Evaluating population histories in Patagonia and Tierra del Fuego, Chile, using ancient mitochondrial and Y-chromosomal DNA

OBJECTIVES

This study aims to characterize the genetic histories of ancient hunter-gatherer groups in Fuego-Patagonia (Chile) with distinct Marine, Terrestrial, and Mixed Economy subsistence strategies. Mitochondrial (mtDNA) and Y-chromosome data were generated to test three hypotheses. H₀: All individuals were drawn from the same panmictic population; H₁: Terrestrial groups first populated the region and gave rise to highly specialized Marine groups by ~7,500 cal BP; or H₂: Marine and Terrestrial groups represent distinct ancestral lineages who migrated independently into the region.

METHODS

Ancient DNA was extracted from the teeth of 50 Fuegian-Patagonian individuals dating from 6,895 cal BP to after European arrival, and analyzed alongside other individuals from previous studies. Individuals were assigned to Marine, Terrestrial, and Mixed Economy groups based on archeological context and stable isotope diet inferences, and mtDNA (HVR1/2) and Y-chromosome variation was analyzed.

RESULTS

Endogenous aDNA was obtained from 49/50 (98%) individuals. Haplotype diversities, F_ST comparisons, and exact tests of population differentiation showed that Marine groups were significantly different from Terrestrial groups based on mtDNA (p < 0.05). No statistically significant differences were found between Terrestrial and Mixed Economy groups. Demographic simulations support models in which Marine groups diverged from the others by ~14,000 cal BP. Y-chromosome results showed similar patterns but were not statistically significant due to small sample sizes and allelic dropout.

DISCUSSION

These results support the hypothesis that Marine and Terrestrial economic groups represent distinct ancestral lineages who diverged during the time populations were expanding in the Americas, and may represent independent migrations into Fuego-Patagonia.

Parallel evolution in human populations: A biocultural perspective

Parallel evolution-where different populations evolve similar traits in response to similar environments-has been a topic of growing interest to biologists and biological anthropologists for decades. Parallel evolution occurs in human populations thanks to myriad biological and cultural mechanisms that permit humans to survive and thrive in diverse environments worldwide. Because humans shape and are shaped by their environments, biocultural approaches that emphasize the interconnections between biology and culture are key to understanding parallel evolution in human populations as well as the nuances of human biological variation and adaptation. In this review, we discuss how biocultural theory has been and can be applied to studies of parallel evolution and adaptation more broadly. We illustrate this through four examples of parallel evolution in humans: malaria resistance, lactase persistence, cold tolerance, and high-altitude adaptation.

Unraveling Signatures of Local Adaptation among Indigenous Groups from Mexico

Few studies have addressed how selective pressures have shaped the genetic structure of the current Native American populations, and they have mostly limited their inferences to admixed Latin American populations. Here, we searched for local adaptation signals, based on integrated haplotype scores and population branch statistics, in 325 Mexican Indigenous individuals with at least 99% Native American ancestry from five previously defined geographical regions. Although each region exhibited its own local adaptation profile, only PPARG and AJAP1, both negative regulators of the Wnt/β catenin signaling pathway, showed significant adaptation signals in all the tested regions. Several signals were found, mainly in the genes related to the metabolic processes and immune response. A pathway enrichment analysis revealed the overrepresentation of selected genes related to several biological phenotypes/conditions, such as the immune response and metabolic pathways, in agreement with previous studies, suggesting that immunological and metabolic pressures are major drivers of human adaptation. Genes related to the gut microbiome measurements were overrepresented in all the regions, highlighting the importance of studying how humans have coevolved with the microbial communities that colonize them. Our results provide a further explanation of the human evolutionary history in response to environmental pressures in this region.

Fostering Responsible Research on Ancient DNA

Anticipating and addressing the social implications of scientific work is a fundamental responsibility of all scientists. However, expectations for ethically sound practices can evolve over time as the implications of science come to be better understood. Contemporary researchers who work with ancient human remains, including those who conduct ancient DNA research, face precisely this challenge as it becomes clear that practices such as community engagement are needed to address the important social implications of this work. To foster and promote ethical engagement between researchers and communities, we offer five practical recommendations for ancient DNA researchers: (1) formally consult with communities; (2) address cultural and ethical considerations; (3) engage communities and support capacity building; (4) develop plans to report results and manage data; and (5) develop plans for long-term responsibility and stewardship. Ultimately, every member of a research team has an important role in fostering ethical research on ancient DNA.

