Subspecies composition and founder contribution of the captive U.S. chimpanzee (Pan troglodytes) population

Chimpanzees are presently classified into three subspecies: Pan troglodytes verus from west Africa, P.t. troglodytes from central Africa, and P.t. schweinfurthii from east Africa. A fourth subspecies (P.t. vellerosus), from Cameroon and northern Nigeria, has been proposed. These taxonomic designations are based on geographical origins and are reflected in sequence variation in the first hypervariable region (HVR-I) of the mtDNA D-loop. Although advances have been made in our understanding of chimpanzee phylogenetics, little has been known regarding the subspecies composition of captive chimpanzees. We sequenced part of the mtDNA HVR-I region in 218 African-born population founders and performed a phylogenetic analysis with previously characterized African sequences of known provenance to infer subspecies affiliations. Most founders were P.t. verus (95.0%), distantly followed by the troglodytes schweinfurthii clade (4.6%), and a single P.t. vellerosus (0.4%). Pedigree-based estimates of genomic representation in the descendant population revealed that troglodytes schweinfurthii founder representation was reduced in captivity, vellerosus representation increased due to prolific breeding by a single male, and reproductive variance resulted in uneven representation among male P.t.verus founders. No increase in mortality was evident from between-subspecies interbreeding, indicating a lack of outbreeding depression. Knowledge of subspecies and their genomic representation can form the basis for phylogenetically informed genetic management of extant chimpanzees to preserve rare genetic variation for research, conservation, or possible future breeding.     

Detection and molecular characterization of foamy viruses in Central African chimpanzees of the Pan troglodytes troglodytes and Pan troglodytes vellerosus subspecies

BACKGROUND: Foamy viruses are exogenous retroviruses that are highly endemic in non-human primates (NHPs). Recent studies, mainly performed in North America, indicated frequent simian foamy virus (SFV) infection in persons occupationally exposed to NHPs. This zoonotic infection was demonstrated mainly after bites by chimpanzees [Pan troglodytes (P. t.)] of the West African P. t. verus subspecies in primatology centers or zoos in the USA. METHODS: We studied 32 chimpanzees from the Central African subspecies P. t. troglodytes and P. t. vellerosus, originating from Cameroon (29 cases) or Gabon (3 cases). We screened first plasma or sera of the animals with a Western blot detecting the SFVs Gag doublet proteins. Then, we performed two nested polymerase chain reactions (PCRs) amplifying a fragment of the integrase and LTR regions and, finally, we made phylogenetical analyses on the sequences obtained from the integrase PCR products. RESULTS: By serological and/or molecular assays, we detected foamy viruses (FVs) infection in 14 chimpanzees. Sequence comparison and phylogenetic analyses of a 425 bp fragment of the integrase gene obtained for 10 of the 14 positive apes, demonstrated a wide diversity of new FVs strains that belong phylogenetically either to the P. t. troglodytes or P. t. vellerosus foamy viral clade. CONCLUSIONS: This study shows that chimpanzees living in these areas of Central Africa are infected by several specific foamy viruses. This raises, in such regions, the potential risk of a human retroviral infection of zoonotic origin linked to chimpanzees contacts, as already exemplified for STLV-1 and SIV infections.     

Dental variability of the Liberian chimpanzee,Pan troglodytes verus

Dental variability was studied in a collection of Liberian chimpanzee (Pan troglodytes verus) crania from one geographic area of Central Liberia. Morphological and metric data were compared to another population of the same subspecies studied by Schuman and Brace (1955) as well as to other pongid taxa. It appears that of all living pongids, chimpanzees are the most derived in their lower molar patterns, particularly, P. t. verus. It is clear, however that the mandibular molar patterns of contemporary chimpanzees are more similar to other pongids that to humans which is contra to the suggestions of Schuman and Brace. Hypocone reduction from M1 to M3 is the common pattern in all hominoids. Complete absence of the M3 hypocone is rare in pongids but it is present in the Frankfurt collection. Of living pongids, the gorilla expresses the least amount of hypocone reduction from M1 to M3. A cusp of Carabelli is recorded bilaterally in one P.t. verus. There is less odontometric variation in P.t. verus than in other pongids as indicated by the CV’s which may suggest the greater dental variability present when different geographic groups are included in the sample.     

