall features of mitosis and meiosis are often assumed to be similar in mechanism, it is now clear that they differ in several important aspects. These include spindle structure and assembly, and timing of chromosome segregation to opposite poles. (...) Here we review progress in the functional characterization of several newly identified microtubule motor proteins, emphasizing their possible roles in spindle structure and function. (shrink)

The origin and progress of multicellularity, which is one of the crucial steps in the evolution of life, remains unclear and stringent phylogenetic reconstruction of the process is difficult. However, further theoretical considerations of the problem could be useful for the creation of a verifiable hypothesis. Sex as a ubiquitous biological phenomenon is usually considered as something entirely linked with reproduction. This is mostly true for modem multicellular organisms, but at the earliest stage of evolution of eukaryotes it was not (...) so. At that time the sexual process had nothing to do with reproduction, and only later, sex and reproduction merged together.One of the aims of this paper is to consider the sexual process as a likely basis for the establishment of multicellularity and to discuss the early stages of evolution of the multicellularity from this perspective. It is suggested that mitotic reproduction of cells at different stages of the sexual cycle of unicellular ancestors might be the starting points for independent transition to multicellularity in different taxa. Numerous consequences of these transitions, including evolution of bisexuality and development of novel meiotic functions in animals, are discussed. (shrink)

Several DNA-damage detection and repair mechanisms have evolved to repair double-strand breaks induced by mutagens. Later in evolutionary history, DNA single- and double-strand cuts made possible immune diversity by V(D)J recombination and recombination at meiosis. Such cuts are induced endogenously and are highly regulated and controlled. In meiosis, DNA cuts are essential for the initiation of homologous recombination, and for the formation of joint molecule and crossovers. Many proteins that function during somatic DNA-damage detection and repair are also (...) active during homologous recombination. However, their meiotic functions may be altered from their somatic roles through localization, posttranslational modifications and/or interactions with meiosis-specific proteins. Presumably, somatic repair functions and meiotic recombination diverged during evolution, resulting in adaptations specific to sexual reproduction. BioEssays 27:795–808, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

The transition from mitotic cell division to meiosis in yeast is governed by both the mating‐type genes and signals from the environment. Analysis of mutants that are unable to regulate entry into meiosis has identified many genes that function in this process and in some cases, the biochemical activity of their protein products has been described. At least two of the the mating‐type genes of Saccharomyces cerevisiae encode DNA binding proteins that regulate transcription of unlinked genes required for (...) entry into meiosis. Meiotic development of the distantly related yeast, Schizosaccharomyces pombe, is also controlled by the mating‐type genes but in this yeast, their role is to regulate expression of a protein that acts as an inhibitor of a protein kinase. The ability to use the powerful tool of genetics in yeast has provided us with many new insights into the problem of meiotic development. (shrink)

Diploid germ cells produce haploid gametes through meiosis, a unique type of cell division. Independent reassortment of parental chromosomes and their recombination leads to ample genetic variability among the gametes. Importantly, new mutations also occur during meiosis, at frequencies much higher than during the mitotic cell cycles. These meiotic mutations are associated with genetic recombination and depend on double‐strand breaks (DSBs) that initiate crossing over. Indeed, sequence variation among related strains is greater around recombination hotspots than elsewhere in (...) the genome, presumably resulting from recombination‐associated mutations. Significantly, enhanced mutagenicity in meiosis may lead to faster divergence during evolution, as germ‐line mutations are the ones that are transmitted to the progeny and thus have an evolutionary impact. The molecular basis for mutagenicity in meiosis may be related to the repair of meiotic DSBs by polymerases, or to the exposure of single‐strand DNA to mutagenic agents during its repair. (shrink)

