bluuebunny/crossref_metadata_2025_split huggingface dataset:
Sympatric Speciation
Authors: Veltsos Paris , Ritchie Michael G 
Published: April 2013

Abstract

Sympatric and parapatric speciation refer to the origin of biological species in the absence of complete geographic isolation between the diverging taxa. Until recently, most biologists believed that geographic isolation was almost universal in the development of species, i.e. most species originated in allopatry. However, new empirical and theoretical studies have shown that speciation may occur despite the diverging populations having adjacent or overlapping geographic ranges and despite on‐going gene flow. Attention in speciation studies has shifted to the mechanisms responsible for reducing gene flow, regardless of the extent of geographic range overlap.

Key Concepts:

The generation of new species has historically been classified according to geographic context, based on the overlap of different populations.

Geographic overlap is usually thought to influence gene flow between populations.

Of all possible cases, diversification in sympatry has been considered the most unlikely.

Theoretical models and examples from nature have shown sympatric speciation to be possible, but rare. A sympatric stage following allopatric divergence may be more common.

Divergence in the presence of gene flow may be common, especially if disruptive selection and/or assortative mating are strong.

A modern view of speciation focuses on processes that generate divergence, rather than a strict geographical classification.

Speciation is usually the result of complex interactions between different genetic, environmental and geographic processes, and must be understood in terms of this more complicated reality.
Homology in Character Evolution
Authors: Staton Joseph L 
Published: April 2013

AbstractHomology forms the basis of organisation for comparative biology. Richard Owen's simple definition of homology as the ‘same organ in different animals under every variety of form and function’ takes on new meaning in light of Darwin's concept of descent with modification. The modern study of comparative biology and phylogenetics is grounded in the notion that organisms share a greater proportion of homologous (i.e. ‘derived’) characteristics the more recently they share a common ancestor. Instead of solely being the purview of anatomical study, homology of these characteristics can be applied at the level of molecular evolution, genomic organisation and modern evolutionary developmental biology (‘evo/devo’). Aspects of homology help us formulate hypotheses about the meaning of changes in homologous structures in the inference of evolution of organisms at all levels of biological organisation.Key Concepts:Homology was originally a measure of correspondence between anatomical structures.Descent with modification explains homology as a result of shared ancestry that reflects modifications in characteristics within related lineages.Comparative methods use changes in homologous characteristics to infer evolution within groups (phylogenetics).Recent advances in molecular biology allow for the comparison of homologous characteristics at more diverse levels, including DNA sequences, genomic arrangements and developmental pathways.Recent work highlights homologous characteristics at a fundamental or ancient level, termed ‘deep homology’.
Heterochrony
Authors: Horder Tim 
Published: April 2013

AbstractHeterochrony has often been proposed as a leading mechanism for macroevolutionary change in morphology of organisms, based simply on a relative shift in the timing of events during embryonic development in the evolving organisms. Its most recent formulation was in Gould's ‘clock model’. Such timing shifts were seen as being controlled genetically (by ‘rate genes’) or by hormones (such as thyroxine). Heterochrony has much in common with the concept of allometry. The concept originated in the context of the idea of recapitulation where embryonic development was interpreted as a repetition of the organism's evolutionary history but in accelerated form; deviations from this recapitulation were identified as instances of heterochrony. In a later version used to explain macroevolutionary change (paedomorphosis), a new adult morphology is derived from ancestral larval forms by truncation of ancestral adult stages. However, it is difficult to see how genuine structural novelty can be explained, given that timing changes during development can in themselves do no more than redeploy preexisting characters. The explanatory value of the concept of heterochrony is increasingly questioned; at best it only indirectly relates to, or identifies, specific underlying mechanisms. It is in effect a form of redescription.Key Concepts:The extension across time of stages and component events in embryonic development is an inherent feature because development involves a cumulative succession of cell multiplication and differentiative processes, all starting from the single cell stage of the egg.The controversial concept of heterochrony attempts to characterise a mechanism controlling variations in developmental timing of the embryo or its component parts, which could potentially mediate and explain the evolution of new morphologies.Neoteny is a paradigm example of heterochrony, in which the adult morphology of a species is thought to be derived from the larval form of an ancestral species whose adult form is lost, perhaps by abandonment of the preexisting developmental events of metamorphosis.It has been proposed that a heterochronic process such as neoteny could be the basis for major innovations of new taxa (macroevolution); in a form of heterochrony termed paedomorphosis, a new body plan can be established by elaboration of a larval morphology rather than being restricted to modifications to the preexisting adult body plan.
Genetic Load
Authors: Whitlock MC , Davis B 
Published: April 2013

