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https://www.nature.com/nature/journal/v502/n7469/full/nature12489.html
LETTER doi: 10.1038/nature12489 Life history trade-offs at a single locus maintain sexually selected genetic variation 24, Jacob Gratteni.3t, Camillo Berenos2, Jill G. Pilkington Susan E. Johnston Tim H. Clutton-Brock Josephine M. Pemberton & Jon Slatel Sexual selection, through intra-male competition or female choice, One wild population where the genetic architecture of a sexually assumed to be a source of strong and sustained directional sel elected trait has been characterized is the Soay sheep ofSt Kilda (Ovis tion in the wild 3. In the presence of such strong directional selec aries), a population of primitive domestic sheep that has existed com tion, alleles enhancing a particular t are predicted to become pletely ged for around 4,000 years and has been intensively unmana fixed within a population, leading to a decrease in the underlying studied since 1985. During the mating season (rut), there is strong genetic variations. However, there is often considerable genetic competition between males for access to oestrous females. Most male variation underlying sexually selected traits in wild populations, develop normal horns, but around 13% develop ves igial horns (scurs and consequently, this phenomenon has become a long-discussed conferring reduced reproductive success (Fig. 1). The horn size of wild Soay sheep, no issue in the field of evolutionary biology ,4,5 In horned males is positively correlated with matingsuccessi, yet large horns confer an advantage in strong intra-sexual competi there is substantial heritable variatio trait (h -0.37), yet males show an inherited polymorphism for horn type and Females develop much smaller horns, and are either normal-horned tion (32%), scurred (40%), or polled (lacking horns or scurs, 28%; Sup- have substantial genetic variation in their horn sizee. Here we show that most genetic variation in this trait is maintained by atrade-off plementary Fig. 1). Recent studies have revealed that a sin gle gene gle gene, relaxin-like relaxin-like receptor 2 (RXFP2), explain most of the genetic variation between natural and sexual selection n at a sin receptor 2 (RXFP2). We found that an allele conferring larger inhorn morphology in Soay sheep and domestic sheep' Two RXFP2 sassociated with higher reproductive success, whereas aneles, Hot and HoP, have been identified in Soay sheep: Ho+ confers horns, Ho a smaller horn allele, Ho nfers increased survival, resulting in a larger, normal horns, whereas Ho' confers smaller horns, with around net effect of overdominance (that is, heterozygote advantage for half of HopHop males developing scurs (Fig. 1). Furthermore, RXFP2 fitness at RXFP2. The nature of this trade-off is simple relative to contributes ~76% of the additive genetic variation in horn size in nor ally mal-horned males, including normal-horned HoPHoh maless (Fig. 1 and commonly proposed explanations for the maintenance of sexu selected traits, such as genic capture (good genes) and sexu ally Supplement tary Note 1). This discovery provides acritical opportunity understand the relative importance of sexual and natural selection in antagonistic selections Our results demonstrate that by identify- maintaining genetic variation underlying horn development. ng the genetic architecture of trait variation, we can determine the principal mechanisms maintaining genetic variation in traits under strong selection and explain apparently counter-evolutionary bservations The persistence of genetic variation in traits under sustained sexual election is a fundamental paradox in evolutionary biology, for which several explanationshavebeen proposed. First, sexually dimorphicchar acters could be an honest signal of male quality or condition, in which the best-condition males develop the largest traits A. Under this genic capture' model, many loci contribute to male condition, creating a large mutational target for the sexually selected trait. As a result, genetic variation will persis despite strong directional selection'. Second, vari ation may be maintained by genetic trade-offs that constrain evolution of the focal trait. For example, sexually selected traits often exceed the point at which they would be optimal for survival indicating that trade-offs existbetween sexual and non-sexual fitness. A variant on this model is intra-locus sexual conflict, driven by sexuallyantagonisticselec- in this scenario, alleles that increase male fitness are associated tion with decreased female fitness (and vice versa) Testing and disentangling which of the theories explains em patterns remains difficult owing to the relatively limited knowledge of the genetic architecture (that is, the number of genes and the magni ude of their effects) ofrelevant traits as there are remarkably few Figure 1 Horn morphology variation with RXF geno type. Examples of systems where the genes responsible for sexuallyselected trait variation adult male horn mo rphology with their corresponding RXFP2 genotypes. are known. However, the advent of affordable genomic technologies ld normal HopHoP for fitness-related variation in wild populations Department otAnimal and Plant Scences, university otShettield, shetteld sio2TN, nstitute of Evolutionary Biology. University of Edinburgh,Edinburgh eH93JT,uK.3Qu ensland Brain Institute, University of Queensland, Brisbane 4072 Australia. Dep rtment of Zoology, University of Cambridge,Cambridge CB2 3E ,UK tPresent addresses Institute of Evolutionary Biology, University of Edinburgh, Edinburgh EH9 3JT, UK (SEJ) Queensland Brain itute, University of Queensland, Brisbane 4072, Australia C.G).Explanation / Answer
(1) A health individual must have a cell-free DNA in the blood plama. In the patients with metastatic cancer, there is an elevated level of cell-free circulating tumor DNA (ctDNA) in blood plasma.
(2) (a) The nucleosomal DNA is protected against MNase digestion and so the regions surrounding the nucleosome are less sensitive to MNase digestion. The number of DNA fragments in a given locus might increase or decrease with increasing MNase concentration. The increasing nucleosome signal shows decreasing MNase levels and decreasing nucleosome signal will show decreasing MNase levels.
(b) The nucleosomes are usually positioned at canonical distances from the transcription start site (TSS).
(c) The TSS of highly expressed housekeeping genes are usually conserved in comparison to unexpressed or lowly expressed genes.
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