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)

Wheat is an allopolyploid containing three distinct but genetically related (homoeologous) genomes, A, B and D. Because of polyploid inheritance and large genome size (16×1012 bp), the wheat genome is thought to be intractable to map‐based cloning of agronomic and other genes of interest. We propose a targeted geneti mapping strategy that combines linkage and physical mapping and may facilitate map‐based cloning. High‐density linkage maps are either generated in wheat or in diploid Triticum tauschii, the donor of the D genome (...) to wheat. Molecular marker‐based chromosome maps are constructed, using an array of deletion lines in wheat. The conventional genetic linkage maps are aligned with chromosome maps to construct cytogenetic ladder maps (CLMs). The CLMs allow region‐specific mapping and convert genetic distances into physical distances. The information from CLMs suggests that many genes in wheat are present in clusters that are highly recombiogenic, small, and may be amenable to cloning by chromosome walking. Therefore, the effective genome size of wheat is relatively small in comparison to the whole genome. The utility of using the smaller genome of rice for mapping and homologous gene cloning is discussed. (shrink)

The maintenance of cell size homeostasis has been studied for years in different cellular systems. With the focus on ‘what regulates cell size’, the question ‘why cell size needs to be maintained’ has been largely overlooked. Recent evidence indicates that animal cells exhibit nonlinear cell size dependent growth rates and mitochondrial metabolism, which are maximal in intermediate sized cells within each cell population. Increases in intracellular distances and changes in the relative cell surface area impose biophysical limitations on cells, which (...) can explain why growth and metabolic rates are maximal in a specific cell size range. Consistently, aberrant increases in cell size, for example through polyploidy, are typically disadvantageous to cellular metabolism, fitness and functionality. Accordingly, cellular hypertrophy can potentially predispose to or worsen metabolic diseases. We propose that cell size control may have emerged as a guardian of cellular fitness and metabolic activity. Cell size is intimately connected to metabolism and growth rate, which are influenced by mitochondrial activity and biophysical constraints. We discuss that cellular fitness is often cell-size dependent. While the current evidence relies largely on proliferating cells, this connection from cell size to fitness may also help explain metabolic diseases. (shrink)

Classic genetics studies found that genomic imbalance caused by changing the dosage of part of the genome (aneuploidy) has more detrimental effects than altering the dosage of the whole genome (ploidy). Previous analysis revealed global modulation of gene expression triggered by aneuploidy across various species, including maize (Zea mays), Arabidopsis, yeast, mammals, etc. Plant microRNAs (miRNAs) are a class of 20‐ to 24‐nt endogenous small noncoding RNAs that carry out post‐transcriptional gene expression regulation. That miRNAs and their putative targets are (...) preferentially retained as duplicates after whole‐genome duplication, as are many transcription factors and signaling components, indicates miRNAs are likely to be dosage‐sensitive and potentially involved in genomic balance networks. This review addresses the following questions regarding the role of miRNAs in genomic imbalance. (1) How do aneuploidy and polyploidy impact the expression of miRNAs? (2) Do miRNAs play a regulatory role in modulating the expression of their targets under genomic imbalance? (shrink)

Polyploid cells contain multiple copies of all chromosomes. Polyploidization can be developmentally programmed to sustain tissue barrier function or to increase metabolic potential and cell size. Programmed polyploidy is normally associated with terminal differentiation and poor proliferation capacity. Conversely, non‐programmed polyploidy can give rise to cells that retain the ability to proliferate. This can fuel rapid genome rearrangements and lead to diseases like cancer. Here, the mechanisms that generate polyploidy are reviewed and the possible challenges upon polyploid (...) cell division are discussed. The discussion is framed around a recent study showing that asynchronous cell cycle progression (an event that is named “chronocrisis”) of different nuclei from a polyploid cell can generate DNA damage at mitotic entry. The potential mechanisms explaining how mitosis in non‐programmed polyploid cells can generate abnormal karyotypes and genetic instability are highlighted. (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)

In a very general sense, hybrid can be understood to be any organism that is the product of two (or more) organisms where each parent belongs to a different kind. For example; the offspring from two or more parent organisms, each belonging to a separate species (or genera), is called a “hybrid”. “Hybridity” refers to the phenomenal character of being a hybrid. And “hybridization ” refers to both natural and artificial processes of generating hybrids. These processes include mechanisms of selective (...) cross-breeding and cross-fertilization of parents of different species for the purpose of producing hybrid offspring. In addition to these processes, “hybridization ” also refers to natural and artificial processes of whole genome duplication that result in the doubling or trebling of the sets of chromosomes of the organism. This entry provides an overview of the impact of hybridity on agriculture. It begins with an historical sketch that traces the early horticulturalists’ and naturalists’ investigations of hybrids. This starts with the observations of Thomas Fairchild and Georges-Louis Leclerc, Comte de Buffon; and leads to the explanation of its mechanism by Gregor Mendel, James Watson and Francis Crick, and Ernst Mayr; and the eventual manipulation of hybrids and hybridization by Barbara McClintock. Following this, the reader is introduced to a number of key terms and concepts in use within current research as well as highlighting diverse ethical concerns that center on hybridization. Recent research that attempts to ascertain the role of hybridization in adaptive change will be introduced. This will include research on the evolution of crop species, increased biodiversity, and the use of hybrids to manipulate phenotypically desirable traits in agricultural crops. The focus of the discussion is on a particularly significant type of naturally occurring hybridization, polyploidy hybridization. Polyploids are organisms which have more than two complete genomes in each cell. This kind of hybridization is ubiquitous among crop plants. The role of polyploidy in plant evolution and the affects of polyploidy on plants and animals will be reviewed. A critical discussion of its agricultural value in the production of fertile polyploid hybrids highlights key epistemological, ontological, and ethical issues. These are illuminated with reference to the distinct processes of artificial and natural hybridization. A survey of these different kinds of hybridization includes the ethical and economic impacts of hybridity on global nutrition, the environment, and considerations of some practical implications for the agricultural industry. Tracking the role of hybrids, the process of hybridization, and the current impacts of it for agriculture requires knowledge of the history of its early conceptualization, understanding, and use. This is the topic of the following section. (shrink)