Genetic subspecies diversity of the chimpanzee CD4 virus-receptor gene

Chimpanzees are naturally and asymptomatically infected by simian immunodeficiency virus (SIV). Pathogenic properties of SIV/HIV vary and differences in susceptibility and pathogenicity of SIV/HIV depend in part on host-specific factors such as virus-receptor/co-receptor interactions. Since CD4 plays a primary role in virus binding and since SIVcpz have been found only in two African chimpanzee subspecies, we characterized the genetic diversity of CD4 receptors in all four recognized subspecies of chimpanzees. We found noticeable variation in the first variable region V1 of CD4 and in intron six among the subspecies of chimpanzees. We found the CD4 receptor to be conserved in individuals belonging to the P. t. verus subspecies and divergent from the other three subspecies, which harbored highly variable CD4 receptors. The CD4 receptor of chimpanzees differed from that of humans. We question whether the observed diversity can explain the species-specific differences in susceptibility to and pathogenicity of SIV/HIV.     

Mitochondrial DNA genealogy of chimpanzees in the Nimba mountains and Bossou, West Africa

The chimpanzee populations of the Bossou and Nimba regions in West Africa were genetically surveyed to 1) reveal the genetic relationship between the Bossou and Nimba populations, and 2) elucidate the evolutionary relationship between the Bossou-Nimba and other West African populations. The chimpanzee group at Bossou is characterized by its small population size, no evidence of contact with neighboring populations, and no female immigration. It is believed that most females and adolescent males emigrate from this population. To reveal the genetic signature of these characteristics, we examined the genetic diversity of Bossou and two neighboring populations (Seringbara and Yealé) in the Nimba Mountains by sequencing approximately 605 bp of the mitochondrial DNA (mtDNA) control region. A total of 20 distinct mtDNA variants were observed from 56 sequences of noninvasively collected, anonymous samples. Nucleotide diversity in the Nimba Mountain populations was 0.03-0.04, and did not differ significantly from that in the Bossou population. Very few mitochondrial variants are shared among the sites sampled, which suggests that there is little gene flow involving mtDNA. Nevertheless, no clear population structures were revealed in either population. A comparison with published sequences from West African chimpanzees (Pan troglodytes verus) indicates that the variants observed in the Bossou and Nimba regions are scattered throughout the subspecies, rather than clustered according to geographic region. This suggests that the Bossou-Nimba populations derived only recently from the common ancestral population of the West African chimpanzees, and did not pass through a bottleneck.     

Eco-Geographical Diversification of Bitter Taste Receptor Genes (TAS2Rs) among Subspecies of Chimpanzees (Pan troglodytes)

Chimpanzees (Pan troglodytes) have region-specific difference in dietary repertoires from East to West across tropical Africa. Such differences may result from different genetic backgrounds in addition to cultural variations. We analyzed the sequences of all bitter taste receptor genes (cTAS2Rs) in a total of 59 chimpanzees, including 4 putative subspecies. We identified genetic variations including single-nucleotide variations (SNVs), insertions and deletions (indels), gene-conversion variations, and copy-number variations (CNVs) in cTAS2Rs. Approximately two-thirds of all cTAS2R haplotypes in the amino acid sequence were unique to each subspecies. We analyzed the evolutionary backgrounds of natural selection behind such diversification. Our previous study concluded that diversification of cTAS2Rs in western chimpanzees (P. t. verus) may have resulted from balancing selection. In contrast, the present study found that purifying selection dominates as the evolutionary form of diversification of the so-called human cluster of cTAS2Rs in eastern chimpanzees (P. t. schweinfurthii) and that the other cTAS2Rs were under no obvious selection as a whole. Such marked diversification of cTAS2Rs with different evolutionary backgrounds among subspecies of chimpanzees probably reflects their subspecies-specific dietary repertoires.     

The ciliate, Troglodytella abrassarti, contributes to polysaccharide hydrolytic activities in the chimpanzee colon

Entodiniomorphid ciliates are intestinal protists inhabiting the colons of African great apes. The participation of intestinal entodiniomorphid ciliates in ape hindgut digestion has been proposed, but little data have been available to support the hypothesis. We measured the specific activities of carboxymethyl cellulase, xylanase, inulinase, and α-amylase against different polysaccharides in the feces of captive chimpanzees and evaluated the participation of the entodiniomorphid ciliate, Troglodytella abrassarti, in these activities. T. abrassarti contributed to the total fecal hydrolytic activities of CM-cellulase by 16.2%, α-amylase by 5.95%, and xylanase by 0.66%. Inulinase activity in T. abrassarti samples was not measurable at reaction conditions used. The ciliates, T. abrassarti, actively participate in the chimpanzee hindgut fermentation of fiber and starch.     

