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Analyses of the genomes of Neanderthals and Denisovans, the closest evolutionary relatives of present-day humans, suggest that our ancestors were part of a web of now-extinct populations linked by limited, but intermittent or sometimes perhaps even persistent, gene flow.

Several studies have suggested that introgressed Neandertal DNA was subjected to negative selection in modern humans. A striking observation in support of this is an apparent monotonic decline in Neandertal ancestry observed in modern humans in Europe over the past 45,000 years. Here, we show that this decline is an artifact likely caused by gene flow between modern human populations, which is not taken into account by statistics previously used to estimate Neandertal ancestry. When we apply a statistic that avoids assumptions about modern human demography by taking advantage of two high-coverage Neandertal genomes, we find no evidence for a change in Neandertal ancestry in Europe over the past 45,000 years. We use whole-genome simulations of selection and introgression to investigate a wide range of model parameters and find that negative selection is not expected to cause a significant long-term decline in genome-wide Neandertal ancestry. Nevertheless, these models recapitulate previously observed signals of selection against Neandertal alleles, in particular the depletion of Neandertal ancestry in conserved genomic regions. Surprisingly, we find that this depletion is strongest in regulatory and conserved noncoding regions and in the most conserved portion of protein-coding sequences.

To utilize the power of high-throughput sequencers, target enrichment methods have been developed. The majority of these require reagents and equipment that are only available from commercial vendors and are not suitable for the targets that are a few kilobases in length. We describe a novel and economical method in which custom made long-range PCR products are used to capture complete human mitochondrial genomes from complex DNA mixtures. We use the method to capture 46 complete mitochondrial genomes in parallel and we sequence them on a single lane of an Illumina GA(II) instrument. This method is economical and simple and particularly suitable for targets that can be amplified by PCR and do not contain highly repetitive sequences such as mtDNA. It has applications in population genetics and forensics, as well as studies of ancient DNA.

In the modern human genome, elevated Neanderthal ancestry is found at genes affecting keratin filaments, suggesting that gene flow with Neanderthals helped modern humans to adapt to non-African environments; deficiencies of Neanderthal ancestry are also found, particularly on the X chromosome and in genes expressed highly in testes, suggesting that some Neanderthal mutations were not tolerated on a modern human genetic background as they reduced male fertility. Neanderthal genes today The modern human genome contains traces of Neanderthal ancestry. But is Neanderthal DNA distributed uniformly across the human genome, or is it concentrated more in some parts than in others? Sriram Sankararaman et al . show that parts of the human genome enriched for genes affecting keratin filaments (in hair, for example) also contain a relatively high concentration of Neanderthal DNA, suggesting that this DNA helped modern humans adapt to the chillier non-African environment. On the downside, many Neanderthal-derived alleles are associated with disease risk. Other parts of the human genome contain a deficiency of Neanderthal alleles, implying their active removal in evolution. Among the 'lost' genes are a number expressed in the testis and on the X chromosome, implying that Neanderthal DNA reduced human fertility when moved to a modern human genetic background. Genomic studies have shown that Neanderthals interbred with modern humans, and that non-Africans today are the products of this mixture 1 , 2 . The antiquity of Neanderthal gene flow into modern humans means that genomic regions that derive from Neanderthals in any one human today are usually less than a hundred kilobases in size. However, Neanderthal haplotypes are also distinctive enough that several studies have been able to detect Neanderthal ancestry at specific loci 1 , 3 , 4 , 5 , 6 , 7 , 8 . We systematically infer Neanderthal haplotypes in the genomes of 1,004 present-day humans 9 . Regions that harbour a high frequency of Neanderthal alleles are enriched for genes affecting keratin filaments, suggesting that Neanderthal alleles may have helped modern humans to adapt to non-African environments. We identify multiple Neanderthal-derived alleles that confer risk for disease, suggesting that Neanderthal alleles continue to shape human biology. An unexpected finding is that regions with reduced Neanderthal ancestry are enriched in genes, implying selection to remove genetic material derived from Neanderthals. Genes that are more highly expressed in testes than in any other tissue are especially reduced in Neanderthal ancestry, and there is an approximately fivefold reduction of Neanderthal ancestry on the X chromosome, which is known from studies of diverse species to be especially dense in male hybrid sterility genes 10 , 11 , 12 . These results suggest that part of the explanation for genomic regions of reduced Neanderthal ancestry is Neanderthal alleles that caused decreased fertility in males when moved to a modern human genetic background.