Objective: To determine cryopreservation-induced alterations in the cytoskeleton of metaphase II mouse oocytes and the implications of these alterations in functionality of the cytoskeleton and polyploidy after fertilization.Design: Comparative study.Setting: Clinical and academic research environment at a medical school teaching hospital.Intervention : Oocytes were frozen using a slow-cooling and slow-thawing protocol in 1.5 M dimethyl sulfoxide and 0.2 M sucrose and were analyzed before and after fertilization.Main Outcome Measure : Cytoskeletal alterations, fertilization, and polyploidy rates.Result : When analyzed (...) immediately after thawing, the oocytes displayed dramatic cytoskeletal alterations. Only slight recovery was observed upon removal of the cryoprotectants. However, incubation after thawing of 1 hour at 37oC completely reestablished a normal microfilament and microtubule pattern while partially restoring normal spindle morphology and chromosome alignment. Accordingly, insemination immediately after removal of cryoprotectants resulted in a significantly decreased fertilization rate and aberrant dynamics of cytoskeleton-dependent events, whereas oocytes inseminated after the post-thaw incubation displayed fertilization rates and cytoskeletal dynamics comparable to those in controls. Cryopreservation did not increase polyspermy but significantly increased digyny when the oocytes were inseminated after the post-thaw incubation. All digynic eggs displayed an abnormal spindle remnant in comparison with diploid or polyspermic eggs.Conclusion : A brief period of incubation after thawing allows recovery and positively affects fertilization and cytoskeletal dynamics. Cryopreservation does not impair the functionality of microfilaments and cytoplasmic microtubules during postfertilization events. Our findings suggest that the increased rate of digyny in cryopreserved oocytes may be related to the spindle disorganization, leading to failure in segregation of the chromosomes, rather than to direct malfunction of the microfilaments in polar body formation. (shrink)

Graphical AbstractThe origins of asexuality in plants as well as animals have long puzzled researchers. In article 2000111, Diego Hojsgaard and Manfred Schartl integrate old ideas with recent molecular and genomic data and provide a single mechanistic model for this phenomenon. They highlight two usually overlooked conditions to understand the molecular nature of clonal organisms to explain asexuals' developmental diversity and biologically vague cases such as automixis and polyploidy.

Since the beginning of time, the pre-biological and the biological world have seen a steady increase in complexity of form and function based on a process of combination and re-combination. The current modern synthesis of evolution known as the neo-Darwinian theory emphasises population genetics and does not explain satisfactorily all other occurrences of evolutionary novelty. The authors suggest that symbiosis and hybridisation and the more obscure processes such as polyploidy, chimerism and lateral transfer are mostly overlooked and not featured (...) sufficiently within evolutionary theory. They suggest, therefore, a revision of the existing theory including its language, to accommodate the scientific findings of recent decades. (shrink)

Knowledge of eukaryotic life cycles and associated genome dynamics stems largely from research on animals, plants, and a small number of “model” (i.e., easily cultivable) lineages. This skewed sampling results in an underappreciation of the variability among the many microeukaryotic lineages, which represent the bulk of eukaryotic biodiversity. The range of complex nuclear transformations that exists within lineages of microbial eukaryotes challenges the textbook understanding of genome and nuclear cycles. Here, we look in‐depth at Foraminifera, an ancient (∼600 million‐year‐old) lineage (...) widely studied as proxies in paleoceanography and environmental biomonitoring. We demonstrate that Foraminifera challenge the “rules” of life cycles developed largely from studies of plants and animals. To this end, we synthesize data on foraminiferal life cycles, focusing on extensive endoreplication within individuals (i.e., single cells), the unusual nuclear process calledZerfall, and the separation of germline and somatic function into distinct nuclei (i.e., heterokaryosis). These processes highlight complexities within lineages and expand our understanding of the dynamics of eukaryotic genomes. (shrink)

There are many layers of regulation governing DNA replication to ensure that genetic information is accurately transmitted from mother cell to daughter cell. While much of the control occurs at the level of origin selection and firing, less is known about how replication fork progression is controlled throughout the genome. In Drosophila polytene cells, specific regions of the genome become repressed for DNA replication, resulting in underreplication and decreased copy number. Importantly, underreplicated domains share properties with common fragile sites. The (...) Suppressor of Underreplication protein SUUR is essential for this repression. Recent work established that SUUR functions by directly inhibiting replication fork progression, raising several interesting questions as to how replication fork progression and stability can be modulated within targeted regions of the genome. Here we discuss potential mechanisms by which replication fork inhibition can be achieved and the consequences this has on genome stability and copy number control. (shrink)