Divergence population genetics of chimpanzees

The divergence of two subspecies of common chimpanzees (Pan troglodytes troglodytes and P. t. verus) and the bonobo (P. paniscus) was studied using a recently developed method for analyzing population divergence. Under the isolation with migration model, the posterior probability distributions of divergence time, migration rates, and effective population sizes were estimated for large multilocus DNA sequence data sets drawn from the literature. The bonobo and the common chimpanzee are estimated to have diverged approximately 0.86 to 0.89 MYA, and the divergence of the two common chimpanzee subspecies is estimated to have occurred 0.42 MYA. P. t. troglodytes appears to have had a larger effective population size (22,400 to 27,900) compared with P. paniscus, P. t. verus, and the ancestral populations of these species. No evidence of gene flow was found in the comparisons involving P. paniscus; however a clear signal of unidirectional gene flow was found from P. t. verus to P. t. troglodytes (2Nm = 0.51).     

Genetic diversity,evolutionary history and implications for conservation of the lion (Panthera leo) in West and Central Africa

Aim In recent decades there has been a marked decline in the numbers of African lions (Panthera leo), especially in West Africa where the species is regionally endangered. Based on the climatological history of western Africa, we hypothesize that West and Central African lions have a unique evolutionary history, which is reflected by their genetic makeup. Location Sub‐Saharan Africa and India, with special focus on West and Central Africa. Method In this study 126 samples, throughout the lion’s complete geographic range, were subjected to phylogenetic analyses. DNA sequences of a mitochondrial region, containing cytochrome b, tRNAPro, tRNAThr and the left part of the control region, were analysed. Results Bayesian, maximum likelihood and maximum parsimony analyses consistently showed a distinction between lions from West and Central Africa and lions from southern and East Africa. West and Central African lions are more closely related to Asiatic lions than to the southern and East African lions. This can be explained by a Pleistocene extinction and subsequent recolonization of West Africa from refugia in the Middle East. This is further supported by the fact that the West and Central African clade shows relatively little genetic diversity and is therefore thought to be an evolutionarily young clade. Main conclusions The taxonomic division between an African and an Asian subspecies does not fully reflect the overall genetic diversity within lions. In order to conserve genetic diversity within the species, genetically distinct lineages should be prioritized. Understanding the geographic pattern of genetic diversity is key to developing conservation strategies, both for in situ management and for breeding of captive stocks.     

Chaînes opératoires and resource-exploitation strategies in chimpanzee (Pan troglodytes) nut cracking

We apply archaeological methods to extend our knowledge of chimpanzee material culture. The cha?ne opératoire conceptual framework, as introduced by ethnography, established technology as a phased process. Prehistoric archaeology adopted this concept to elucidate technological variability in tool-making procedures, based on knowledge of tool functions or subsistence patterns. We focused on the detection of operational sequences by wild chimpanzees (Pan troglodytes verus) when nut cracking with lithic implements at the sites of Bossou and Diecké, Guinea, West Africa. Thus, while it has recently been claimed that chimpanzees leave behind recognizable assemblages of stone hammers that can be morphologically distinguished from Oldowan hammers, this is the first study to focus specifically on the existence of operational sequences during the utilization of stone tools by wild chimpanzees. By combining primatological and archaeological methods and examining ecological areas inhabited by different chimpanzee groups, we sought technological variability and identified variables influencing regional diversity in tool typology and technology. We compared three case studies: (1) Bossou-direct recording of experimental nut-cracking sessions; (2) Bossou- direct and indirect monitoring of nut-cracking sites in the wild; (3) Diecké-indirect monitoring of nut-cracking sites in the wild. Results suggest that chimpanzees perform sequences of repeated tool transport and nut cracking. Data show discrimination of tool functions based on tool features. We identified the most technologically complex tool for nut cracking, which was composed of four stones. We found regional diversity in chimpanzee stone assemblages. Raw-material type and tool mobility constrain technological development in human and nonhuman primates. Spatial analysis of tool distribution indicates a pattern of resource-exploitation strategy, revealing affinities with Oldowan.     

