Population-wide lineage frequencies predict genetic load in the seed-harvester ant Pogonomyrmex

Many populations of the seed-harvester ant Pogonomyrmex barbatus exhibit genetic caste determination (GCD) generated by the interbreeding of two distinct yet interdependent lineages. Same-lineage matings are genetically predestined to become female reproductives (gynes) whereas alternate-lineage matings become workers. The perpetuation of this system requires that reproductives of both lineages are available for mating and are thus part of the effective population. We label these dependent lineage populations, because each lineage depends on the alternate lineage for worker production. Here we investigate the potential costs associated with GCD in a population with highly skewed lineage frequencies. We reared colonies using newly mated queens from a GCD population and an ecologically equivalent Pogonomyrmex rugosus population with environmental caste determination. GCD founding queens suffer a genetic load from mating randomly and produce fewer brood with advanced development compared with environmental caste determination queens. Our results indicate that GCD queens acquiring a high proportion of same-lineage sperm are unlikely to found a colony successfully. Given model parameters of random mating and founding queens mating with three males on average, there was a close fit between theoretical expectations of variation in colony worker production based on mating and lineage frequencies and empirical deficits in worker production. As expected, severely decreased worker production was specific to the common lineage, suggesting that negative frequency-dependent selection acts to stabilize a dependent lineage system.     

Genetic basis for queen-worker dimorphism in a social insect

Eusocial insects are characterized by reproductive division of labor, cooperative brood care, and the presence of a sterile worker caste. It is generally accepted that caste determination, including the differentiation of females into sterile workers and reproductive queens, is determined by environmental factors. In contrast, we find that in the red harvester ant, Pogonomyrmex barbatus, an individual's genotype at a particular microsatellite locus predicts its caste. We propose that this microsatellite locus is in tight linkage disequilibrium with at least one locus that plays an important role in caste determination. We call this the caste locus. We hypothesize that the system of caste determination we observe segregates the population into two distinct genetic lineages, each of which has distinct alleles at the microsatellite locus and also has distinct alleles, we propose, at caste. Workers are the offspring of parents from different lineages, and are thus heterozygous at caste, whereas queens are the offspring of parents from the same lineage, and are, therefore, homozygous at caste. This mode of caste determination has important consequences for the evolution of multiple mating by females and for control of the sex ratio and reproductive allocation in social insect colonies.     

Patriline shifting leads to apparent genetic caste determination in harvester ants

The harvester ant, Pogonomyrmex occidentalis, is characterized by high levels of intracolonial genetic diversity resulting from multiple mating by the queen. Within reproductively mature colonies, the relative frequency of different male genotypes (patrilines) is not stable. The difference between samples increases with time, nearing an asymptote after a year. Patriline distributions in gynes and workers display similar patterns of change. A consequence of changing patriline distributions is that workers and gynes appear to have different fathers. However, apparent genetic differences between castes are caused by changing paternity among all females. Temporal variation in the relative frequency of patrilines may be a consequence of processes that reflect sexual conflict, such as sperm clumping. Recent work documenting genotype differences between physical castes (workers and gynes; major and minor workers) in several species of ants has been interpreted as evidence of genetic caste determination. Reanalysis of these studies found little support for this hypothesis. Apparent caste determination may result from temporal variation in sperm use, rather than from fertilization bias among male ejaculates.     

Identification of a pheromone regulating caste differentiation in termites

The hallmark of social insects is their caste system: reproduction is primarily monopolized by queens, whereas workers specialize in the other tasks required for colony growth and survival. Pheromones produced by reining queens have long been believed to be the prime factor inhibiting the differentiation of new reproductive individuals. However, there has been very little progress in the chemical identification of such inhibitory pheromones. Here we report the identification of a volatile inhibitory pheromone produced by female neotenics (secondary queens) that acts directly on target individuals to suppress the differentiation of new female neotenics and identify n-butyl-n-butyrate and 2-methyl-1-butanol as the active components of the inhibitory pheromone. An artificial pheromone blend consisting of these two compounds had a strong inhibitory effect similar to live neotenics. Surprisingly, the same two volatiles are also emitted by eggs, playing a role both as an attractant to workers and an inhibitor of reproductive differentiation. This dual production of an inhibitory pheromone by female reproductives and eggs probably reflects the recruitment of an attractant pheromone as an inhibitory pheromone and may provide a mechanism ensuring honest signaling of reproductive status with a tight coupling between fertility and inhibitory power. Identification of a volatile pheromone regulating caste differentiation in a termite provides insights into the functioning of social insect colonies and opens important avenues for elucidating the developmental pathways leading to reproductive and nonreproductive castes.     