The functions of the Bloom syndrome helicase and its orthologs are well characterized in mitotic DNA damage repair, but their roles within the context of meiotic recombination are less clear. In meiotic recombination, multiple repair pathways are used to repair meiotic DSBs, and current studies suggest that BLM may regulate the use of these pathways. Based on literature from Saccharomyces cerevisiae, Arabidopsis thaliana, Mus musculus, Drosophila melanogaster, and Caenorhabditis elegans, we present a unified model for a critical meiotic role of (...) BLM and its orthologs. In this model, BLM and its orthologs utilize helicase activity to regulate the use of various pathways in meiotic recombination by continuously disassembling recombination intermediates. This unwinding activity provides the meiotic program with a steady pool of early recombination substrates, increasing the probability for a DSB to be processed by the appropriate pathway. As a result of BLM activity, crossovers are properly placed throughout the genome, promoting proper chromosomal disjunction at the end of meiosis. This unified model can be used to further refine the complex role of BLM and its orthologs in meiotic recombination. In meiotic recombination, Blm utilizes its helicase activity to disassemble early recombination intermediates, thereby retaining a pool of recombination sites from which some can be selected to enter the Class I crossover pathway while simultaneously promoting SDSA and preventing the use of the Class II pathway. (shrink)

In budding yeast, commitment to meiosis is attained when meiotic cells cannot return to the mitotic cell cycle even if the triggering cue (nutrients deprivation) is withdrawn. Commitment is arrived at gradually, and different aspects of meiosis may be committed at different times. Cells become fully committed to meiosis at the end of Prophase I, long after DNA replication and just before the first meiotic division (MI). Whole‐genome gene expression analysis has shown that committed cells have a (...) distinct and rapid response to nutrients, and are not simply insulated from environmental signals. Thus becoming committed to meiosis is an active process. The cellular event most likely to be associated with commitment to meiosis is the separation of the duplicated spindle‐pole bodies (SPBs) and the formation of the spindle. Commitment to the mitotic cell cycle is also associated with the separation of SPBs, although it occurs in G1, before DNA replication. (shrink)

Sexual reproduction is the dominant reproductive mode in eukaryotes but, in many taxa, it has never been observed. Molecular methods that detect evidence of sex are largely based on the genetic consequences of sexual reproduction. Here we describe a powerful new approach to directly search genomes for genes that function in meiosis. We describe a “meiosis detection toolkit”, a set of meiotic genes that represent the best markers for the presence of meiosis. These genes are widely present (...) in eukaryotes, function only in meiosis and can be isolated by degenerate PCR. The presence of most, or all, of these genes in a genome would suggest they have been maintained for meiosis and, implicitly, sexual reproduction. In contrast, their absence would be consistent with the loss of meiosis and asexuality. This approach will help to understand both meiotic gene evolution and the capacity for meiosis and sex in putative obligate asexuals. BioEssays 30:579–589, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

From 1956, when the complex ultrastructure of meiotic chromosomes was discovered, 1 until 1985, when the isolation of meiotic chromosome cores was reported, knowledge of the molecular structure of the meiotic chromosome was at best a dream. The dissection of meiotic chromosome structures has become a realistic challenge through the arrival of isolated symptonemal complexes (SCs), monoclonal and polyclonal antibodies against SCs, the possibility for screening expression libraries for genes that encode SC proteins, the isolation of SC‐associated DNA, and the (...) development of techniques for the in situ recognition of DNA sequences in the context of the meiotic chromosome structure. (shrink)

One distinguishing feature of vertebrate oocyte meiosis is its discontinuity; oocytes are released from their prophase I arrest, usually by hormonal stimulation, only to again halt at metaphase II, where they await fertilization. The product of the c‐mos proto‐oncogene, Mos, is a key regulator of this maturation process. Mos is a serine‐threonine kinase that activates and/or stabilizes maturation‐promoting factor (MPF), the master cell cycle switch, through a pathway that involves the mitogen‐activated protein kinase (MAPK) cascade. Oocytes arrested at prophase (...) I lack detectable levels of Mos, which must be synthesized from a pool of maternal mRNAs for proper maturation. While Mos is necessary throughout maturation in Xenopus, it seems to be required only for meiosis II in the mouse. The translational activation of c‐mos mRNA at specific times during meiosis requires cytoplasmic polyadenylation. Cis‐ and trans‐acting factors for polyadenylation are, therefore, essential elements of maturation. (shrink)

Normal meiosis consists of a single round of DNA replication followed by two nuclear divisions. In the 1st division the chromosomes segregate reductionally whereas in the 2nd division they segregate equationally (as they do in mitosis). In certain yeast mutants, a single‐division meiosis takes place, in which some chromosomes segregate reductionally while others divide equationally. This autonomous segregation behaviour of individual chromosomes on a common spindle is determined by the centromeres they carry. The relationship between reductional segregation of (...) a pair of chromosomes and their earlier recombinational history is also discussed. (shrink)

