A significant role for hybridization (i.e. interbreeding) between individuals from different biological populations is increasingly being recognized in the evolution of human and non-human primates. However, the full evolutionary impact of hybridization between modern humans and other members of the genus Homo remains unclear and highly debatable. Other questions regarding actual levels of genetic exchange and interbreeding within this genus also remain. Unfortunately, the relative antiquity of such events makes these finer grain, yet important details regarding human evolution more difficult than more recent evolutionary events.
Studying genetic aspects of hybridization in extinct, non-human primates provides one way to tackle this challenge because non-human primates serve as important analogs of human evolution. This study proposes to use genetic data from two Neotropical monkeys, common (Callithrix jacchus) and black-tufted marmosets (C. penicillata) as anthropological models of hybridization to address the following question: What is the role of past and present hybridization and genetic exchange in shaping primate population histories? To examine this question, genetic material will be sampled within two marmoset hybrid zones, as well from non-interbreeding populations to make inferences about the evolutionary effects of hybridization on the genomes of the both species.
C. jacchus and C. penicillata are small, closely-related Brazilian primates that are an especially informative system for the anthropological study of hybridization because (1) in certain parts of their ranges, they are thought to be interbreeding under population conditions similar to that of early humans; (2) parts of their genomes that are conserved and homologous to certain regions within the human genome can be used to study genomic effects of interbreeding; and (3) they exhibit similar flexibility in mating systems as do humans. Additionally, marmosets are of interest to many researchers because they possess a unique suite of biological and social traits such as twinning, cooperative breeding and social suppression of ovulation in females. Yet, few details about the evolutionary history of marmosets are currently available in the primatological literature.
The intellectual merits of this project are threefold: (1) this project will provide a multicontext view of primate hybridization in terms of time and space, (2) the project's results will broaden current knowledge about marmoset evolution, and (3) project findings will allow researchers to formulate and test new hypotheses about hybridization in human and other nonhuman primates.
This project has broader impacts on primate conservation, international collaboration, and outreach. The data for this project will assist in decisions about primate population management and conservation for invasive marmoset species and highly endangered primates. The proposed work creates ample opportunities for the exchange and dissemination of ideas about primate hybridization through international collaboration with several Brazilian primatologists. Outreach efforts of this project are aimed to train both American and Brazilian undergraduate students in molecular biology. Additionally, the results of this project will form the basis of scientific inquiry lessons designed for American K-12 students. Finally, this project is part of dissertation research which will aid in the training of a female doctoral student.
The recognition of hybridization as an important force in shaping the population histories of human and non-human primates has gained a large amount of anthropological traction. However, the full impact of hybridization on primate evolution, particularly among early hominins, remains unclear and highly debatable; thus, the study of non-human primate hybridization can help elucidate such issues. In this study, we are employing various neutral genetic markers to help us understand the role of hybridization in primate evolution. Specifically, we examine hybridization between two closely related Brazilian species, black-tufted and common marmosets to elucidate patterns of genetic exchange between the two species after divergence from their last common ancestor. We looked at how on-going hybridization affected the genetic diversity of common and black-tufted marmosets at two hybrid zones, one zone occurring at a natural species border and at a second completely man-made zone. The natural hybrid zone lies between the northeastern Brazilian cities of Petrolina and Juazeiro along the banks of the San Francisco River. The man-made hybrid zone lies in the state of Rio de Janeiro, far from the natural ranges of common and black-tufted marmosets, where the two study species were introduced illegaly. Our phenotypic and genotypic data from these two zones paints a complicated picture of marmoset hybridization. In the natural hybrid zone, marmosets caught on the north side of the San Francisco river look superficially like common marmosets, in accordance to the natural border of that species. Marmosets caught on the south side of the San Francisco River mostly look like black-tufted marmosets, again agreeing with the natural border of that species. However, a large number of marmosets sampled in the Rio de Janeiro state hybrid zone looked like intermiediate versions of the two parental species. A respectable few of the Rio de Janeiro marmosets looked like pure versions of the two study species. Under the genetic "hood," the hybridization picture looks even more complicated. We found evidence for genetic transfer of mitochondrial control region (a quickly evolving marker found on the mitochondrial genome) between common and black-tufted marmosets across the San Francisco River in the Petrolina-Juazeiro hybrid zone. We also found that at microsatellite loci (quickly evolving loci found on the nuclear genome) most sampled animals had 75% ancestry from one species and 25% ancestry from the other species. Levels of genetic diversity at the natural hybrid zone were almost equal to what we observed in pure parental populations of either study species. We find these genetic patterns despite most of the sample marmosts having a pure looking phenotype! In the Rio de Janeiro hybrid zone, we found from microsatellite loci that sampled marmosets had about 60% of their ancestry from one species and 40% of their ancestry from the other species. A major point of concern was the highly reduced levels of genetic diversity at mitochondrial and microsatellite markers in the Rio de Janeiro zone when compared to levels in the other hybrid zone and what we see in pure, parental populations. Our data show that when it comes to hybridization, it is not wise to judge a book just by its cover. Pure looking individuals may have much more complicated ancestry than what we can tell from just looking at an individual. Currently, reports are increasing of introductions and hybridization of our two study species in areas where other species of marmoset are found; so much so that common and black-tufted marmosets are considered pest or invasive species in many places in Brazil. Not only that, but many other marmoset species with whom introduced common and black-tufted marmosets hybridize are also endangered. Based on our results, we predict that natural hybridization does not seem to take a toll on the biodiversity of marmosets, where as human-mediate hybridization may. This is an area of concern for the present evolutionary history of many primate populations, including marmosets, because human alteration of habitats increases the incidence of hybridization. However, we must also understand the effects of hybridization on primate reproductive fitness to fully gauge both the positive and negative effects of primate hybridization. During the course of our project, we were able to collaborate with a number of Brazilian biologists. We also carried out outreach work with Brazilian NGOs, Brazilian grade schools and an American high school to talk about our findings, and discuss the general importance of animal hybridization. Additionally, this project has supported the training of several graduate and undergraduate students in field and molecular biology/primatology.