Worker caste polymorphism has a genetic basis in Acromyrmex leaf-cutting ants

Division of labor is fundamental to the success of all societies. The most striking examples are the physically polymorphic worker castes in social insects with clear morphological adaptations to different roles. These polymorphic worker castes have previously been thought to be a classic example of nongentically controlled polymorphism, being mediated entirely by environmental cues. Here we show that worker caste development in the leaf-cutting ant Acromyrmex echinatior has a significant genetic component. Individuals of different patrilines within the same colony differ in their propensities to develop into minor or major workers. The mechanism appears to be plastic, with caste destiny resulting from interplay between nurture and nature. Unlike the few other recently discovered examples of a genetic influence on caste determination, the present result does not relate to any rare or exceptional circumstances, such as interspecific hybridization. The results suggest that a significant role of genetics may have been overlooked in our understanding of other complex polymorphisms of social insects.     

Evolution of eusociality and the soldier caste in termites: influence of intraspecific competition and accelerated inheritance

We present new hypotheses and report experimental evidence for powerful selective forces impelling the evolution of both eusociality and the soldier caste in termites. Termite ancestors likely had a nesting and developmental life history similar to that of the living family Termopsidae, in which foraging does not occur outside the host wood, and nonsoldier helpers retain lifelong options for differentiation into reproductives. A local neighborhood of families that live exclusively within a limited resource results in interactions between conspecific colonies, high mortality of founding reproductives, and opportunities for accelerated inheritance of the nest and population by offspring that differentiate into nondispersing neotenic reproductives. In addition, fertile reproductive soldiers, a type of neotenic previously considered rare and docile, frequently develop in this intraspecific competitive context. They can be highly aggressive in subsequent interactions, supporting the hypothesis that intercolonial battles influenced the evolution of modern sterile termite soldier weaponry and behaviors.     

Social exploitation of hexamerin: RNAi reveals a major caste-regulatory factor in termites

Lower termites express a unique form of eusocial polyphenism in that totipotent workers can differentiate into either soldier or reproductive caste phenotypes. In this initial effort using RNA interference in termites, we found that two hexamerin genes, Hex-1 and Hex-2, participate in the regulation of caste polyphenism. Our methodology involved a dual gene-silencing approach that used a single short-interfering RNA fragment to silence the two homologous hexamerin genes. We performed validation studies that evaluated effects on nontarget housekeeping genes, silencing of a nonhousekeeping control gene, and effects at the protein level. We found that the two hexamerin proteins, which are inducible by the morphogenetic juvenile hormone and which constitute a significant proportion of total termite protein, suppress juvenile-hormone-dependent worker differentiation to the soldier caste phenotype. This mechanism allows termite colonies to retain high proportions of altruistic worker caste members, thus apparently enhancing colony-inclusive fitness. These findings demonstrate a unique status quo regulatory mechanism for termite worker caste retention and provide an example of previously undescribed preadult developmental/caste-regulatory genes from any social insect.     

Genetic royal cheats in leaf-cutting ant societies

Social groups are vulnerable to cheating because the reproductive interests of group members are rarely identical. All cooperative systems are therefore predicted to involve a mix of cooperative and cheating genotypes, with the frequency of the latter being constrained by the suppressive abilities of the former. The most significant potential conflict in social insect colonies is over which individuals become reproductive queens rather than sterile workers. This reproductive division of labor is a defining characteristic of eusocial societies, but individual larvae will maximize their fitness by becoming queens whereas their nestmates will generally maximize fitness by forcing larvae to become workers. However, evolutionary constraints are thought to prevent cheating by removing genetic variation in caste propensity. Here, we show that one-fifth of leaf-cutting ant patrilines cheat their nestmates by biasing their larval development toward becoming queens rather than workers. Two distinct mechanisms appear to be involved, one most probably involving a general tendency to become a larger adult and the other relating specifically to the queen-worker developmental switch. Just as evolutionary theory predicts, these "royal" genotypes are rare both in the population and within individual colonies. The rarity of royal cheats is best explained as an evolutionary strategy to avoid suppression by cooperative genotypes, the efficiency of which is frequency-dependent. The results demonstrate that cheating can be widespread in even the most cooperative of societies and illustrate that identical principles govern social evolution in highly diverse systems.     