During meiosis, homologous chromosomes must pair in order to permit recombination and correct chromosome segregation to occur. Two recent papers1,2 show that meiotic pairing is also important for correct gene expression during meiosis. They describe data for the filamentous fungus Neurospora crassa that show that a lack of pairing generated by ectopic integration of genes can result in silencing of genes expressed during meiosis. This can result in aberrant meioses whose defects are specific to the function of (...) the unpaired gene. Furthermore, mutations affecting the silencing mechanism have been selected in a gene encoding a putative RNA‐dependent RNA polymerase. This finding indicates the involvement of a meiotic specific post‐transcriptional gene silencing mechanism (PTGS) similar to that observed in vegetative cells in N. crassa and other organisms. Finally, this gene product is essential for normal meiosis, suggesting that RNA‐dependent processes are fundamental to the sexual cycle. BioEssays 25:99–103, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

The distribution pattern of the meiotic machinery in known eukaryotes is most parsimoniously explained by the hypothesis that all eukaryotes are ancestrally sexual. However, this assumption is questioned by preliminary results, in culture conditions. These suggested that Acanthamoeba, an organism considered to be largely asexual, constitutively expresses meiosis genes nevertheless—at least in the lab. This apparent disconnect between the “meiosis toolkit” and sexual processes in Acanthamoeba led to the conclusion that the eukaryotic ancestor is asexual. In this review, (...) the “meiosis toolkit” is rigorously defended, drawing on numerous research articles. Additionally, the claim of constitutive meiotic gene expression is probed in Acanthamoeba via the same transcriptomics data. The results show that the expression of the meiotic machinery is not constitutive in Acanthamoeba as claimed before. Furthermore, it is argued that this would have no implications for understanding the nature of the eukaryotic ancestor, regardless of the result. (shrink)

Premeiotic association of homologous chromosomes in the yeast, Saccharomyces cerevisiae has been shown, by means of fluorescent in situ hybridization (FISH)(1,2). Time course and mutant studies show that the premeiotic associations are disrupted upon entry into meiosis, to be reestablished shortly before synapsis. The data are consistent with a model in which multiple, unstable interactions bring homologues together, prior to stable joining by recombination(3).

The most‐critical point of reproductive development in all sexually reproducing species is the transition from mitotic to meiotic cell cycle. Studies in unicellular fungi have indicated that the decision to enter meiosis must be made before the beginning of the premeiotic S phase. Recent data from the mouse1 suggest that this timing of meiosis initiation is a universal feature shared also by multicellular eukaryotes. In contrast, the signaling cascade that leads to meiosis initiation shows great diversity among (...) species. BioEssays 29:511–514, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

Meiotin‐1 is a protein found in developing microsporocytes of Lilium longiflorum, and immunological assays indicate that cognates exist in both mono‐ and dicotyledonous plants. Its temporal and spatial expression pattern, coupled with its unusual distribution in chromatin and the properties it shares with histone H1, encourages speculation that it is involved in regulating meiotic chromatin structure. Molecular analyses provide support for the hypothesis that meiotin‐1 arose from histone H1 by an exon shuffling mechanism, as meiotin‐1 is an H1‐like protein that (...) lacks the amino‐terminal domain shared by H1 molecules. We have proposed that meiotin‐1 serves to limit chromatin condensation in order to foster the unique cytological and molecular events which occur during meiotic prophase. As such, meiotin‐1 fits the role of a ‘meiosis readiness factor’, and its accumulation to a threshold level may commit mitotically dividing progenitor cells to differentiate into meiocytes. (shrink)

Chromosome ends have been implicated in the meiotic processes of the nematode Caenorhabditis elegans. Cytological observations have shown that chromosome ends attach to the nuclear membrane and adopt kinetochore functions. In this organism, centromeric activity is highly regulated, switching from multiple spindle attachments all along the chromosome during mitotic division to a single attachment during meiosis. C. elegans chromosomes are functionally monocentric during meiosis. Earlier genetic studies demonstrated that the terminal regions of the chromosomes are not equivalent in (...) their meiotic potentials. There are asymmetries in the abilities of the ends to recombine when duplicated or deleted. In addition, mutations in single genes have been identified that mimic the meiotic effects of a terminal truncation of the X chromosome. The recent cloning and characterization of the C. elegans telomeres has provided a starting point for the study of chromosomal elements mediating the meiotic process. (shrink)