Analysis of DNA from a 37,000–42,000-year-old modern human from Romania reveals that 6–9% of the genome is derived from Neanderthals, with the individual having a Neanderthal ancestor as recently as four to six generations back. A Neanderthal in the family This paper presents an analysis of the DNA from a modern human individual from Peştera cu Oase in Romania who lived 37,000–42,000 years ago. This individual male carries 6–9% Neanderthal DNA — compared to just 1–3% reported in other modern Eurasian humans — and given the size of the segments of Neanderthal ancestry in his genome, the authors conclude that he had a Neanderthal among his ancestors four to six generations back in his family tree. This shows that interbreeding between modern humans and Neanderthals was not limited to the Africa and the Near East, but that it occurred in Europe as well. But the population to which the Oase individual belonged did not make a substantial genetic contribution to later humans in Europe. This provides evidence for an early initial migration of modern humans who mixed with Neanderthals but did not contribute to present-day Europeans. Neanderthals are thought to have disappeared in Europe approximately 39,000–41,000 years ago but they have contributed 1–3% of the DNA of present-day people in Eurasia 1 . Here we analyse DNA from a 37,000–42,000-year-old 2 modern human from Peştera cu Oase, Romania. Although the specimen contains small amounts of human DNA, we use an enrichment strategy to isolate sites that are informative about its relationship to Neanderthals and present-day humans. We find that on the order of 6–9% of the genome of the Oase individual is derived from Neanderthals, more than any other modern human sequenced to date. Three chromosomal segments of Neanderthal ancestry are over 50 centimorgans in size, indicating that this individual had a Neanderthal ancestor as recently as four to six generations back. However, the Oase individual does not share more alleles with later Europeans than with East Asians, suggesting that the Oase population did not contribute substantially to later humans in Europe.

The hormone progesterone is important for preparing the uterine lining for egg implantation and for maintaining the early stages of pregnancy. The gene encoding the progesterone receptor (PGR) carries introgressed Neandertal haplotypes with two missense substitutions and a mobile Alu element. These Neandertal gene variants have reached nearly 20% frequency in non-Africans and have been associated with preterm birth. Here, we show that one of the missense substitutions appears fixed in Neandertals, while the other substitution as well as the Alu insertion were polymorphic among Neandertals. We show that two Neandertal haplotypes carrying the PGR gene entered the modern human population and that present-day carriers of the Neandertal haplotypes express higher levels of the receptor. In a cohort of present-day Britons, these carriers have more siblings, fewer miscarriages, and less bleeding during early pregnancy suggesting that the Neandertal progesterone receptor alleles promote fertility. This may explain their high frequency in modern human populations.

Comparisons of DNA sequences between Neandertals and present-day humans have shown that Neandertals share more genetic variants with non-Africans than with Africans. This could be due to interbreeding between Neandertals and modern humans when the two groups met subsequent to the emergence of modern humans outside Africa. However, it could also be due to population structure that antedates the origin of Neandertal ancestors in Africa. We measure the extent of linkage disequilibrium (LD) in the genomes of present-day Europeans and find that the last gene flow from Neandertals (or their relatives) into Europeans likely occurred 37,000-86,000 years before the present (BP), and most likely 47,000-65,000 years ago. This supports the recent interbreeding hypothesis and suggests that interbreeding may have occurred when modern humans carrying Upper Paleolithic technologies encountered Neandertals as they expanded out of Africa.

DNA that survives in museum specimens, bones and other tissues recovered by archaeologists is invariably fragmented and chemically modified. The extent to which such modifications accumulate over time is largely unknown but could potentially be used to differentiate between endogenous old DNA and present-day DNA contaminating specimens and experiments. Here we examine mitochondrial DNA sequences from tissue remains that vary in age between 18 and 60,000 years with respect to three molecular features: fragment length, base composition at strand breaks, and apparent C to T substitutions. We find that fragment length does not decrease consistently over time and that strand breaks occur preferentially before purine residues by what may be at least two different molecular mechanisms that are not yet understood. In contrast, the frequency of apparent C to T substitutions towards the 5'-ends of molecules tends to increase over time. These nucleotide misincorporations are thus a useful tool to distinguish recent from ancient DNA sources in specimens that have not been subjected to unusual or harsh treatments.

The first step in CRISPR-Cas9-mediated genome editing is the cleavage of target DNA sequences that are complementary to so-called spacer sequences in CRISPR guide RNAs (gRNAs). However, some DNA sequences are refractory to CRISPR-Cas9 cleavage, which is at least in part due to gRNA misfolding. To overcome this problem, we have engineered gRNAs with highly stable hairpins in their constant parts and further enhanced their stability by chemical modifications. The ‘Genome-editing Optimized Locked Design’ (GOLD)-gRNA increases genome editing efficiency up to around 1000-fold (from 0.08 to 80.5%) with a mean increase across different other targets of 7.4-fold. We anticipate that this improved gRNA will allow efficient editing regardless of spacer sequence composition and will be especially useful if a desired genomic site is difficult to edit. Some DNA sequences are refractory to CRISPR-Cas9 cleavage, partially due to gRNA misfolding. Here the authors engineer gRNAs to prevent misfolding and further enhanced their stability by chemical modifications allowing robust genome editing regardless of target sequence.

At its core, genetics is a historical discipline. Mutations are passed on from generation to generation and accumulate as a result of chance as well as of selection within and between populations and species. However, until recently, geneticists were confined to the study of present-day genetic variation and could only indirectly make inferences about the historical processes that resulted in the variation in present-day gene pools. This “time trap” has now been overcome thanks to the ability to analyze DNA extracted from ancient remains, and this is about to revolutionize several aspects of genetics.