Palaeo-Eskimo genetic ancestry and the peopling of Chukotka and North America

Much of the American Arctic was first settled 5,000 years ago, by groups of people known as Palaeo-Eskimos. They were subsequently joined and largely displaced around 1,000 years ago by ancestors of the present-day Inuit and Yup'ik1-3. The genetic relationship between Palaeo-Eskimos and Native American, Inuit, Yup'ik and Aleut populations remains uncertain4-6. Here we present genomic data for 48 ancient individuals from Chukotka, East Siberia, the Aleutian Islands, Alaska, and the Canadian Arctic. We co-analyse these data with data from present-day Alaskan Iñupiat and West Siberian populations and published genomes. Using methods based on rare-allele and haplotype sharing, as well as established techniques4,7-9, we show that Palaeo-Eskimo-related ancestry is ubiquitous among people who speak Na-Dene and Eskimo-Aleut languages. We develop a comprehensive model for the Holocene peopling events of Chukotka and North America, and show that Na-Dene-speaking peoples, people of the Aleutian Islands, and Yup'ik and Inuit across the Arctic region all share ancestry from a single Palaeo-Eskimo-related Siberian source.

Comparing signals of natural selection between three Indigenous North American populations

While many studies have highlighted human adaptations to diverse environments worldwide, genomic studies of natural selection in Indigenous populations in the Americas have been absent from this literature until very recently. Since humans first entered the Americas some 20,000 years ago, they have settled in many new environments across the continent. This diversity of environments has placed variable selective pressures on the populations living in each region, but the effects of these pressures have not been extensively studied to date. To help fill this gap, we collected genome-wide data from three Indigenous North American populations from different geographic regions of the continent (Alaska, southeastern United States, and central Mexico). We identified signals of natural selection in each population and compared signals across populations to explore the differences in selective pressures among the three regions sampled. We find evidence of adaptation to cold and high-latitude environments in Alaska, while in the southeastern United States and central Mexico, pathogenic environments seem to have created important selective pressures. This study lays the foundation for additional functional and phenotypic work on possible adaptations to varied environments during the history of population diversification in the Americas.

Chaco Canyon Dig Unearths Ethical Concerns

The field of paleogenomics (the study of ancient genomes) is rapidly advancing, with more robust methods of isolating ancient DNA and increasing access to next-generation DNA sequencing technology. As these studies progress, many important ethical issues have emerged that should be considered when ancient Native American remains, whom we refer to as ancestors, are used in research. We highlight a 2017 article by Kennett et al., "Archaeogenomic evidence reveals prehistoric matrilineal dynasty," that brings to light several ethical issues that should be addressed in paleogenomics research. The study helps elucidate the matrilineal relationships in ancient Chacoan society through ancient DNA analysis. However, we, as Indigenous researchers and allies, raise ethical concerns with the study's scientific conclusions that can be problematic for Native American communities: (1) the lack of tribal consultation, (2) the use of culturally insensitive descriptions, and (3) the potential impact on marginalized groups. Further, we explore the limitations of the Native American Graves Protection and Repatriation Act, which addresses repatriation but not research, because clear ethical guidelines have not been established for research involving Native American ancestors, especially those deemed "culturally unaffiliated." Multiple studies of "culturally unaffiliated" remains have been initiated recently, so it is imperative that researchers consider the ethical ramifications of paleogenomics research. Past research indiscretions have created a history of mistrust and exploitation in many Native American communities. To promote ethical engagement of Native American communities in research, we therefore suggest careful attention to ethical considerations, strong tribal consultation requirements, and greater collaborations among museums, federal agencies, researchers, scientific journals, and granting agencies.

Advancing the ethics of paleogenomics

Ancestral remains should be regarded not as “artifacts” but as human relatives who deserve respect

Health and genetic ancestry testing: time to bridge the gap

BACKGROUND

It is becoming increasingly difficult to keep information about genetic ancestry separate from information about health, and consumers of genetic ancestry tests are becoming more aware of the potential health risks associated with particular ancestral lineages. Because some of the proposed associations have received little attention from oversight agencies and professional genetic associations, scientific developments are currently outpacing governance regimes for consumer genetic testing.