Abstract

Genetic load is the reduction in the mean fitness of a population relative to a population composed entirely of individuals having optimal genotypes. Load can be caused by recurrent deleterious mutations, genetic drift, recombination affecting epistatically favourable gene combinations, or other genetic processes. Genetic load potentially can cause the mean fitness of a population to be greatly reduced relative to populations without sources of less fit genotypes. Mutation load can be difficult or impossible to measure. Many species have mutation rates low enough that substantial genetic load is not expected, but for others, such as humans, the mutation rate may be great enough that load can be substantial. In extremely small populations, drift load, caused by the fixation by drift of weakly deleterious mutations, can threaten the probability of persistence of the population. Migration from other populations adapted to different local conditions can bring in locally maladapted alleles, resulting in migration load.

Key Concepts:

Genetic load is the reduction in mean fitness of a population caused by some population genetic process.

Mutation load is the reduction in fitness caused by recurrent deleterious mutations.

Mutation load may be as great as 95% for the human population.

Drift load is the reduction in mean fitness caused by genetic drift. In extreme cases, deleterious alleles can reach a frequency of one in a population because of genetic drift.

Genetic load can also be caused by recombination breaking up beneficial combinations of alleles, segregation reducing the frequency of fit heterozygotes, or migration bringing less fit alleles into a local population.
Malaria
Authors: Bannister Lawrence H , Sherman Irwin W 
Published: April 2013

Abstract

Malaria is a disease of the blood resulting from infection by protozoan parasites of the genus
Plasmodium
, transmitted by female Anopheline mosquitoes. Five species of
Plasmodium
infect humans,

Plasmodium falciparum

,

Plasmodium vivax

,

Plasmodium ovale

,

Plasmodium malariae

and

Plasmodium knowlesi

. The parasite life cycle includes stages in the mosquito and in human liver cells and red blood cells, which they penetrate and feed on.

Plasmodium falciparum

is the main cause of malaria deaths, especially of young African children. Cerebral malaria, renal and pulmonary failure are major pathologies. Frequent infections gradually confer immunity over several years, but this is rarely complete. Many antimalarial drugs are available for prophylaxis and treatment, but emerging parasite resistance limits their use. Several vaccines are being tested, with some moderate success. Efforts to control malaria include breeding grounds drainage, insecticide sprays and use of insecticide‐impregnated bednets. With the help of several major global initiatives, there are reports of significant reductions of malaria cases.

Key Concepts:

Malaria is a disease caused by infection of red blood cells (erythrocytes) by protozoan parasites of the genus
Plasmodium.

Malaria is a major global disease affecting many millions worldwide and causing up to a million or more deaths a year, mostly children under 5 years in sub‐Saharan Africa.

Malaria is caused by five species of
Plasmodium
, the most lethal being
Plasmodium falciparum
.

Malaria parasites are transmitted to humans by the bite of infected female mosquitoes, and then the parasite multiplies first in the liver and later in the bloodstream before passing back to the mosquito as it feeds.

Malaria has many symptoms, which include cyclic episodes of fever, the frequency depending on malaria parasite species.

Severe malaria is frequently fatal if not treated rapidly.

The pathology of malaria is related to blood infections, either by destroying erythrocytes or by blocking small blood vessels (sequestration) as occurs in cerebral malaria.