Long-term balancing selection at the antiviral gene OAS1 in Central African chimpanzees

Oligoadenylate synthetases (OAS) are interferon-induced enzymes that participate in the first line of defense against a wide range of viral infection in animals. Upon activation by viral double-stranded RNA, OAS synthesizes (2-5) oligoadenylates, which activate RNase L, leading to the nonspecific degradation of cellular and viral RNA. Some association studies in humans suggest that variation at one of the OAS genes, OAS1, could be influencing host susceptibility to viral infection. We assessed the diversity of OAS1 in hominoid primates with a focus on chimpanzees. We found that the OAS1 gene is extremely polymorphic in Central African chimpanzee and exhibits levels of silent and replacement diversity much higher than neutral regions of the chimpanzee genome. This level of variation strongly suggests that balancing selection is acting on OAS1, and indeed, this conclusion was validated by several tests of neutrality. We further demonstrated that balancing selection has been acting at this locus since the split between chimpanzees, humans, and gorillas (~8.6 Ma) and caused the persistence of two deeply divergent allelic lineages in Central African chimpanzees. These two groups of OAS1 alleles differ by a large number of amino acids (a.a.), including several a.a. putatively involved in RNA binding. It is therefore very likely that variation at the OAS1 locus affects the innate immune response of individuals to specific viral infection. Our data strongly suggest that interactions between viral RNA and OAS1 are responsible for the maintenance of ancestral polymorphisms at this locus for at least 13.2 My.     

Autosomal and mtDNA Markers Affirm the Distinctiveness of Lions in West and Central Africa

The evolutionary history of a species is key for understanding the taxonomy and for the design of effective management strategies for species conservation. The knowledge about the phylogenetic position of the lion (Panthera leo) in West/Central Africa is largely based on mitochondrial markers. Previous studies using mtDNA only have shown this region to hold a distinct evolutionary lineage. In addition, anthropogenic factors have led to a strong decline in West/Central African lion numbers, thus, the conservation value of these populations is particularly high. Here, we investigate whether autosomal markers are concordant with previously described phylogeographic patterns, and confirm the unique position of the West/Central African lion. Analysis of 20 microsatellites and 1,454 bp of the mitochondrial DNA in 16 lion populations representing the entire geographic range of the species found congruence in both types of markers, identifying four clusters: 1) West/Central Africa, 2) East Africa, 3) Southern Africa and 4) India. This is not in line with the current taxonomy, as defined by the IUCN, which only recognizes an African and an Asiatic subspecies. There are no indications that genetic diversity in West/Central Africa lions is lower than in either East or Southern Africa, however, given this genetic distinction and the recent declines of lion numbers in this region, we strongly recommend prioritization of conservation projects in West/Central Africa. As the current taxonomic nomenclature does not reflect the evolutionary history of the lion, we suggest that a taxonomic revision of the lion is warranted.     

Patterns of microsatellite polymorphism in the range-restricted bonobo (Pan paniscus): considerations for interspecific comparison with chimpanzees (P. troglodytes)

The endangered great ape, Pan paniscus (bonobo) has the smallest range of the African apes. Virtually nothing is known about the genetic diversity or genetic structure of this species, while substantial amounts of polymorphism have been reported for the bonobo’s widespread congener, the chimpanzee (P. troglodytes). Given its restricted range, what is the extent of genetic variation in the bonobo relative to the chimpanzee, and is the bonobo genetically depauperate? To investigate patterns of genetic polymorphism, bonobos of wild origin were genotyped for 28 microsatellite loci. The mean number of alleles per locus (5.2) and the mean observed heterozygosity (0.52) in bonobos were similar to variation observed in a wild chimpanzee community (P. t. schweinfurthii). The rarer bonobo is not genetically depauperate and may have genetic diversity comparable to the eastern chimpanzee subspecies. Bonobos have approximately 55% of the allelic diversity and 66% of the observed heterozygosity exhibited by all three chimpanzee subspecies sampled across equatorial Africa. Resampling techniques were used to quantify the effects of sample size differences and number and choice of loci between bonobos and chimpanzees. The examination of these variables underscores their importance in accurately interpreting interspecific comparisons of diversity estimates.     

Primatology: advanced ape technology

New findings from African rainforests show chimpanzees to have impressively advanced technology. They make tools of vegetation to harvest termites as in East and West Africa, but some apes in Central Africa show different techniques and tool sets geared for different tasks.     