DNA double‐strand breaks (DSBs) are extremely hazardous lesions for all DNA‐bearing organisms and the mechanisms of DSB repair are highly conserved. In the eukaryotic mitotic cell cycle, DSBs are often present following DNA replication while, in meiosis, hundreds of DSBs are generated as a prelude to the reshuffling of the maternally and paternally derived genomes. In both cases, the DSBs are repaired by a process called homologous recombinational repair (HRR), which utilises an intact DNA molecule as the repair template. (...) Mitotic and meiotic HRR are managed by ‘checkpoints’ that inhibit cell division until DSB repair is complete. Here we attempt to summarise the substantial recent progress in understanding the checkpoint management of HRR in mitosis (focussing mainly on mammals) and then go on to use this information as a framework for understanding the presumed checkpoint management of HRR in mammalian meiosis. BioEssays 29:974–986, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

Ideas about the mechanisms that regulate chromosome pairing, recombination, and segregation during meiosis have gained in molecular detail over the last few years. The purpose of this article is to survey briefly the shifts in paradigms and experiments that have generated new perspectives. It has never been very clear what it is that brings together the homologous chromosomes at meiotic prophase. For a while it appeared that the synaptonemal complex might be the nuclear organelle responsible for synapsis, but the (...) supporting evidence has not been entirely convincing. Whatever the mechanism, it has always been assumed that homologous synapsis creates the opportunity for homologous DNA sequences to initiate recombination. At present, alternative ideas are developing. Attractive is the concept that double strand DNA repair mechanisms, that find and use the undamaged homologue for repair, have evolved into a meiotic mechanism for the recognition and pairing of homologous sequences. Subsequent intimate synapsis of homologous chromosomes in the context of the synaptonemal complex may serve later functions in the regulation of interference and segregation at first anaphase. A number of areas that are being tested at present and some that may be investigated in the future are discussed at the end of the review. (shrink)

A reduction of chromosome number in meiosis is essential for genome transmission in diploid organisms. Reduction depends on a change in kinetochore configuration.1 A recent study2 connects changes in kinetochores with other changes in chromosome structure and raises the intriguing possibility that topoisomerase II, the DNA untangling enzyme, is involved. BioEssays 25:309–312, 2003. © 2003 Wiley Periodicals, Inc.

Mammalian sex chromosomes exhibit marked sexual dimorphism in behavior during gametogenesis. During oogenesis, the X chromosomes pair and participate in unrestricted recombination; both are transcriptionally active. However, during spermatogenesis the X and Y chromosomes experience spatial restriction of pairing and recombination, are transcriptionally inactive, and form a chromatin domain that is markedly different from that of the autosomes. Thus the male germ cell has to contend with the potential loss of X‐encoded gene products, and it appears that coping strategies have (...) evolved. Genetic control of sex‐chromosome inactivation during spermatogenesis does not involve pairing or the presence of the Y chromosome or an intact X chromosome, and may therefore be under exogenous control by the gonad. Sex‐chromosome reactivation during oogenesis and inactivation during spermatogenesis probably reflect specific meiotic events such as recombination. Understanding these phenomena may help explain other sex‐related differences in genetic recombination. (shrink)

The notion that eukaryotes are ancestrally sexual has been gaining attention. This idea comes in part from the discovery of sets of “meiosis‐specific genes” in the genomes of protists. The existence of these genes has persuaded many that these organisms may be engaging in sex, even though this has gone undetected. The involvement of sex in protists is supported by the view that asexual reproduction results in the accumulation of mutations that would inevitably result in the decline and extinction (...) of such lineages. It is argued that this phenomenon can be obviated by polyploidy and that the “meiosis‐specific genes” are used in other processes, including polyploidy control and homologous recombination, independent of meiosis. These phenomena account for the finding that these genes are expressed in cultures devoid of apparent cell fusion events. Hence, it is also proposed that asexual, and not sexual, reproduction is the ancestral condition. (shrink)