Nonrelatives inherit colony resources in a primitive termite

The evolution of eusociality, especially how selection would favor sterility or subfertility of most individuals within a highly social colony, is an unresolved paradox. Eusociality evolved independently in diverse taxa, including insects (all ants and termites; some bees, wasps, thrips, and beetles), snapping shrimp, and naked mole rats. Termites have received comparatively less focus than the haplodiploid Hymenoptera (ants, bees, and wasps); however, they are the only diploid group with highly complex colonies and an extraordinary diversity of castes. In this study we staged encounters between unrelated colonies of primitive dampwood termites, Zootermopsis nevadensis, mimicking natural meetings that occur under bark. During encounters, kings and/or queens were killed and surviving members merged into one colony. After encounters, members of both unrelated colonies cooperated as a single social unit. We determined the colony of origin of replacement reproductives that emerged after death of kings and/or queens. Here, we document that replacement reproductives developed from workers in either or both original colonies, inherited the merged resources of the colony, and sometimes interbred. Because this species shares many characteristics with ancestral termites, these findings demonstrate how ecological factors could have promoted the evolution of eusociality by accelerating and enhancing direct fitness opportunities of helper offspring, rendering relatedness favoring kin selection less critical.     

Worker policing without genetic conflicts in a clonal ant

In group-living animals, mutual policing to suppress reproduction is an important mechanism in the resolution of conflict between selfish group members and the group as a whole. In societies of bees, ants, and wasps, policing against the production of males by other workers is expected when egg laying by workers decreases the average inclusive fitness of individual group members. This may result (i) from the relatedness of workers being lower to worker than to queen-derived males or (ii) from a lowered overall colony efficiency. Whereas good evidence exists for policing behavior caused by genetic conflicts, policing caused by efficiency factors has not been demonstrated. We investigated the regulation of reproduction in the ant Platythyrea punctata, a species in which colonies are clones because workers are capable of producing female offspring by thelytokous parthenogenesis. Reproductive conflicts resulting from differences in genetic relatedness are therefore not expected, but uncontrolled reproduction by all workers could lead to the destruction of sociality. Here we show that worker policing by aggressive attacks against additionally reproducing workers keeps the number of reproducing workers low. Furthermore, through experimental manipulation of the number of brood items per colony, we show that worker policing can enhance group efficiency.     

Structural analysis of the ADHS electromorph of Drosophila melanogaster.

Population geneticists have often determined the fitness differences that account for the dynamics of naturally occurring genetic polymorphisms. However, to understand causal aspects of evolutionary processes requires, in addition, investigation of the physiological and molecular structural differences underlying adaptively significant genetic polymorphisms. The characteristics of the alcohol dehydrogenase gene--enzyme system in Drosophila melanogaster make it well suited for this kind of study. Natural populations of this species are polymorphic for two electrophoretically detectable variants, ADHF and ADHS, of the enzyme. Structural studies reported here reveal that the two variants differ by (at least) a single amino acid replacement, threonine in ADHF for lysine in ADHS.     

Genetic introgression as a potential to widen a species' niche: insights from alpine Carex curvula

Understanding what causes the decreasing abundance of species at the margins of their distributions along environmental gradients has drawn considerable interest, especially because of the recent need to predict shifts in species distribution patterns in response to climatic changes. Here, we address the ecological range limit problem by focusing on the sedge, Carex curvula, a dominant plant of high-elevation grasslands in Europe, for which two ecologically differentiated but crosscompatible taxa have been described in the Alps. Our study heuristically combines an extensive phytoecological survey of alpine plant communities to set the niche attributes of each taxon and a population genetic study to assess the multilocus genotypes of 177 individuals sampled in typical and marginal habitats. We found that ecological variation strongly correlates with genetic differentiation. Our data strongly suggest that ecologically marginal populations of each taxon are mainly composed of individuals with genotypes resulting from introgressive hybridization. Conversely, no hybrids were found in typical habitats, even though the two taxa were close enough to crossbreed. Thus, our results indicate that genotype integrity is maintained in optimal habitats, whereas introgressed individuals are favored in marginal habitats. We conclude that gene flow between closely related taxa might be an important, although underestimated, mechanism shaping species distribution along gradients.     

Demographic predisposition to the evolution of eusociality: a hierarchy of models.

