SYSTEMATIC REVISION OF POCKET GOPHERS OF THE CRATOGEOMYS GYMNURUS SPECIES GROUP

Multivariate analyses of skull morphology inform the taxonomy and evolution of geomyoid rodents

Morphological analyses are critical to quantify phenotypic variation, identify taxa, inform phylogenetic relationships, and shed light on evolutionary patterns. This work is particularly important in groups that display great morphological disparity. Such is the case in geomyoid rodents, a group that includes 2 of the most species-rich families of rodents in North America: the Geomyidae (pocket gophers) and the Heteromyidae (kangaroo rats, pocket mice, and their relatives). We assessed variation in skull morphology (including both shape and size) among geomyoids to test the hypothesis that there are statistically significant differences in skull measurements at the family, genus, and species levels. Our sample includes 886 specimens representing all geomyoid genera and 39 species. We used the geometric mean to compare size across taxa. We used 14 measurements of the cranium and lower jaw normalized for size to compare shape among and within taxa. Our results show that skull measurements enable the distinction of geomyoids at the family, genus, and species levels. There is a larger amount of size variation within Geomyidae than within Heteromyidae. Our phylomorphospace analysis shows that the skull shape of the common ancestor of all geomyoids was more similar to the common ancestor of heteromyids than that of geomyids. Geomyid skulls display negative allometry whereas heteromyid skulls display positive allometry. Within heteromyids, dipodomyines, and non-dipodomyines show significantly different allometric patterns. Future analyses including fossils will be necessary to test our evolutionary hypotheses.

Sexual dimorphism in cranial shape and size in geomyoid rodents: multivariate and evolutionary perspectives

Geomyoid rodents provide a great study system for the analysis of sexual dimorphism. They are polygynic and many inhabit harsh arid environments thought to promote sexual dimorphism. In fact, there has been extensive work published on the sexual size dimorphism of individual populations and species within this rodent clade. However, little work has been undertaken to assess the evolutionary patterns and processes associated with this sexual dimorphism. We use multivariate analyses of cranial measurements in a phylogenetic framework to determine the distribution of size and shape dimorphism among geomyoids and test for Rensch’s rule. Our results suggest that sexual dimorphism is more common in geomyids than heteromyids, but it is not in fact universal. There is evidence for variation in sexual dimorphism across populations. Additionally, in many taxa, geographic variation appears to overwhelm existing sexual dimorphism. We find support for the repeated independent evolution of shape and size dimorphism across geomyoid taxa, but we do not find support for an association between size and shape dimorphism. There is no evidence for Rensch’s rule in geomyoids, whether at the superfamily or family level. Together, our findings suggest that there is no single explanation for the evolution of sexual dimorphism in geomyoids and that, instead, it is the product of numerous evolutionary events. Future studies incorporating phylogenetic relationships will be necessary to paint a more complete picture of the evolution of sexual dimorphism in geomyoids.

Systematic revision of the pocket gopher genus Orthogeomys

Pocket gophers of the genus Orthogeomys show unusually high morphological and ecological diversity compared to other genera in the family Geomyidae. Whereas this diverse group once was divided into 3 genera ( Merriam 1895 ), a revision by Russell (1968) recognized only Orthogeomys , with Merriam’s original genera relegated to subgeneric status as Heterogeomys , Macrogeomys , and Orthogeomys . Recent studies have called into question the monophyly of Orthogeomys , as well as the validity of 4 currently recognized Orthogeomys species. To date, the taxonomic validity of only 1 of these species has been verified ( Hafner et al. 2014 ). In this analysis, the first to include all 11 recognized species of the genus, we examine 3 mitochondrial and 2 nuclear gene sequences (4,352 base pairs) and analyze cranial morphology to explore relationships within the genus. Our data support a taxonomic revision that restricts the genus Orthogeomys to a single species ( O. grandis ) and combines the subgenera Heterogeomys and Macrogeomys into the resurrected genus, Heterogeomys (7 species). In addition, 3 currently recognized species of Orthogeomys are synonymized as follows: O. cuniculus with O. grandis ; H. thaeleri with H. dariensis ; and H. matagalpae with H. cherriei . A synonymy and a key to the species of the genera Orthogeomys and Heterogeomys are provided.

Las tuzas del género Orthogeomys muestran una diversidad morfológica y ecológica inusual en comparación con otros géneros de la familia Geomyidae. Aunque este diverso grupo fue alguna vez dividido en 3 géneros ( Merriam 1895 ), la revisión de Russell (1968) reconoció solo a Orthogeomys , mientras que los géneros originales de Merriam fueron relegados a estatus subgenérico como Heterogeomys , Macrogeomys y Orthogeomys . Estudios recientes han cuestionado la monofilia de Orthogeomys , así como la validez de 4 de las especies actualmente reconocidas. A la fecha, la validez taxonómica de sólo una de estas especies ha sido verificada. En este análisis, el primero en incluir las 11 especies reconocidas en el género, examinamos secuencias de 3 genes mitocondriales y 2 nucleares y analizamos la morfología craneal para explorar las relaciones dentro del género. Nuestras 4,352 pares de bases de secuencias de ADN apoyan una revisión taxonómica que retiene al género Orthogeomys (incluyendo sólo a O. grandis ) y combina los subgéneros Heterogeomys y Macrogeomys en un género recuperado, Heterogeomys (7 especies). Además, 3 especies de Orthogeomys actualmente reconocidas son sinonimizadas de la siguiente forma: O. cuniculus con O. grandis ; H. thaeleri con H. dariensis ; y H. matagalpae con H. cherriei . Se incluye sinonimia y una clave para las especies de los géneros Orthogeomys y Heterogeomys .

Cophylogeny on a fine scale: Geomydoecus chewing lice and their pocket gopher hosts, Pappogeomys bulleri

Many species of pocket gophers and their ectoparasitic chewing lice have broadly congruent phylogenies, indicating a history of frequent codivergence. For a variety of reasons, phylogenies of codiverging hosts and parasites are expected to be less congruent for more recently diverged taxa. This study is the first of its scale in the pocket gopher and chewing louse system, with its focus entirely on comparisons among populations within a single species of host and 3 chewing louse species in the Geomydoecus bulleri species complex. We examined mitochondrial DNA from a total of 46 specimens of Geomydoecus lice collected from 11 populations of the pocket gopher host, Pappogeomys bulleri. We also examined nuclear DNA from a subset of these chewing lice. Louse phylogenies were compared with a published pocket gopher phylogeny. Contrary to expectations, we observed a statistically significant degree of parallel cladogenesis in these closely related hosts and their parasites. We also observed a higher rate of evolution in chewing louse lineages than in their corresponding pocket gopher hosts. In addition, we found that 1 louse species (Geomydoecus burti) may not be a valid species, that subspecies within G. bulleri are not reciprocally monophyletic, and that morphological and genetic evidence support recognition of a new species of louse, Geomydoecus pricei.