Species association of hepatitis B virus (HBV) in non-human apes; evidence for recombination between gorilla and chimpanzee variants

Hepatitis B virus (HBV) infections are widely distributed in humans, infecting approximately one third of the world's population. HBV variants have also been detected and genetically characterised from Old World apes; Gorilla gorilla (gorilla), Pan troglodytes (chimpanzee), Pongo pygmaeus (orang-utan), Nomascus nastusus and Hylobates pileatus (gibbons) and from the New World monkey, Lagothrix lagotricha (woolly monkey). To investigate species-specificity and potential for cross species transmission of HBV between sympatric species of apes (such as gorillas and chimpanzees in Central Africa) or between humans and chimpanzees or gorillas, variants of HBV infecting captive wild-born non-human primates were genetically characterised. 9 of 62 chimpanzees (11.3%) and two from 11 gorillas (18%) were HBV-infected (15% combined frequency), while other Old world monkey species were negative. Complete genome sequences were obtained from six of the infected chimpanzee and both gorillas; those from P. t .ellioti grouped with previously characterised variants from this subspecies. However, variants recovered from P. t. troglodytes HBV variants also grouped within this clade, indicative of transmission between sub-species, forming a paraphyletic clade. The two gorilla viruses were phylogenetically distinct from chimpanzee and human variants although one showed evidence for a recombination event with a P.t.e.-derived HBV variant in the partial X and core gene region. Both of these observations provide evidence for circulation of HBV between different species and sub-species of non-human primates, a conclusion that differs from the hypothesis if of strict host specificity of HBV genotypes.     

The effect of low- and high-fiber diets on the population of entodiniomorphid ciliates Troglodytella abrassarti in captive chimpanzees (Pan troglodytes)

Troglodytella abrassarti is an intestinal entodiniomorphid ciliate commonly diagnosed in the feces of wild and captive chimpanzees (Pan troglodytes). Entodiniomorphids could be considered to have a mutualistic relationship with the great apes, in that the ciliates benefit from the intestinal ecosystem of the host, while also contributing to the fiber fermentation process. We examined the effect of diet on the infection intensities of T. abrassarti in two captive chimpanzees in the Liberec Zoo, Czech Republic. The chimpanzees were fed a low-fiber diet (LFD) with 14% neutral detergent fiber (NDF) and a high-fiber diet (HFD; 26% NDF) for 10 days with one transition, and two 10-day adaptation periods. Fecal samples were examined coproscopically with the merthiolate-iodine-formaldehyde concentration (MIFC) technique, in order to quantify the number of ciliates per gram of feces. A significant trend of increasing T. abrassarti numbers was observed when the animals were fed the LFD, compared to when they were fed the HFD. Our results suggest, however, that infection intensities of T. abrassarti in captive chimpanzees are not influenced primarily by the amount of fiber in the diet, but rather by the dietary starch concentration (HFD: 1%; LFD: 8%).     

Survey of savanna chimpanzees (Pan troglodytes verus) in Southeastern Sénégal

A survey of the western subspecies of chimpanzee (Pan troglodytes verus) was conducted from 1 February to 9 April 2000 in Sénégal, West Africa, by the Miami Assirik Pan Project (MAPP). In addition to the Assirik area of the Parc National du Niokolo Koba (PNNK), areas south and east of the park were surveyed. Nests made by chimpanzees were used to estimate chimpanzee distribution and densities. Within the PNNK, chimpanzees were estimated to occur at an average of 0.13 individuals/km(2). Chimpanzee nests were recorded in nine of 10 locales surveyed outside of the PNNK, as well as within the park. Data on 994 nests made by chimpanzees were recorded outside the PNNK, while 736 nests were recorded inside the park. Nest density in areas surveyed outside the PNNK, such as Bandafassi, Tomboronkoto, and Segou, was comparable to that of Assirik in habitats where nests were concentrated (i.e., evergreen gallery forest). The purpose of MAPP was to initiate long-term research of chimpanzees in southeastern Sénégal, as a follow-up to the Stirling African Primate Project (SAPP) of the 1970s. We sought to replicate the standards set by the SAPP project, except when technological innovations allowed improvement in data collection procedures (e.g., the global positioning system (GPS)).     

