CRYPTIC SPECIES IN THE MEXICAN POCKET GOPHER CRATOGEOMYS MERRIAMI

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.

Phylogeography and taxonomic revision of Nelson’s pocket mouse (Chaetodipus nelsoni)

Chaetodipus nelsoni occurs on rocky substrates across the Mexican Altiplano. We investigated phylogeographic diversity within the species using morphologic, karyotypic, and molecular data. Data from nuclear (AFLP) and mitochondrial DNA support three distinct genetic groups with minimal substructuring coincident with biogeographic barriers previously identified in the Chihuahuan Desert and drainage basins of the Altiplano. We examined the morphological and karyotypic data in light of the molecular data. The results support recognition of three species within the currently accepted widespread C. nelsoni: 1) C. nelsoni restricted to a distribution centered on the El Salado River Basin; 2) elevation of C. n. collis to species, with two subspecies: one centered on Trans-Pecos Texas, the other on the Mapimí Basin (new subspecies); and 3) recognition of a new species, C. durangae, centered on the Nazas Basin and upper Río Mezquital drainage.

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 .

Co1 DNA supports conspecificity of Geomyphiluspierai and G.barrerai (Coleoptera, Scarabaeidae, Aphodiinae) and is a good marker for their phylogeographic investigation in Mexican mountains

Members of Geomyphilus are associated with rodent burrows, such as pocket gophers and prairie dogs. In Mexico, they are found in the mountains of the Mexican Volcanic Belt and in Sierra Madre Oriental. Our study aims to initiate the exploration of the dispersal modes of Geomyphiluspierai and Geomyphilusbarrerai from burrows of pocket gophers. In order to estimate the dispersal scale of the beetles, the utility of mitochondrial and nuclear molecular markers for studying the phylogeographic structure of this complex of species (Geomyphiluspierai and Geomyphilusbarrerai) was tested from 49 beetle individuals. High intraspecific and intra-mountain nucleotidic diversity was captured from this sample using Co1 mitochondrial sequences, whilst the ITS2 nuclear ribosomal sequence did not allow observing informative variation. Mitochondrial phylogenetic analysis revealed that the specific delineation between the two species under study was doubtful. In this preliminary study, Co1 was shown to be a good marker for elucidating dispersal routes of the burrowing rodent-associated beetles.

Phylogeography of the dark kangaroo mouse, Microdipodops megacephalus: cryptic lineages and dispersal routes in North America's Great Basin

AIM: The rodent genus Microdipodops (kangaroo mice) includes two sand-obligate endemics of the Great Basin Desert: M. megacephalus and M. pallidus. The dark kangaroo mouse, M. megacephalus, is distributed throughout the Great Basin and our principal aims were to formulate phylogenetic hypotheses for this taxon and make phylogeographical comparisons with its congener. LOCATION: The Great Basin Desert of western North America. METHODS: DNA sequence data from three mitochondrial genes were examined from 186 individuals of M. megacephalus, representing 47 general localities. Phylogenetic inference was used to analyse the sequence data. Directional analysis of phylogeographical patterns was used to examine haplotype sharing patterns and recover routes of gene exchange. Haplotype-area curves were constructed to evaluate the relationship between genetic variation and distributional island size for M. megacephalus and M. pallidus. RESULTS: Microdipodops megacephalus is a rare desert rodent (trapping success was 2.67%). Temporal comparison of trapping data shows that kangaroo mice are becoming less abundant in the study area. The distribution has changed slightly since the 1930s but many northern populations now appear to be small, fragmented, or locally extinct. Four principal phylogroups (the Idaho isolate and the western, central and eastern clades) are evident; mean sequence divergence between phylogroups for cytochrome b is c. 8%. Data from haplotype sharing show two trends: a north-south trend and a web-shaped trend. Analyses of haplotype-area curves reveal significant positive relationships. MAIN CONCLUSIONS: The four phylogroups of M. megacephalus appear to represent morphologically cryptic species; in comparison, a companion study revealed two cryptic lineages in M. pallidus. Estimated divergence times of the principal clades of M. megacephalus (c. 2-4 Ma) indicate that these kangaroo mice were Pleistocene invaders into the Great Basin coincident with the formation of sandy habitats. The north-south and web patterns from directional analyses reveal past routes of gene flow and provide evidence for source-sink population regulation. The web pattern was not seen in the companion study of M. pallidus. Significant haplotype-area curves indicate that the distributional islands are now in approximate genetic equilibrium. The patterns described here are potentially useful to conservation biologists and wildlife managers and may serve as a model for other sand-obligate organisms of the Great Basin.