MAIN TEXT

We highlight the recent and unremarked upon emergence of biomedical studies linking markers of genetic ancestry to disease risks, and show that this body of scientific research is becoming part of public discourse connecting ancestry and health. For instance, data on genome-wide ancestry informative markers are being used to assess health risks, and we document over 100 biomedical research articles that propose associations between mitochondrial DNA and Y chromosome markers of genetic ancestry and a wide variety of disease risks. Taking as an example an association between coronary heart disease and British men belonging to Y chromosome haplogroup I, we show how this science was translated into mainstream and online media, and how it circulates among consumers of genetic tests for ancestry. We find wide variations in how the science is interpreted, which suggests the potential for confusion or misunderstanding.

CONCLUSION

We recommend that stakeholders involved in creating and using estimates of genetic ancestry reconsider their policies for communicating with each other and with the public about the health implications of ancestry information.

Group benefit associated with polymorphic trichromacy in a Malagasy primate (Propithecus verreauxi)

In some primate lineages, polymorphisms in the X-linked M/LWS opsin gene have produced intraspecific variation in color vision. In these species, heterozygous females exhibit trichromacy, while males and homozygous females exhibit dichromacy. The evolutionary persistence of these polymorphisms suggests that balancing selection maintains color vision variation, possibly through a 'trichromat advantage' in detecting yellow/orange/red foods against foliage. We identified genetic evidence of polymorphic trichromacy in a population of Verreaux's sifaka (Propithecus verreauxi) at Kirindy Mitea National Park in Madagascar, and explored effects of color vision on reproductive success and feeding behavior using nine years of morphological, demographic, and feeding data. We found that trichromats and dichromats residing in social groups with trichromats exhibit higher body mass indices than individuals in dichromat-only groups. Additionally, individuals in a trichromat social group devoted significantly more time to fruit feeding and had longer fruit feeding bouts than individuals in dichromat-only groups. We hypothesize that, due to small, cohesive sifaka social groups, a trichromat advantage in detecting productive fruit patches during the energetically stressful dry season also benefits dichromats in a trichromat's group. Our results offer the first support for the 'mutual benefit of association' hypothesis regarding the maintenance of polymorphic trichromacy in primates.

Native American Genomics and Population Histories

Studies of Native American genetic diversity and population history have been transformed over the last decade by important developments in anthropological genetics. During this time, researchers have adopted new DNA technologies and computational approaches for analyzing genomic data, and they have become increasingly sensitive to the views of research participants and communities. As new methods are applied to long-standing questions, and as more research is conducted in collaboration with indigenous communities, we are gaining new insights into the history and diversity of indigenous populations. This review discusses the recent methodological advances and genetic studies that have improved our understanding of Native American genomics and population histories. We synthesize current knowledge about Native American genomic variation and build a model of population history in the Americas. We also discuss the broader implications of this research for anthropology and related disciplines, and we highlight challenges and other considerations for future research.

Detection of Cytosine methylation in ancient DNA from five native american populations using bisulfite sequencing

While cytosine methylation has been widely studied in extant populations, relatively few studies have analyzed methylation in ancient DNA. Most existing studies of epigenetic marks in ancient DNA have inferred patterns of methylation in highly degraded samples using post-mortem damage to cytosines as a proxy for cytosine methylation levels. However, this approach limits the inference of methylation compared with direct bisulfite sequencing, the current gold standard for analyzing cytosine methylation at single nucleotide resolution. In this study, we used direct bisulfite sequencing to assess cytosine methylation in ancient DNA from the skeletal remains of 30 Native Americans ranging in age from approximately 230 to 4500 years before present. Unmethylated cytosines were converted to uracils by treatment with sodium bisulfite, bisulfite products of a CpG-rich retrotransposon were pyrosequenced, and C-to-T ratios were quantified for a single CpG position. We found that cytosine methylation is readily recoverable from most samples, given adequate preservation of endogenous nuclear DNA. In addition, our results indicate that the precision of cytosine methylation estimates is inversely correlated with aDNA preservation, such that samples of low DNA concentration show higher variability in measures of percent methylation than samples of high DNA concentration. In particular, samples in this study with a DNA concentration above 0.015 ng/μL generated the most consistent measures of cytosine methylation. This study presents evidence of cytosine methylation in a large collection of ancient human remains, and indicates that it is possible to analyze epigenetic patterns in ancient populations using direct bisulfite sequencing approaches.

Anthropology. Response to Comment on "Late Pleistocene human skeleton and mtDNA link Paleoamericans and modern Native Americans"

Prüfer and Meyer raise concerns over the mitochondrial DNA (mtDNA) results we reported for the Hoyo Negro individual, citing failure of a portion of these data to conform to their expectations of ancient DNA (aDNA). Because damage patterns in aDNA vary, outright rejection of our findings on this basis is unwarranted, especially in light of our other observations.