Loss of the Y‐chromosome is a common feature of species with chromosomal sex determination. However, our understanding of why some lineages frequently lose Y‐chromosomes while others do not is limited. The fragile Y hypothesis proposes that in species with chiasmatic meiosis the rate of Y‐chromosome aneuploidy and the size of the recombining region have a negative correlation. The fragile Y hypothesis provides a number of novel insights not possible under traditional models. Specifically, increased rates of Y aneuploidy may impose (...) positive selection for (i) gene movement off the Y; (ii) translocations and fusions which expand the recombining region; and (iii) alternative meiotic segregation mechanisms (achiasmatic or asynaptic). These insights as well as existing evidence for the frequency of Y‐chromosome aneuploidy raise doubt about the prospects for long‐term retention of the human Y‐chromosome despite recent evidence for stable gene content in older non‐recombining regions. (shrink)

Here a wide distribution of meiotic machinery is shown, indicating the occurrence of sexual processes in all major eukaryotic groups, without exceptions, including the putative “asexuals.” Meiotic machinery has evolved from archaeal DNA repair machinery by means of ancestral gene duplications. Sex is very conserved and widespread in eukaryotes, even though its evolutionary importance is still a matter of debate. The main processes in sex are plasmogamy, followed by karyogamy and meiosis. Meiosis is fundamentally a chromosomal process, which (...) implies recombination and ploidy reduction. Several eukaryotic lineages are proposed to be asexual because their sexual processes are never observed, but presumed asexuality correlates with lack of study. The authors stress the complete lack of meiotic proteins in nucleomorphs and their almost complete loss in the fungus Malassezia. Inversely, complete sets of meiotic proteins are present in fungal groups Glomeromycotina, Trichophyton, and Cryptococcus. Endosymbiont Perkinsela and endoparasitic Microsporidia also present meiotic proteins. (shrink)

Meiotic defects cause abnormal chromosome segregation leading to aneuploidy in mammalian oocytes. Chromosome segregation is particularly error‐prone in human oocytes, but the mechanisms behind such errors remain unclear. To explain the frequent chromosome segregation errors, recent investigations have identified multiple meiotic defects and explained how these defects occur in female meiosis. In particular, we review the causes of cohesin exhaustion, leaky spindle assembly checkpoint (SAC), inherently unstable meiotic spindle, fragmented kinetochores or centromeres, abnormal aurora kinases (AURK), and clinical genetic (...) variants in human oocytes. We mainly focus on meiotic defects in human oocytes, but also refer to the potential defects of female meiosis in mouse models. (shrink)

Meiosis is the process by which diploid germ cells divide to produce haploid gametes for sexual reproduction. The process is highly conserved in eukaryotes, however the recent availability of mouse models for meiotic recombination has revealed surprising regulatory differences between simple unicellular organisms and those with increasingly complex genomes. Moreover, in these higher eukaryotes, the intervention of physiological and sex-specific factors may also influence how meiotic recombination and progression are monitored and regulated. This review will focus on the recent (...) studies involving mouse mutants for meiosis, and will highlight important differences between traditional model systems for meiosis (such as yeast) and those involving more complex cellular, physiological and genetic criteria. BioEssays 23:996–1009, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

The organism is one of the fundamental concepts of biology and has been at the center of many discussions about biological individuality, yet what exactly it is can be confusing. The definition that we find generally useful is that an organism is a unit in which all the subunits have evolved to be highly cooperative, with very little conflict. We focus on how often organisms evolve from two or more formerly independent organisms. Two canonical transitions of this type—replicators clustered in (...) cells and endosymbiotic organelles within host cells—demonstrate the reality of this kind of evolutionary transition and suggest conditions that can favor it. These conditions include co-transmission of the partners across generations and rules that strongly regulate and limit conflict, such as a fair meiosis. Recently, much attention has been given to associations of animals with microbes involved in their nutrition. These range from tight endosymbiotic associations like those between aphids and Buchnera bacteria, to the complex communities in animal intestines. Here, starting with a reflection about identity through time, we consider the distinctions between these kinds of animal–bacteria interactions and describe the criteria by which a few can be considered jointly organismal but most cannot. (shrink)