Acquired immunity builds up gradually and its maintenance depends on repeated infections; innate immunity is often associated with genetic diseases of the blood.

Numerous antimalarial drugs have been developed including those related to quinine and more recently artmesinin, but parasites have developed resistance to most of these in some parts of the world; several vaccines are now under trial.

Other efforts to control malaria include elimination of mosquitoes and draining their breeding grounds plus the use of insecticide‐impregnated bednets to minimise transmission.
Leishmaniases
Authors: Chang Kwang Poo 
Published: April 2013

Abstract

Leishmaniases are parasitic diseases, which are transmitted by the blood‐sucking sand flies as the vectors. These diseases are largely zoonotic, but considered as anthroponotic in some endemic sites. The reservoir animals include canines, rodents, edentates and other wild animals. A spectrum of clinical manifestations is associated with these diseases, ranging from self‐healing cutaneous lesions to facial mucocutaneous disfiguration to visceralization with fatal consequence. According to the World Health Organisation, there are 12 million cases of these diseases in approximately 90 countries with an annual incidence of approximately 2 million. Treatment of leishmaniases depends on very few drugs for chemotherapy, which are either expensive or toxic and have lost effectiveness due to drug‐resistance. The causative agents are the trypanosomatid protozoa
Leishmania
, widespread in tropical and subtropical areas. Intraphagolysosomal parasitism of macrophages by these parasites is a key feature in considering molecular mechanisms of their virulence relevant to developing effective drugs and vaccines needed.

Key Concepts:

Trypanosomatid protozoa in the genus of
Leishmania
cause Leishmaniases.

They have evolved mechanisms to live in macrophage endosomes/lysosomes.

The diseases are wide‐spread mainly in tropical and subtropical areas of the world.

The diseases are largely zoonotic and transmitted by blood‐sucking sand flies.

Reservoirs include domestic and wild animals, for example, canines, rodents & edentates.

Clinically, there are cutaneous, mucocutaneous and visceral Leishmaniasis.

The clinical spectrum results from host immunopathology to
Leishmania
infection.

Chemotherapy relies on toxic drugs and loses its effectiveness to drug‐resistance.

Drug development programs by targeting unique parasite enzymes are very few.

Control programs include the use of anti‐vector pesticide/repellents.
Eimeria
Authors: Duszynski Donald W 
Published: April 2013

AbstractThe protist phylum Apicomplexa Levine, 1970, comprises >4000 described species, all obligate parasites, that infect a wide range of vertebrates and invertebrates. Within the phylum, the genusEimeriaSchneider, 1875, (Family Eimeriidae) is the mostspecioselineage (>1800 species) of intracellular parasites found in all classes of vertebrates and some invertebrates. All eimerians have direct life cycles that include both asexual (merogony) and sexual (gamogony) reproduction, the latter resulting in the production of resistant propagules, the oocysts, which are discharged into the environment by the infected host. Endogenous development usually occurs in epithelial or endothelial cells of the intestine or in related structures (e.g. gall bladder, bile ducts, renal tubular epithelium, esophageal epithelium, oviduct epithelium uterine wall epithelium, etc.). Some eimerian species are highly pathogenic, especially in domesticated animals, but some also in wild animal species.Key Concepts:Within the coccidia proper, the Family Eimeriidae comprises about 15 genera, includingEimeria; all are obligate intracellular parasites known to infect every animal group which has been examined for them.Untapped Biodiversity: The genusEimeria, with >1800 described forms, may be the most speciose group of parasites on Earth, but it has been little studied in the Earth's app. 62 150 vertebrate species.It can be estimated that 98.6% (or more) of the coccidian parasites of vertebrates are yet to be discovered, based on what we know through 2010.Our knowledge of the biodiversity of life on Earth is still badly incomplete, especially of the microbes that inhabit the seas, the soils and the bodies of ‘higher’ organisms that live in and on them.Parasitologists, and most other biologists in general, agree that parasites represent a large proportion of all living species and may, in fact, far exceed the total number of all ‘free‐living’ forms.As human population increases and agricultural development continues to shrink/fragment natural ecosystems, food webs will be disrupted and new and more virulent strains of coccidia may be expected, especially when exacerbated by environmental stressors (pollution), global climate change and invasion of new parasites from edge‐dwelling hosts.
Entamoeba
and
Entamoeba histolytica
Authors: Pritt Bobbi S , Clark C Graham 
Published: April 2013