Ancient DNA from the Schild site in Illinois: Implications for the Mississippian transition in the Lower Illinois River Valley

Archaeologists have long debated whether rapid cultural change in the archaeological record is due to in situ developments, migration of a new group into the region, or the spread of new cultural practices into an area through existing social networks, with the local peoples adopting and adapting practices from elsewhere as they see fit (acculturation). Researchers have suggested each of these explanations for the major cultural transition that occurred at the beginning of the Mississippian period (AD 1050) across eastern North America. In this study, we used ancient DNA to test competing hypotheses of migration and acculturation for the culture change that occurred between the Late Woodland (AD 400-1050) and Mississippian (AD 1050-1500) periods in the Lower Illinois River Valley. We obtained sequences of the first hypervariable segment of the mitochondrial genome (mtDNA) from 39 individuals (17 Late Woodland, 22 Mississippian) interred in the Schild cemetery in western Illinois, and compared these lineages to ancient mtDNA lineages present at other sites in the region. Computer simulations were used to test a null hypothesis of population continuity from Late Woodland to Mississippian times at the Schild site and to investigate the possibility of gene flow from elsewhere in the region. Our results suggest that the Late Woodland to Mississippian cultural transition at Schild was not due to an influx of people from elsewhere. Instead, it is more likely that the transition to Mississippian cultural practices at this site was due to a process of acculturation.

Late Pleistocene human skeleton and mtDNA link Paleoamericans and modern Native Americans

Because of differences in craniofacial morphology and dentition between the earliest American skeletons and modern Native Americans, separate origins have been postulated for them, despite genetic evidence to the contrary. We describe a near-complete human skeleton with an intact cranium and preserved DNA found with extinct fauna in a submerged cave on Mexico's Yucatan Peninsula. This skeleton dates to between 13,000 and 12,000 calendar years ago and has Paleoamerican craniofacial characteristics and a Beringian-derived mitochondrial DNA (mtDNA) haplogroup (D1). Thus, the differences between Paleoamericans and Native Americans probably resulted from in situ evolution rather than separate ancestry.

Brief communication: Evolution of a specific O allele (O1vG542A) supports unique ancestry of Native Americans

In this study, we explore the geographic and temporal distribution of a unique variant of the O blood group allele called O1v(G542A) , which has been shown to be shared among Native Americans but is rare in other populations. O1v(G542A) was previously reported in Native American populations in Mesoamerica and South America, and has been proposed as an ancestry informative marker. We investigated whether this allele is also found in the Tlingit and Haida, two contemporary indigenous populations from Alaska, and a pre-Columbian population from California. If O1v(G542A) is present in Na-Dene speakers (i.e., Tlingits), it would indicate that Na-Dene speaking groups share close ancestry with other Native American groups and support a Beringian origin of the allele, consistent with the Beringian Incubation Model. If O1v(G542A) is found in pre-Columbian populations, it would further support a Beringian origin of the allele, rather than a more recent introduction of the allele into the Americas via gene flow from one or more populations which have admixed with Native Americans over the past five centuries. We identified this allele in one Na-Dene population at a frequency of 0.11, and one ancient California population at a frequency of 0.20. Our results support a Beringian origin of O1v(G542A) , which is distributed today among all Native American groups that have been genotyped in appreciable numbers at this locus. This result is consistent with the hypothesis that Na-Dene and other Native American populations primarily derive their ancestry from a single source population.

Nocturnal light environments influence color vision and signatures of selection on the OPN1SW opsin gene in nocturnal lemurs

Although loss of short-wavelength-sensitive (SWS) cones and dichromatic color vision in mammals has traditionally been linked to a nocturnal lifestyle, recent studies have identified variation in selective pressure for the maintenance of the OPN1SW opsin gene (and thus, potentially dichromacy) among nocturnal mammalian lineages. These studies hypothesize that purifying selection to retain SWS cones may be associated with a selective advantage for nocturnal color vision under certain ecological conditions. In this study, we explore the effect of nocturnal light environment on OPN1SW opsin gene evolution in a diverse sample of nocturnal lemurs (106 individuals, 19 species, and 5 genera). Using both phylogenetic and population genetic approaches, we test whether species from closed canopy rainforests, which are impoverished in short-wavelength light, have experienced relaxed selection compared with species from open canopy forests. We identify clear signatures of differential selection on OPN1SW by habitat type. Our results suggest that open canopy species generally experience strong purifying selection to maintain SWS cones. In contrast, closed canopy species experience weaker purifying selection or a relaxation of selection on OPN1SW. We also found evidence of nonfunctional OPN1SW genes in all Phaner species and in Cheirogaleus medius, implying at least three independent losses of SWS cones in cheirogaleids. Our results suggest that the evolution of color vision in nocturnal lemurs has been influenced by nocturnal light environment.