The genetic code appeared on Earth at the origin of life, and the codes of culture arrived almost four billion years later. For a long time it has been assumed that these are the only codes that exist in Nature, and if that were true we would have to conclude that codes are extraordinary exceptions that appeared only at the beginning and at the end of the history of life. In reality, various other organic codes have been discovered in the (...) past few decades and it is likely that more will come to light in the future. The existence of many organic codes in Nature is therefore an experimental fact, but also more than that. It is one of those facts that have extraordinary implications. In this book it is shown that the genetic code was a precondition for the origin of the first cells, the signal transduction codes divided the first cells into three primary kingdoms (Archaea, Bacteria and Eukarya), the splicing codes were instrumental to the origin of the eukaryotic nucleus, the histone code provided a new regulation system in eukaryotic genomes, and the cytoskeleton codes allowed the Eukarya to perform internal movements, including those of mitosis and meiosis. It is shown, furthermore, that organic codes had a key role in multicellular life, in particular in the origin of animals, the origin of mind and the origin of language. The great events of macroevolution, in other words, were associated with the appearance of new organic codes, and we can easily understand why. The reason is that a new code brings into existence something that has never existed before because it creates arbitrary associations, relationships that are not determined by physical necessity. Another outstanding implication is the fact that codes involve meaning and we need therefore to introduce in biology, with the standard methods of science, not only the concept of biological information but also that of biological meaning. The research on biological codes, in conclusion, is bringing to light new mechanisms in evolution and new fundamental concepts in science. This research is the field of Code Biology, the study of all codes of life, from the genetic code to the codes of culture, from the origin of life to the origin of man. (shrink)

Homologous chromosome pairing is required for proper chromosome segregation and recombination during meiosis. The mechanism by which a pair of homologous chromosomes contact each other to establish pairing is not fully understood. When pairing occurs during meiotic prophase in the fission yeast, Schizosaccharomyces pombe, the nucleus oscillates between the cell poles and telomeres remain clustered at the leading edge of the moving nucleus. These meiosis‐specific activities produce movements of telomere‐bundled chromosomes. Several lines of evidence suggest that these movements (...) facilitate homologous chromosome pairing by aligning homologous chromosomes and promoting contact between homologous regions. Since telomere clustering and nuclear or chromosome movements in meiotic prophase have been observed in a wide range of eukaryotic organisms, it is suggested that telomere‐mediated chromosome movements are general activities that facilitate homologous chromosome pairing. BioEssays 23:526–533, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

Studies in the yeast Saccharomyces cerevisiae have validated the major features of the double‐strand break repair (DSBR) model as an accurate representation of the pathway through which meiotic crossovers (COs) are produced. This success has led to this model being invoked to explain double‐strand break (DSB) repair in other contexts. However, most non‐crossover (NCO) recombinants generated during S. cerevisiae meiosis do not arise via a DSBR pathway. Furthermore, it is becoming increasingly clear that DSBR is a minor pathway for (...) recombinational repair of DSBs that occur in mitotically‐proliferating cells and that the synthesis‐dependent strand annealing (SDSA) model appears to describe mitotic DSB repair more accurately. Fundamental dissimilarities between meiotic and mitotic recombination are not unexpected, since meiotic recombination serves a very different purpose (accurate chromosome segregation, which requires COs) than mitotic recombination (repair of DNA damage, which typically generates NCOs). (shrink)

This philosophical-pragmatic paper discusses several forms of irony which rest on other figures of speech contingent on overt untruthfulness, namely the figures arising as a result of flouting the first maxim of Quality. It is argued that an ironic implicature may be piggybacked on another implicature, called “as if implicature”, originating from flouting the first maxim of Quality occasioned by metaphor. Metaphorical irony, which is subject to the irony-after-metaphor order of interpretation, exhibits a number of manifestations depending on the nature (...) and scope of irony, and the scope of the subordinate metaphor. On the other hand, rather than giving rise to an as if implicature distinct from the irony-based one, hyperbole and meiosis, which are inherently evaluative, most typically overlap with ironically evaluative expressions, promoting meiotic and hyperbolic irony, frequently considered by researchers to rely on flouting Quantity maxims. However, cases of independent use of hyperbole or meiosis in ironic environment are also possible. Such invite two levels of untruthfulness and two implicatures. (shrink)