Abstract

Entamoeba
species are unicellular eukaryotes that parasitise all classes of vertebrates and some invertebrates. Only three species of
Entamoeba
have been proven to cause disease and sometimes death in their hosts:

Entamoeba histolytica

, a parasite of humans,

Entamoeba nuttalli

, a parasite of nonhuman primates and

Entamoeba invadens

, a parasite of reptiles. Other species appear to live as commensals in their hosts and do not cause evident disease. Amoebiasis is defined as infection with

E. histolytica

regardless of symptoms. Most cases of amoebiasis are asymptomatic, but

E. histolytica

can rarely cause intestinal or disseminated disease. Identification of

E. histolytica

is complicated by the existence of two morphologically identical amoebae that may also colonise the human intestinal tract,

Entamoeba dispar

and

Entamoeba moshkovskii

, neither of which are thought to cause disease.
Entamoeba
spp. are commonly studied to gain further insight into protozoal evolution, amoeboid locomotion and cell‐killing ability, among other topics.

Key Concepts:

Entamoeba
species are unicellular eukaryotes in the supergroup Amoebozoa.

Entamoeba
species parasitise all classes of vertebrates and some invertebrates.

The life cycle of most
Entamoeba
species consists of a motile, feeding and reproductive trophozoite stage and an environmentally resistant cyst stage responsible for transmitting the infection.

Most
Entamoeba
species do not cause disease in their host and are considered commensal organisms.

Entamoeba histolytica

infection is referred to as amoebiasis regardless of whether or not symptoms are present.

Most infections with

E. histolytica

in humans are asymptomatic, but

E. histolytica

can rarely cause dysentery or disseminated disease and is associated with significant human morbidity and mortality worldwide.

Diagnosis and epidemiologic studies of

E. histolytica

are complicated by the presence of morphologically identical amoebae in human faeces.

Entamoeba
species are studied to gain insights into aspects of biology, including human disease, parasitism, evolution, cell locomotion and cell‐killing mechanisms.
Vorticella
Authors: Buhse Howard E , McCutcheon Suzanne M , Clamp John C , Sun Ping 
Published: April 2013

AbstractMany species of the genusVorticellaare common ciliates living in many types of aquatic habitats. Their distinctive, contractile stalk anchors the unicellular body to a substrate. Contraction of the cell is calcium‐driven and independent ofadenosine triphosphate(ATP), relying on the presence of calcium‐binding proteins. A coupled mechanochemical model accounts for the coiling of an elastic stalk and the binding of calcium to the calcium‐binding proteins. Stalk elongation is driven by elastic energy stored in the coiled stalk and the rate of elongation is controlled by calcium dissociation from calcium‐binding proteins and its re‐sequestration into internal stores. Recently, the taxonomic status of the subclass Peritichia has changed radically based on molecular studies. The former order Mobilida was removed from the subclass and elevated to the level of a separate subclass. Phylogenetic studies using small subunitribosomal ribonucleic acid(ssu rRNA) and internal transcribed spacer (ITS) sequences place the Peritrichia closer to the subclass Hymenostomatia and the Mobilia closer to the subclass Peniculia.Key Concepts:Contractility inVorticellarelies on a calcium‐driven, ATP‐independent system that depends on calcium‐binding proteins.Vorticellacan be used as a bio‐indicator for the evaluation of polluted waterways.Vorticellaand similar filter‐feeding ciliates are used to clarify sewage in waste treatment plants.Vorticellais a useful organism for the study of exocytosis by examining production and release of granules that form the extracellular matrix of the stalk and the adhesive pad.Vorticellastalk contraction and relaxation is dependent of the presence or absence of calcium ions, this property can be exploited in a microelectromechanical system (MEMS) to perform linear work generated by spasmonemal contraction and controlled by calcium levels.Chemical analysis of the sticky material that anchors the stalk ofVorticellafirmly to its substrate could result in the identification of a new type of adhesive material that sets in water.
Plant Nuclear Genome Composition
Authors: Heslop‐Harrison JS (Pat) , Schmidt Thomas 
Published: April 2013