Nondestructive sampling of human skeletal remains yields ancient nuclear and mitochondrial DNA

Museum curators and living communities are sometimes reluctant to permit ancient DNA (aDNA) studies of human skeletal remains because the extraction of aDNA usually requires the destruction of at least some skeletal material. Whether these views stem from a desire to conserve precious materials or an objection to destroying ancestral remains, they limit the potential of aDNA research. To help address concerns about destructive analysis and to minimize damage to valuable specimens, we describe a nondestructive method for extracting DNA from ancient human remains. This method can be used with both teeth and bone, but it preserves the structural integrity of teeth much more effectively than that of bone. Using this method, we demonstrate that it is possible to extract both mitochondrial and nuclear DNA from human remains dating between 300 BC and 1600 AD. Importantly, the method does not expose the remains to hazardous chemicals, allowing them to be safely returned to curators, custodians, and/or owners of the samples. We successfully amplified mitochondrial DNA from 90% of the individuals tested, and we were able to analyze 1-9 nuclear loci in 70% of individuals. We also show that repeated nondestructive extractions from the same tooth can yield amplifiable mitochondrial and nuclear DNA. The high success rate of this method and its ability to yield DNA from samples spanning a wide geographic and temporal range without destroying the structural integrity of the sampled material may make possible the genetic study of skeletal collections that are not available for destructive analysis.

Ancient DNA perspectives on American colonization and population history

Ancient DNA (aDNA) analyses have proven to be important tools in understanding human population dispersals, settlement patterns, interactions between prehistoric populations, and the development of regional population histories. Here, we review the published results of sixty-three human populations from throughout the Americas and compare the levels of diversity and geographic patterns of variation in the ancient samples with contemporary genetic variation in the Americas in order to investigate the evolution of the Native American gene pool over time. Our analysis of mitochondrial haplogroup frequencies and prehistoric population genetic diversity presents a complex evolutionary picture. Although the broad genetic structure of American prehistoric populations appears to have been established relatively early, we nevertheless identify examples of genetic discontinuity over time in select regions. We discuss the implications this finding may have for our interpretation of the genetic evidence for the initial colonization of the Americas and its subsequent population history.

Opsin gene polymorphism predicts trichromacy in a cathemeral lemur

Recent research has identified polymorphic trichromacy in three diurnal strepsirrhines: Coquerel's sifaka (Propithecus coquereli), black and white ruffed lemurs (Varecia variegata), and red ruffed lemurs (V. rubra). Current hypotheses suggest that the transitions to diurnality experienced by Propithecus and Varecia were necessary precursors to their independent acquisitions of trichromacy. Accordingly, cathemeral lemurs are thought to lack the M/L opsin gene polymorphism necessary for trichromacy. In this study, the M/L opsin gene was sequenced in ten cathemeral blue-eyed black lemurs (Eulemur macaco flavifrons). This analysis identified a polymorphism identical to that of other trichromatic strepsirrhines at the critical amino acid position 285 in exon 5 of the M/L opsin gene. Thus, polymorphic trichromacy is likely present in at least one cathemeral Eulemur species, suggesting that strict diurnality is not necessary for trichromacy. The presence of trichromacy in E. m. flavifrons suggests that a re-evaluation of current hypotheses regarding the evolution of strepsirrhine trichromacy may be necessary. Although the M/L opsin polymorphism may have been independently acquired three times in the lemurid-indriid clade, the distribution of opsin alleles in lemurids and indriids may also be consistent with a common origin of trichromacy in the last common ancestor of either the lemurids or the lemurid-indriid clade.