I present a hierarchy of models that illustrate, within the framework of inclusive fitness theory, how demographic factors can predispose a species to the evolution of eusociality. Delayed reproductive maturation lowers the inclusive fitness of a solitary foundress relative to that of a worker. Variation in age at reproductive maturity makes the worker strategy more profitable to some individuals than to others and thus predicts the coexistence of single-foundress and multiple-foundress nesting associations. Delayed reproductive maturation and variation in age at reproductive maturity also select for mixed reproductive strategies so that some individuals whose reproductive maturation is expected to be delayed can first act as workers and later switch over to the role of a queen or foundress. Assured fitness returns shows how identical mortality rates can have different consequences for workers and solitary nest foundresses because a solitary foundress will have to necessarily survive for the entire duration of development of her brood, whereas a worker can hope to get proportional fitness returns for short periods of work. In concert with assured fitness returns, delayed reproductive maturation and variation in age at reproductive maturity become more powerful in selecting for worker behavior, and mixed reproductive strategies become available to a wider range of individuals. These phenomena provide a consistently more powerful selective advantage for the worker strategy than do genetic asymmetries created by haplodiploidy.     

Ecologically dependent postmating isolation between sympatric host forms of Neochlamisus bebbianae leaf beetles

Ecological speciation is the promotion of reproductive isolation via the divergent adaptation of populations to alternative environments. A prediction peculiar to ecological speciation is that hybrids between such populations should be adapted poorly to parental environments, yielding reduced fitness and postmating isolation. However, F₁ analyses alone cannot demonstrate that ecological (“extrinsic”) factors contribute to such isolation. Rather, this requires documenting a “switch” in the relative fitnesses of reciprocal backcrosses between environments. Specifically, each backcross should exhibit higher fitness in the environment of its pure parent, with which it shares the most genes, including environment-specific ones. In contrast, because genetic proportions are expected to be similar for all backcrosses (≈¾ from one parental type and ≈¼ from the other), the more general genetic incompatibilities responsible for “intrinsic” isolation predict no such environment-specific fitness switches. Thus, although intrinsic isolation may contribute to the fitness reduction and variation underlying such patterns, it offers an insufficient explanation for them. Here, we present a quantitative genetic “backcross” analysis of sympatric Neochlamisus bebbianae leaf beetle populations adapted to maple versus willow host plants. Results statistically supported ecological speciation predictions, notably the switch in relative fitness for backcross types, the expected rank order of cross type fitnesses, and appreciable extrinsic isolation. We additionally documented genetic variation in host-associated fitness, ruled out nongenetic maternal effects, and discuss the maintenance of ecological differentiation in sympatry. In summary, our study provides a rare and strongly supported demonstration of genetically based, ecologically dependent postmating isolation during ecological speciation.     