Abstract

The plant nuclear genome consists of
deoxyribonucleic acid
(DNA) divided among the chromosomes within the cell nucleus. Plant genomes contain coding and regulatory sequences for genes and repetitive DNA. They are evolutionarily dynamic and analysis provides insights into the evolution of genes and genomes, supporting studies of species phylogeny and plant breeding. The amount of DNA present in plant genomes, nearly constant within one species, varies over some 2300 times between species; the majority of the difference is accounted for genome duplication, or by various classes of repetitive DNA which may be dispersed widely along chromosomes or located in arrays at a small number of loci. Bioinformatics and genome sequence analysis show conservation of many gene sequences across all plants. Repetitive DNA motifs may be conserved over large taxonomic groupings or evolve rapidly. Repetitive DNA plays an important role as a structural component of plant chromosomes (e.g. telomeres and centromeres) and affects gene regulation.

Key Concepts:

Plant nuclear genome sizes, constant in a species, vary from 60 000 000 base pairs of DNA (written as 60 Mbp) to 150 000 Mbp, a range of 2300 times.

The DNA of plant nuclear genomes is wrapped around the histone proteins to form nucleosomes, and the resulting chromatin is organised into linear chromosomes with characteristic numbers, sizes and morphology in each species.

Genomes contain coding sequences, typically for 25 000–40 000 genes, and regulatory sequences, as shown by genome sequencing and bioinformatics.

Size variation comes from polyploidisation, genome or chromosome duplication, and particularly amplification of DNA motifs to give repetitive DNA.

Repetitive DNA consists of sequences that amplify via RNA intermediates (retrotransposons), through copying and reinsertion of DNA (DNA transposable elements), or other mechanisms (satellite and microsatellite DNA).

Some repetitive DNA encodes genes, in particular the 45S and the 5S ribosomal RNA genes (rDNA); other repetitive DNA has structural roles at the telomeres or chromosomal ends and the centromeres where chromosomes attach to microtubules during segregation. In most species, much of the repetitive DNA has no known function.
Plant Breeding and Crop Improvement
Authors: Caligari Peter DS , Forster Brian P 
Published: April 2013

AbstractTo respond to the increasing need to feed the world's population, standing in 2011 at 7 billion and predicted to reach 9 billion by 2050, as well as an ever greater demand for a balanced and healthy diet, there is a continuing need to produce improved new cultivars of plants, particularly crop plants. Land available for crop production is limited, with much of the world's best soils already in use and others protected, for example, for environmental concerns. The demand for food brings marginal lands into play for which stress tolerant crops need to be developed. Climate variation is yet another challenge breeders have to respond to. In short, more food, fibre, fuel and forage need to be produced per unit of land. The strategies used to meet these demands are increasingly based on our knowledge of relevant science, particularly genetics and reproductive biology. Success is gained by a multidisciplinary understanding and the deployment of relevant science and technology.Key Concepts:Plant breeders must have access to genetic variation in crop species.Plant breeders must be equipped with the tools to respond quickly to new demands by developing accelerated breeding techniques and the ability to screen for traits of interest rapidly among progeny.Yield and yield stability remain the top priorities for breeders.Increasing production of plant products is essential for food, feed and fibre for the increasing World population.Breeders must be visionary in planning for requirements in the future, at least 7–20 years ahead, as this is the timescale from initiating the breeding programme to releasing cultivars.