John Harsanyi and John Rawls both used the veil of ignorance thought experiment to study the problem of choosing between alternative social arrangements. With his ‘impartial observer theorem’, Harsanyi tried to show that the veil of ignorance argument leads inevitably to utilitarianism, an argument criticized by Sen, Weymark and others. A quite different use of the veil-of-ignorance concept is found in evolutionary biology. In the cell-division process called meiosis, in which sexually reproducing organisms produce gametes, the chromosome number is (...) halved; when meiosis is fair, each gene has only a fifty percent chance of making it into any gamete. This creates a Mendelian veil of ignorance, which has the effect of aligning the interests of all the genes in an organism. This paper shows how Harsanyi's version of the veil-of-ignorance argument can shed light on Mendelian genetics. There turns out to be an intriguing biological analogue of the impartial observer theorem that is immune from the Sen/Weymark objections to Harsanyi's original. View HTML Send article to KindleTo send this article to your Kindle, first ensure [email protected] is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about sending to your Kindle. Find out more about sending to your Kindle. Note you can select to send to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply. Find out more about the Kindle Personal Document Service.SOCIAL JUSTICE, GENOMIC JUSTICE AND THE VEIL OF IGNORANCE: HARSANYI MEETS MENDELVolume 28, Issue 1Samir Okasha DOI: https://doi.org/10.1017/S0266267112000119Your Kindle email address Please provide your Kindle [email protected]@kindle.com Available formats PDF Please select a format to send. By using this service, you agree that you will only keep articles for personal use, and will not openly distribute them via Dropbox, Google Drive or other file sharing services. Please confirm that you accept the terms of use. Cancel Send ×Send article to Dropbox To send this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about sending content to Dropbox. SOCIAL JUSTICE, GENOMIC JUSTICE AND THE VEIL OF IGNORANCE: HARSANYI MEETS MENDELVolume 28, Issue 1Samir Okasha DOI: https://doi.org/10.1017/S0266267112000119Available formats PDF Please select a format to send. By using this service, you agree that you will only keep articles for personal use, and will not openly distribute them via Dropbox, Google Drive or other file sharing services. Please confirm that you accept the terms of use. Cancel Send ×Send article to Google Drive To send this article to your Google Drive account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about sending content to Google Drive. SOCIAL JUSTICE, GENOMIC JUSTICE AND THE VEIL OF IGNORANCE: HARSANYI MEETS MENDELVolume 28, Issue 1Samir Okasha DOI: https://doi.org/10.1017/S0266267112000119Available formats PDF Please select a format to send. By using this service, you agree that you will only keep articles for personal use, and will not openly distribute them via Dropbox, Google Drive or other file sharing services. Please confirm that you accept the terms of use. Cancel Send ×Export citation Request permission. (shrink)

The genomes of virtually all sexually reproducing species contain transposable elements. Although active elements generally transpose more rapidly than they are inactivated by mutation or excision, their number can be kept in check by purifying selection if its effectiveness becomes disproportionately greater as their copy number increases. In sexually reproducing species, such synergistic selection can result from ectopic crossing-over or from homologous recombination under negative epistasis. In addition, there may be controls on transposon activity that are associated with meiosis. (...) Because a sexual lineage that abandons sex must lack such mechanisms, it may be driven to extinction by the unchecked proliferation of deleterious transposons inherited from its sexual progenitor. An important component of the evolutionary advantage of sex over asex may therefore lie in the ability of sex, despite facilitating the spread of deleterious elements within interbreeding populations, also to restrain their intragenomic proliferation. BioEssays 27:76–85, 2005. © 2004 Wiley Periodicals, Inc. (shrink)

We know today that classical eugenics, of an essentially negative nature, was not only an aggressive and brutal practice but, like its positive counterpart, inefficient as well. In fact, numerous biological, sociological, and psychological events beyond our control arise to prevent the realisation of any eugenic plan. Thus, like all human beings, individuals whose procreation is encouraged by positive eugenics suffer unexpected mutations that are transmitted to their offspring by their gametes. Gene distribution among the gametes at meiosis is (...) the result of an uncontrollable, natural lottery. As an effect of this lottery, positive eugenics could allow the birth of defective babies whereas negative eugenics precludes the birth of normal babies. (shrink)