Genetic analysis of early holocene skeletal remains from Alaska and its implications for the settlement of the Americas

Mitochondrial and Y-chromosome DNA were analyzed from 10,300-year-old human remains excavated from On Your Knees Cave on Prince of Wales Island, Alaska (Site 49-PET-408). This individual's mitochondrial DNA (mtDNA) represents the founder haplotype of an additional subhaplogroup of haplogroup D that was brought to the Americas, demonstrating that widely held assumptions about the genetic composition of the earliest Americans are incorrect. The amount of diversity that has accumulated in the subhaplogroup over the past 10,300 years suggests that previous calibrations of the mtDNA clock may have underestimated the rate of molecular evolution. If substantiated, the dates of events based on these previous estimates are too old, which may explain the discordance between inferences based on genetic and archaeological evidence regarding the timing of the settlement of the Americas. In addition, this individual's Y-chromosome belongs to haplogroup Q-M3*, placing a minimum date of 10,300 years ago for the emergence of this haplogroup.

Asymmetric Male and Female Genetic Histories among Native Americans from Eastern North America

Previous studies have investigated the human population history of eastern North America by examining mitochondrial DNA (mtDNA) variation among Native Americans, but these studies could only reconstruct maternal population history. To evaluate similarities and differences in the maternal and paternal population histories of this region, we obtained DNA samples from 605 individuals, representing 16 indigenous populations. After amplifying the amelogenin locus to identify males, we genotyped 8 binary polymorphisms and 10 microsatellites in the male-specific region of the Y chromosome. This analysis identified 6 haplogroups and 175 haplotypes. We found that sociocultural factors have played a more important role than language or geography in shaping the patterns of Y chromosome variation in eastern North America. Comparisons with previous mtDNA studies of the same samples demonstrate that male and female demographic histories differ substantially in this region. Postmarital residence patterns have strongly influenced genetic structure, with patrilocal and matrilocal populations showing different patterns of male and female gene flow. European contact also had a significant but sex-specific impact due to a high level of male-mediated European admixture. Finally, this study addresses long-standing questions about the history of Iroquoian populations by suggesting that the ancestral Iroquoian population lived in southeastern North America.

Unexpected patterns of mitochondrial DNA variation among Native Americans from the Southeastern United States

Mitochondrial DNA (mtDNA) haplogroups were determined by restriction fragment length polymorphism-typing for 66 individuals from four southeastern North American populations, and the HVS I portion of the mtDNA control region was sequenced in 48 of these individuals. Although populations from the same geographic region usually exhibit similar haplogroup frequency distributions (Lorenz and Smith [1996] Am. J. Phys. Anthropol. 101:307-323; Malhi et al. [2001] Hum. Biol. 73:17-55), those from the Southeast instead exhibit haplogroup frequency distributions that differ significantly from one another. Such divergent haplogroup frequency distributions are unexpected for the Muskogean-speaking southeastern populations, which share many sociocultural traits, speak closely related languages, and have experienced extensive admixture both with each other and with other eastern North American populations. Independent origins, genetic isolation from other Native American populations due to matrilocality, differential admixture, or a genetic bottleneck could be responsible for this heterogeneous distribution of haplogroup frequencies. Within a given haplogroup, however, the HVS I sequences from the four Muskogean-speaking populations appear relatively similar to one another, providing evidence for close relationships among them and for reduced diversity within haplogroups in the Southeast. Given additional archaeological, linguistic, and ethnographic evidence, these results suggest that a genetic bottleneck associated with the historical population decline is the most plausible explanation for such patterns of mtDNA variation.

The effect of lead on photoreceptor response amplitude--influence of the light stimulus

The mass receptor potential of the excised, superfused retina of the bullfrog was studied. Photoreceptor responses were isolated by addition of sodium aspartate to the Ringer solutions. Responses of the cones were monitored independently from responses of the rods by employing a two-flash method of stimulation which took advantage of the very different rates of rapid dark adaptation of rods and of cones. Stimulation with paired flashes of white light at regular intervals caused enhancement of rod response amplitude in that the response grew larger with subsequent flashes until reaching a stable plateau. The degree of enhancement was directly proportional to the amount of light exposure and increased with either increasing stimulus intensity or decreasing stimulus interval. Only the rod response was affected; the cone response was not enhanced by continued stimulation. The effects of 12.5 microM PbCl2 on rod response amplitude were complex. There was a small (less than 10%) but consistent depression of rod response amplitude even when the rods were in the unenhanced state. However, the most striking effect of lead was on the enhanced response, where treatment with 12.5 microM PbCl2 led to a depression of about 33%. When added prior to light stimulation, lead significantly decreased the degree to which the rod response could be enhanced, but never prevented enhancement entirely. Removal of lead resulted in a very large increase in the degree to which the rod response was enhanced by light, even when compared to the first, lead-free control. The cone response was unaffected by 12.5 microM PbCl2.