The sampling scheme matters: Pan troglodytes troglodytes and P. t. schweinfurthii are characterized by clinal genetic variation rather than a strong subspecies break

Populations of an organism living in marked geographical or evolutionary isolation from other populations of the same species are often termed subspecies and expected to show some degree of genetic distinctiveness. The common chimpanzee (Pan troglodytes) is currently described as four geographically delimited subspecies: the western (P. t. verus), the nigerian‐cameroonian (P. t. ellioti), the central (P. t. troglodytes) and the eastern (P. t. schweinfurthii) chimpanzees. Although these taxa would be expected to be reciprocally monophyletic, studies have not always consistently resolved the central and eastern chimpanzee taxa. Most studies, however, used data from individuals of unknown or approximate geographic provenance. Thus, genetic data from samples of known origin may shed light on the evolutionary relationship of these subspecies. We generated microsatellite genotypes from noninvasively collected fecal samples of 185 central chimpanzees that were sampled across large parts of their range and analyzed them together with 283 published eastern chimpanzee genotypes from known localities. We observed a clear signal of isolation by distance across both subspecies. Further, we found that a large proportion of comparisons between groups taken from the same subspecies showed higher genetic differentiation than the least differentiated between‐subspecies comparison. This proportion decreased substantially when we simulated a more clumped sampling scheme by including fewer groups. Our results support the general concept that the distribution of the sampled individuals can dramatically affect the inference of genetic population structure. With regard to chimpanzees, our results emphasize the close relationship of equatorial chimpanzees from central and eastern equatorial Africa and the difficult nature of subspecies definitions. Am J Phys Anthropol 156:181–191, 2015. © 2014 Wiley Periodicals, Inc.     

Chimpanzee Malaria Parasites Related to Plasmodium ovale in Africa

Since the 1970''s, the diversity of Plasmodium parasites in African great apes has been neglected. Surprisingly, P. reichenowi, a chimpanzee parasite, is the only such parasite to have been molecularly characterized. This parasite is closely phylogenetically related to P. falciparum, the principal cause of the greatest malaria burden in humans. Studies of malaria parasites from anthropoid primates may provide relevant phylogenetic information, improving our understanding of the origin and evolutionary history of human malaria species. In this study, we screened 130 DNA samples from chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla) from Cameroon for Plasmodium infection, using cytochrome b molecular tools. Two chimpanzees from the subspecies Pan t. troglodytes presented single infections with Plasmodium strains molecularly related to the human malaria parasite P. ovale. These chimpanzee parasites and 13 human strains of P. ovale originated from a various sites in Africa and Asia were characterized using cytochrome b and cytochrome c oxidase 1 mitochondrial partial genes and nuclear ldh partial gene. Consistent with previous findings, two genetically distinct types of P. ovale, classical and variant, were observed in the human population from a variety of geographical locations. One chimpanzee Plasmodium strain was genetically identical, on all three markers tested, to variant P. ovale type. The other chimpanzee Plasmodium strain was different from P. ovale strains isolated from humans. This study provides the first evidence of possibility of natural cross-species exchange of P. ovale between humans and chimpanzees of the subspecies Pan t. troglodytes.     

Contrasting effects of natural selection on human and chimpanzee CC chemokine receptor 5

Human immunodeficiency virus type 1 (HIV-1) evolved via cross-species transmission of simian immunodeficiency virus (SIVcpz) from chimpanzees (Pan troglodytes). Chimpanzees, like humans, are susceptible to infection by HIV-1. However, unlike humans, infected chimpanzees seldom develop immunodeficiency when infected with SIVcpz or HIV-1. SIVcpz and most strains of HIV-1 require the cell-surface receptor CC chemokine receptor 5 (CCR5) to infect specific leukocyte subsets, and, subsequent to infection, the level of CCR5 expression influences the amount of HIV-1 entry and the rate of HIV-1 replication. Evidence that variants in the 5' cis-regulatory region of CCR5 (5'CCR5) affect disease progression in humans suggests that variation in CCR5 might also influence the response of chimpanzees to HIV-1/SIVcpz. To determine whether patterns of genetic variation at 5'CCR5 in chimpanzees are similar to those in humans, we analyzed patterns of DNA sequence variation in 37 wild-born chimpanzees (26 P. t. verus, 9 P. t. troglodytes, and 2 P. t. schweinfurthii), along with previously published 5'CCR5 data from 112 humans and 50 noncoding regions in the human and chimpanzee genomes. These analyses revealed that patterns of variation in 5'CCR5 differ dramatically between chimpanzees and humans. In chimpanzees, 5'CCR5 was less diverse than 80% of noncoding regions and was characterized by an excess of rare variants. In humans, 5'CCR5 was more diverse than 90% of noncoding regions and had an excess of common variants. Under a wide range of demographic histories, these patterns suggest that, whereas human 5'CCR5 has been subject to balancing selection, chimpanzee 5'CCR5 has been influenced by a selective sweep. This result suggests that chimpanzee 5'CCR5 might harbor or be linked to functional variants that influence chimpanzee resistance to disease caused by SIVcpz/HIV-1.