Cancers often express hundreds of genes otherwise specific to germ cells, the germline/cancer (GC) genes. Here, we present and discuss the hypothesis that activation of a “germline program” promotes cancer cell malignancy. We do so by proposing four hallmark processes of the germline: meiosis, epigenetic plasticity, migration, and metabolic plasticity. Together, these hallmarks enable replicative immortality of germ cells as well as cancer cells. Especially meiotic genes are frequently expressed in cancer, implying that genes unique to meiosis may (...) play a role in oncogenesis. Because GC genes are not expressed in healthy somatic tissues, they form an appealing source of specific treatment targets with limited side effects besides infertility. Although it is still unclear why germ cell specific genes are so abundantly expressed in cancer, from our hypothesis it follows that the germline's reproductive program is intrinsic to cancer development. (shrink)

Many aspects of biology, such as population genetics and senescence, are predicated on identifying individuals and generations. Conventional demarcations of individuals and generations, such as physiological autonomy, unicellular bottlenecks, and alternation of generation, are rife with problems. Do physically separated cuttings or plant ramets constitute separate individuals or generations? Are chimaeras one or more individuals? To resolve these problems, Clarke : 321–361, 2012) proposed that individuals are circumscribed by mechanisms that constrain heritable variance in fitness. Simultaneously, Gorelick and Heng : (...) 1088–1098 2011) showed that sex constrains heritable variance Therefore, for eukaryotes, meiosis and karyogamy provide a consistent way to demarcate individuals and generations. Epigenetic reset associated with meiosis and karyogamy rejuvenates the next generation, but not the parent that engaged in the sex act. Wholesale epigenetic resets that probably only occur with meiosis and karyogamy imply that monozygotic twins are two different individuals, but apomictic progeny are diffuse parts of one disaggregated individual. Mitotic heritability circumscribes an individual, whereas meiotic heritability demarcates new individuals and generations. (shrink)

Mammalian oocytes exhibit a series of cell cycle transitions that coordinate the penultimate events of meiosis with the onset of embryogenesis at fertilization. The execution of these cell cycle transitions, at G2/M of meiosis‐I and metaphase/anaphase of meiosis I and II, involve both biosynthetic and post‐translational modifications that directly modulate centrosome and microtubule behavior. Specifically, somatic cells alter the signal transduction pathways in the oocyte and influence the expression of maturation promoting factor (MPF) and cytostatic factor (CSF) (...) activity through a microtubule‐dependent mechanism. The regulation of the oocytes' cell cycle machinery by hormone‐mediated somatic cell signals, involving both positive and negative stimuli, ensures that meiotic cell cycle progression is synchronized with the earliest pivotal events of mammalian reproduction. (shrink)

The mammalian oocyte is a round cell arrested at prophase I of meiosis. It is characterized by the presence of a large nucleus, called the germinal vesicle, in the middle of which is the nucleolus. Before it can be fertilized, the oocyte must resume meiosis, enter metaphase II and be ovulated. The nucleolus is dissolved during this process. However, the nucleoli of the male and female pronuclei in the zygote are both of maternal origin. A recent paper1 demonstrates (...) that the maternal nucleolus, together with other nucleoplasmic elements, is essential for early embryonic development. These nucleolar and nucleoplasmic factors remain undetermined. BioEssays 30:613–616, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

Mouse embryonic stem cells have an unlimited lifespan in cultures if they are prevented from differentiating. After differentiating, they produce cells which divide only a limited number of times. These changes seen in cultures parallel events that occur in the developing embryo, where immortal embryonic cells differentiate and produce mortal somatic ones. The data strongly suggest that differentiation initiates senescence, but this view entails additional assumptions in order to explain how the highly differentiated sexual gametes manage to remain potentially immortal. (...) Cells differentiate by blocking expression from large parts of their genome and it is suggested that losses or gains of genetic totipotency determine cellular lifespans. Cells destined to be somatic do not regain totipotency and senesce, while germ‐line cells regain complete genome expression and immortality after meiosis and gamete fusions. Losses of genetic totipotency could induce senescence by lowering the levels of repair and maintenance enzymes. (shrink)