The effects of lead and of cadmium on the mass photoreceptor potential: the dose-response relationship

The effects of micromolar (1.0-60.0 microM) amounts of lead and of cadmium on the light response of photoreceptors were studied using the isolated, perfused bullfrog retina. The effect of lead in depressing rod activity is readily and fully reversible. Lead is very effective in depressing the rod response at concentrations as low as 1.0 microM, but the effect of lead saturates at about 25.0 microM with about 34% depression of the rod response. At the higher concentrations some spontaneous recovery of rod response amplitude is observed shortly after exposure of the retina to lead begins. The cone response is affected by lead only rarely and then only at the higher concentrations. When affected, the cone response is enhanced rather than depressed. The effects of cadmium are generally similar to those of lead. However, saturating concentrations of cadmium depress the rod response to a greater degree than lead and the cadmium effects are not as readily reversible as the effects of lead at the higher concentrations. Cones are much more sensitive to cadmium than they are to lead and 60.0 microM cadmium always enhances cone response amplitude. The results of experiments in which lead and cadmium were both added to the retinal perfusate indicate to us that cadmium and lead both affect the same sensitive site or sites responsible for generating the rod response, but that cadmium affects an additional site that is not sensitive to lead. The depressive effects of lead and of cadmium on the rod response are discussed in terms of the postulated mechanisms of transduction in the vertebrate photoreceptor.

Rapid dark adaptation of bullfrog rods is delayed by barium

With use of sodium aspartate, the late receptor potential of the excised, perfused bullfrog retina was isolated. By means of a two-flash technique, rapid dark adaptation of rods was monitored. As in cones, barium ions were found to delay the onset of rapid dark adaptation of rods, but the rate of recovery, once begun, was virtually unaffected. The effect of barium on the amplitude-intensity relationship of rods was also determined. Unlike its ability to dramatically increase the amplitude of the receptor potential of rods, barium had no effect on the absolute threshold of rods. We propose that barium ions act on the enzyme system postulated to govern the onset of rapid dark adaptation of rods and suggest that a reduction in the activity of an ATP protein kinase might be the basis for this effect.

The influence of calcium on the aspartate-isolated mass receptor potential of bullfrog cones

The aspartate-isolated receptor potential was studied in the excised, perfused bullfrog retina. Cones were monitored without interference from rods by employing conditioning and test stimuli in a manner previously described (23,24). Lowering extracellular calcium by switching from a perfusate containing 0.4mM CaCl2 to one having no added calcium resulted in an increase in cone response amplitude. Conversely, elevating extracellular calcium by perfusing with a Ringer containing 0.8mM CaCl2 resulted in a decrease in cone response amplitude. These changes were sustained and fully reversible. In contrast, perfusing the retina with a Ringer solution containing EGTA resulted in a transient increase in cone response amplitude. Decreasing external calcium by simple depletion also shortened the delay prior to onset of rapid dark adaptation of the cones, thereby hastening the entire process of recovery. Increasing external calcium had little effect on rapid dark adaptation. Decreasing external calcium with EGTA led to extremely rapid response recovery, but the effect was not reversible. In no case did EGTA lead to a complete suppression of the response. The results of this study are interpreted as being inconsistent with the view that calcium is the internal transmitter responsible for the generation of the receptor potential in cones. They are consistent with the view that calcium functions to modulate recovery of the cones' ability to generate a response following a stimulus, perhaps by affecting the activity of a cyclic nucleotide.

Barium delays the onset of rapid dark adaptation in bullfrog cones

The late receptor potential of the excised, perfused bullfrog retina was isolated with sodium aspartate. By employing a three-flash technique, cone responses were monitored without interference from rods. In cones barium ions were found to delay the onset of rapid dark adaptation, but the rate of recovery, once begun, was unaffected. We propose that barium ions act directly upon the enzyme system postulated to govern the onset of rapid dark adaptation of cones. In addition, barium was found to affect the amplitude of the rod receptor potential differently from that of cones, increasing the former but decreasing the latter. The effect of barium upon photoreceptor potential amplitude is discussed in terms of a reduction in the potassium conductance of the photoreceptors and the mechanisms postulated for photoreceptor excitation and rapid dark adaptation.