Craniofacial Adaptations to Tree-Gouging Among Marmosets

Pelage variation and morphometrics of closely related Callithrix marmoset species and their hybrids

BACKGROUND

Hybrids are expected to show greater phenotypic variation than their parental species, yet how hybrid phenotype expression varies with genetic distances in closely-related parental species remains surprisingly understudied. Here, we investigate pelage and morphometric trait variation in anthropogenic hybrids between four species of Brazilian Callithrix marmosets, a relatively recent primate radiation. Marmoset species are distinguishable by pelage phenotype and morphological specializations for eating tree exudates. In this work, we (1) describe qualitative phenotypic pelage differences between parental species and hybrids; (2) test whether significant quantitative differences exist between parental and hybrid morphometric phenotypes; and (3) determine which hybrid morphometic traits show heterosis, dysgenesis, trangression, or intermediacy relative to the parental trait. We investigated cranial and post-cranial morphometric traits, as most hybrid morphological studies focus on the former instead of the latter. Finally, we estimate mitogenomic distances between marmoset species from previously published data.

RESULTS

Marmoset hybrid facial and overall body pelage variation reflected novel combinations of coloration and patterns present in parental species. In morphometric traits, C. jacchus and C. penicillata were the most similar, while C. aurita was the most distinct, and C. geoffroyi trait measures fell between these species. Only three traits in C. jacchus x C. penicillata hybrids showed heterosis. We observed heterosis and dysgenesis in several traits of C. penicillata x C. geoffroyi hybrids. Transgressive segregation was observed in hybrids of C. aurita and the other species. These hybrids were also C. aurita-like for a number of traits, including body length. Genetic distance was closest between C. jacchus and C. penicillata and farthest between C. aurita and the other species.

CONCLUSION

We attributed significant morphometric differences between marmoset species to variable levels of morphological specialization for exudivory in these species. Our results suggest that intermediate or parental species-like hybrid traits relative to the parental trait values are more likely in crosses between species with relatively lesser genetic distance. More extreme phenotypic variation is more likely in parental species with greater genetic distance, with transgressive traits appearing in hybrids of the most genetically distant parental species. We further suggest that fewer developmental disturbances can be expected in hybrids of more recently diverged parental species, and that future studies of hybrid phenotypic variation should investigate selective pressures on Callithrix cranial and post-cranial morphological traits.

Evolutionary morphology, platyrrhine evolution, and systematics

This special volume of the Anatomical Record focuses on the evolutionary morphology of New World monkeys. The studies range from three-dimensional surface geometry of teeth to enamel ultrastructure; from cranioskeletal adaptations for eating leaves and seeds to the histology of taste bud proxies; from the architecture of its bones to the mechanoreceptors of the tail's skin; from the physical properties of wild foods to the feeding biomechanics of jaws and skull; from the shapes of claws and fingertips, and of elbows, to the diversity and morphology of positional behavior; from the vomeronasal organ and its biological roles to links between brains, guts, sociality, and feeding; from the gum-eating adaptations of the smallest platyrrhines to the methods used to infer how big the largest fossil platyrrhines were. They demonstrate the power of combining functional morphology, behavior, and phylogenetic thinking as an approach toward reconstructing the evolutionary history of platyrrhine primates. While contributing new findings pertaining to all the major clades and ecological guilds, these articles reinforce the view that platyrrhines are a coherent ecophylogenetic array that differentiated along niche dimensions definable principally by body size, positional behavior, and feeding strategies. In underlining the value of character analysis and derived morphological and behavioral patterns as tools for deciphering phylogenetic and adaptational history, doubts are raised about a competing small-bore morphological method, parsimony-based cladistic studies. Intentionally designed not to enlist the rich reservoir of platyrrhine evolutionary morphology, an empirical assessment of the costs incurred by this research stratagem reveals inconsistent, nonrepeatable, and often conflicting results.