Introgression in hybrid ants is favored in females but selected against in males

Hybridization is not a mere reproductive dead end but has been suggested to play a central role in speciation, for example, by introducing adaptive genetic variation. Our previous study uncovered a unique consequence of hybridization in Formica ants. In a population including two isolated but partially introgressed genetic groups, the females have an apparent hybrid background, whereas the males do not. This situation results in large-scale differences between male and female genomes that are stable throughout generations. Here, we compare genotypes from different developmental stages to investigate how sex-specific introgression and genetic differences between sexes are maintained. We show that strong selection rather than sex-dependent transmission maintains the genetic differences between sexes. All genotype combinations are produced and observed in the eggs of both sexes, but the alleles acquired through hybridization disappear from the haploid males during development from egg to adult as their frequencies drop toward zero. However, the same introgressed alleles increase in frequency and are favored when heterozygous in the females. Genotypes eliminated from males most likely represent incompatibilities arising from hybridization. Our results show an unusual situation of opposite selection, where introgression is favored in diploid females but selected against in haploid males. This finding suggests that introgressed genomic regions harbor both fitness-enhancing and -reducing elements. Our work highlights the complex consequences of hybridization and provides a rare opportunity to observe natural selection in real time in nature.Recent studies suggest hybridization in animals to be more common than previously thought and play a role in both speciation and adaptation (1–3). Toward the end of the speciation process, hybridization commonly leads to inviability and reduced fertility because of incompatible epistatic interactions between genes from different species (4–6). Alternatively, it can result in adaptive introgression of genetic material across the species boundary, where fitness-enhancing gene complexes are transferred from one taxon into another (2, 3).Ants offer unique opportunities to study the process of speciation. Because of sociality and haplodiploidy, some of the consequences of hybridization in ants differ from those in other species. Ants are haplodiploid, as the females (queens and workers) develop through normal sexual reproduction and are diploid, whereas the males develop from unfertilized eggs and are haploid. Therefore, even if a female mates with a male of another species, her haploid sons are not hybrids (7). Furthermore, haploid sons of a hybrid female reveal both recessive and dominant hybrid incompatibilities genome-wide. Haploid male hybrids lack the intact genome of both parental species and thus, face the incompatibility problems seen in F2 hybrids or backcrossed individuals of a diploid species (8, 9). These findings are in accordance with Haldane’s rule, which suggests that the effect of hybrid incompatibilities is strongest in not only the heterogametic but also, the hemizygous (haploid) sex (10). In previously described cases of ant hybridization, the workers are hybrids, but reproductive individuals are produced by mating within a pure line or a species as, for example, in the Pogonomyrmex harvester ants (11) and Solenopsis fire ants (12). Another system (dubbed social hybridogenesis) has been described in Cataglyphis desert ants, where new queens are produced parthenogenetically, whereas workers are hybrids between two genetic lineages (13).Our earlier study based on a population sample of adults revealed yet another exceptional example of hybridization in ants. The population has a history of hybridization between the mound-building wood ants Formica aquilonia and Formica polyctena (9), which has resulted in two genetic groups (R and W) that occupy the same nests and both have diagnostic alleles of their own (Fig. 1). The past history of hybridization has led to introgression of alleles between the groups in such a way that alleles present in one genetic group are also found in the diploid individuals (i.e., females) but not the haploid males of the other genetic group (Fig. 1). In other words, the females (both workers and queens) are apparent hybrids, but the males are not.Open in a separate windowFig. 1.Schematic representation of the male (haploid) and the female (diploid) genomes in the two genetic groups (R and W) of the hybrid ant population. Red rectangles represent genome areas originating from the R group, and blue rectangles represents genome areas originating from the W group. Allele categories are (1) allele diagnostic to W, (2) allele introgressed from W to R, (3) allele introgressed from R to W, and (4) allele diagnostic to R. Introgression is apparent in the females of both groups but not in the males.The parental species of this hybrid population belong to the recently speciated Formica rufa group. At the European scale, F. polyctena is a southern species, whereas F. aquilonia has a boreoalpine distribution occurring in northern and mountainous areas. Their distributions overlap in southern Finland, where our study population is also located. Hybridization may readily take place in their sympatric populations, and the ants are taxonomically difficult to identify (14). Therefore, finding pure parental populations for comparative purposes is difficult. Furthermore, both F. polyctena and F. aquilonia form large supercolonies, a single nest containing many and up to hundreds of queens. The parental species can show large genetic differences within a species even locally because of founder effects and restricted dispersal (15).Introgression into females but not into males is compatible with Haldane’s rule and has resulted in large-scale genetic differences between sexes within the two groups, R and W, of the hybrid population. Mating and sexual reproduction within a group should wipe out such genetic differences within a few generations. The intriguing question thus remains: how are genetic differences between the sexes within the genetic groups created and maintained? Here, we will test two hypotheses that were proposed to explain the situation (9).The selection hypothesis predicts that the genetic differences between male and female genomes result from strong postzygotic selection eliminating incompatible genotype combinations. Strong selection against hybrid males could create large-scale differentiation between sexes, because recessive incompatibilities can be masked in diploid heterozygous females but not haploid males. Consequently, the hybrid males die, but females can survive. The observed genotype frequencies in the adult ants suggest that selection should be very strong and eliminate even most of the offspring genotypes depending on the genetic group and the sex (9).The segregation hypothesis proposes that hybridization has led to the formation of two independently segregating allelic sets, one of which is always transmitted from the mother queens to their sons and the other one is always transmitted to daughters (as if females would be of type XY and males would be of type Y). When the allelic set transmitted to future daughters (X) is fertilized by sperm carrying the paternal complement (Y), the females remain hybrids, and differentiation between sexes is maintained. Two independently segregating allelic sets could be created by chromosomal rearrangements, which can contribute to hybrid incompatibilities and reproductive isolation (16, 17).     

Hybridization as a stimulus for the evolution of invasiveness in plants?

