Hybridization is the process by which two genetically distinct lineages interbreed and produce, at least, some offspring. Although this is a phenomenon well known in nature, studies of hybridization in primates have been scant. Through the increased use of molecular tools, a larger number of cases of primate hybridization in the wild have been recognized. Our group at the University of Michigan and our collaborators in different institutions are focused on the study of one of the first primate hybridization systems for which genetic confirmation of natural hybridization has been obtained.

Hybridization is currently viewed as a powerful evolutionary force. On the one hand it may be a way to increase genetic diversity for one or both of the hybridizing taxa while still maintaining their taxonomic integrity. On the other hand, it may take one or both species to extinction, or create vigorous genetic combinations producing new lineages, distinct to their parental forms. Understanding the mechanisms through which hybridization may work to generate variation, limit species survival, or affect individual fitness is relevant for our understanding of how species originate and maintain over time. Molecular tools have enhanced hybridization research in the last 30 years. We have used different types of markers to understand the genotypic variation in hybrid howler monkeys (Alouatta palliata x A. pigra), as well as the extent at which hybrid incompatibility and genetic introgression are occurring in this hybrid system.

Reproductive Isolation in Alouatta contact zone in Mexico

We are interested in understanding the role of reproductive isolation in speciation. In allopatry, populations diverge through adaptation to the local environment, drift, or differential mutation. During secondary contact and hybridization, intrinsic (genetic) differences that arose in allopatry are tested in a common environment in admixed individuals of varying hybrid backgrounds. Some allele combinations may reduce hybrid fitness and further production of hybrid offspring and subsequently reduced genetic exchange. In the Alouatta hybrid zone (A. palliata x A. pigra), we have used genomic data and found evidence that divergent selection and reinforcement maintain species boundaries between these two sister taxa. We also have demonstrated reduce introgression of the sex chromosomes, consistent with the hypothesis that the sex chromosomes have a disproportionate contribution to reproductive isolation. Our current work seeks to further understand how these processes affect the evolutionary history and trajectory of these two New World monkeys.

Causes and Consequences of Vocal Variation in a Howler Monkey (Alouatta spp.) Hybrid Zone

This is a collaborative research between researchers from the University of Michigan, Ohio State University and Universidad Veracruzana.  The project aims to integrate molecular, morphological, and behavioral data to understand the role of vocal communication in the hybridization patterns between A. palliata and A. pigra in Mexico. In this project we want to understand both how hybridization effects communication and how communication structures hybridization and gene flow in a primate system.

Some preliminary data suggest that vocalizations of A. palliata and A. pigra differ in many aspects and that A. palliata x A. pigra hybrids are also different than either purebred species. Based on our multi-locus molecular approach to genetically characterize purebred and hybrid individuals (see genetics section), we are currently building a large database of vocalizations for a number of individuals with a wide gradient of genetic backgrounds, ranging from purebred A. palliata through individuals with different levels of hybridity, to purebred A. pigra. These vocalizations will be analyzed to quantify differences between species and between purebred and hybrid individuals.  Additionally, playback experiments will be conducted to assess competitive ability of males and female choice within and outside the hybrid zone.

How does hybridization affect primate morphology?

Although there are a number of recent reports of hybridization in the primate literature, there is a great lack of understanding on the morphological variation associated to the hybridization process in primates. In particular, recent reviews show the need to conduct long-term studies combining morphological and genetic traits in hybrid individuals to understand the extent of the variation in phenotypic expression in hybrids, determine the longevity of hybrid morphologies, and understand the universality of hybrid traits.

As a first step in understanding differences between purebred and hybrid individuals, we have analyzed morphological variation found within each of the species involved in this hybrid system outside the hybrid zone, and analyzed the differences between them. Our analyses include comparisons of body mass, body length, canine size and testicular volume. We have also analyzed sexual size dimorphism using three of the variables as a way to understand possible differences in social interactions within each species, as it is assumed that sexual size dimorphism is larger in species with higher degrees of male-male competition for access to reproductive females. In brief, we have found that A. pigra males and females are larger in body mass and length than their A. palliata counterparts. Interestingly, male and female canine size is not significantly different between the two species, despite the fact that they differ in overall body size. Although A. pigra males are the larger of the two, A. palliata males have testes that are twice as large as their counterparts. Both species show sexual dimorphism in body mass,  trunk length, and canine size, but there is no statistically significant difference between the two species. The lack of sexual dimorphism differences and the fact that A. palliata males may outcompete A. pigra males if both fertilize the same female, may have interesting implications for our understanding of the possible causes of the patterns of hybridization observed with genetic data.>

We are currently conducting analyses of metric (continuous trait) data for adult individuals of A. palliata, A. pigra and their hybrids, both within and outside the putative hybrid zone in Mexico. Our dataset includes 18 morphometric traits, microsatellite genotypes and mitochondrial DNA and SRY gene data fro 224 individuals, 129 of which have some degree of hybridity.

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