Invasive species are of great interest to evolutionary biologists and ecologists because they represent historical examples of dramatic evolutionary and ecological change. Likewise, they are increasingly important economically and environmentally as pests. Obtaining generalizations about the tiny fraction of immigrant taxa that become successful invaders has been frustrated by two enigmatic phenomena. Many of those species that become successful only do so (i) after an unusually long lag time after initial arrival, and/or (ii) after multiple introductions. We propose an evolutionary mechanism that may account for these observations. Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness. We present and review a remarkable number of cases in which hybridization preceded the emergence of successful invasive populations. Progeny with a history of hybridization may enjoy one or more potential genetic benefits relative to their progenitors. The observed lag times and multiple introductions that seem a prerequisite for certain species to evolve invasiveness may be a correlate of the time necessary for previously isolated populations to come into contact and for hybridization to occur. Our examples demonstrate that invasiveness can evolve. Our model does not represent the only evolutionary pathway to invasiveness, but is clearly an underappreciated mechanism worthy of more consideration in explaining the evolution of invasiveness in plants.     

Reproductive cooperation between queens and their mated workers: the complex life history of an ant with a valuable nest

The life history of Harpegnathos saltator is exceptional among ants because both queens and workers reproduce sexually. Recently mated queens start new colonies alone, but later some of the offspring workers also become inseminated and take over the egg-laying role. This alternation seems associated with the existence of very complex underground nests, which are designed to survive floods. Longevity of ponerine queens is low (a consequence of limited caste dimorphism in this "primitive" subfamily), and upon the death of an H. saltator foundress, the nest represents a substantial investment. The queen's progeny should thus be strongly selected to retain the valuable nests. Unlike the flying queens, the workers copulate with males from their own colonies, and, thus, their offspring are expected to be highly related to the foundress. Colony fission appears not to occur because a daughter fragment would lack an adequate nest for protection. Thus, the annual production of queens in colonies with reproductive workers remains essential for the establishment of new colonies. This contrasts with various other ponerine species in which the queens no longer exist.     

Reproductive protein protects functionally sterile honey bee workers from oxidative stress

Research on aging shows that regulatory pathways of fertility and senescence are closely interlinked. However, evolutionary theories on social species propose that lifelong care for offspring can shape the course of senescence beyond the restricted context of reproductive capability. These observations suggest that control circuits of aging are remodeled in social organisms with continuing care for offspring. Here, we studied a circuit of aging in the honey bee (Apis mellifera). The bee is characterized by the presence of a long-lived reproductive queen caste and a shorter-lived caste of female workers that are life-long alloparental care givers. We focus on the role of the conserved yolk precursor gene vitellogenin that, in Caenorhabditis elegans, shortens lifespan as a downstream element of the insulin/insulin-like growth factor signaling cascade. Vitellogenin protein is synthesized at high levels in honey bee queens and is abundant in long-lived workers. We establish that vitellogenin gene activity protects worker bees from oxidative stress. Our finding suggests that one mechanistic explanation for patterns of longevity in bees is that a reproductive regulatory pathway has been remodeled to extend life. This perspective is of considerable relevance to research on longevity regulation that builds largely on inference from solitary model species.     

A prehistory of Indian Y chromosomes: evaluating demic diffusion scenarios

Understanding the genetic origins and demographic history of Indian populations is important both for questions concerning the early settlement of Eurasia and more recent events, including the appearance of Indo-Aryan languages and settled agriculture in the subcontinent. Although there is general agreement that Indian caste and tribal populations share a common late Pleistocene maternal ancestry in India, some studies of the Y-chromosome markers have suggested a recent, substantial incursion from Central or West Eurasia. To investigate the origin of paternal lineages of Indian populations, 936 Y chromosomes, representing 32 tribal and 45 caste groups from all four major linguistic groups of India, were analyzed for 38 single-nucleotide polymorphic markers. Phylogeography of the major Y-chromosomal haplogroups in India, genetic distance, and admixture analyses all indicate that the recent external contribution to Dravidian- and Hindi-speaking caste groups has been low. The sharing of some Y-chromosomal haplogroups between Indian and Central Asian populations is most parsimoniously explained by a deep, common ancestry between the two regions, with diffusion of some Indian-specific lineages northward. The Y-chromosomal data consistently suggest a largely South Asian origin for Indian caste communities and therefore argue against any major influx, from regions north and west of India, of people associated either with the development of agriculture or the spread of the Indo-Aryan language family. The dyadic Y-chromosome composition of Tibeto-Burman speakers of India, however, can be attributed to a recent demographic process, which appears to have absorbed and overlain populations who previously spoke Austro-Asiatic languages.