James W. Demastes

DIFFERENCES IN RATE OF CYTOCHROME-b EVOLUTION AMONG SPECIES OF RODENTS

Abstract Although molecular evolution often appears to proceed in a clocklike fashion, examples to the contrary are increasing in number. Our study compares rate of cytochrome-b evolution in 21 rodent species, each of which belongs to a different genus. In these comparisons, substitutions at synonymous sites appear to be saturated, precluding inferences about rate of synonymous substitution. Rate of nonsynonymous substitution differs significantly among many of the rodents studied. However, the cause or causes of these differences in substitution rate remains in question. Differences in generation time, body size, or metabolic rate do not seem to be associated with rate of nonsynonymous substitution in these rodents. Effective population size remains a viable explanation of the observed rate heterogeneity. However, we suggest that a search for simple causes of differences in rate of molecular evolution may be difficult in light of numerous aspects of an organisms biology that may together influence evolutionary rates over space and time.

Cospeciation of Pocket Gophers (Geomys) and their Chewing Lice (Geomydoecus)

Comparison of independently derived phylogenies for pocket gophers ( Geomys ) and their chewing lice ( Geomydoecus ) from Texas and Louisiana indicates a history of widespread cospeciation in this host-parasite assemblage. Inference of cospeciation is supported by statistical comparison of genetic-distance matrices for gophers and lice based on allozyme data. Although similar, host and parasite phylogenies are not identical; inconsistencies likely result from host-switching by the parasites, retention of ancestral taxa of parasites on recently evolved hosts, or poorly delineated taxonomic boundaries. The current disjunct distribution of Geomydoecus ewingi suggests that this chewing louse once parasitized the common ancestor of Geomys breviceps and G. attwateri . Combined protein and morphologic evidence suggests that the population of Geomydoecus ewingi hosted by G. breviceps breviceps in northeastern Louisiana may be a cryptic species of louse.

Cryptic Species in the Mexican Pocket Gopher Cratogeomys merriami

Abstract A study of chromosomal variation in the Mexican pocket gopher Cratogeomys merriami revealed differences in diploid number that correspond to 3 major genetic and morphological clades within the species. Each of these 3 clades is diagnosable based on multiple characters, including chromosomal diploid number, quantitative and qualitative morphological characters, and mitochondrial DNA. Accordingly, we restrict the name C. merriami (Thomas) to include only pocket gophers of this genus from the states of México, México D.F., northern Morelos, and west-central Puebla. We resurrect the species name C. fulvescens Merriam to represent members of this genus from southern Tlaxcala, east-central Puebla, and parts of west-central Veracruz. Finally, we resurrect the species name C. perotensis Merriam to represent members of this genus from southern Hidalgo, northern Tlaxcala, north-central Puebla, and parts of west-central Veracruz. Based on the observation that differences in diploid number usually signal reproductive isolation between populations of pocket gophers, we hypothesize that C. merriami, C. fulvescens, and C. perotensis are reproductively incompatible. We provide synonymies and descriptions for these 3 species, along with a key to the C. castanops species group to which these species belong.

Evolutionary Relationships of Pocket Gophers (Cratogeomys castanops Species Group) of the Mexican Altiplano

Abstract The Southern Coahuila Filter-Barrier (SCFB) effectively subdivides the mammalian fauna of the Mesa del Norte, the northern and most extensive section of the Mexican Altiplano. Pocket gophers of the genus Cratogeomys north and south of this filter-barrier have been informally recognized as 2 distinct species, C. castanops and C. goldmani, respectively. Support for species recognition derives from early morphological comparisons and recent chromosomal and ectoparasite studies. Contradictory conclusions based on the only comprehensive morphometric study have prevented formal recognition of C. goldmani. A morphometric reevaluation based on ratio-transformed data reveals that the previous analysis was unduly biased by size, an ecophenotypically plastic character. When this factor is removed, morphometric variation is fully concordant with chromosomal diploid number and sequence data of mitochondrial and nuclear DNA. We provide synonymies and descriptions for C. goldmani apart from C. castanops, and revise the number of subspecies from a total of 26 to 2 subspecies in each species. The SCFB is most effective in its central portion (Desierto Mayrán), and least effective in its western portion (Río Nazas), which should be geographically broadened to include the neighboring Río Aguanaval.

SYSTEMATICS OF A RARE SPECIES OF POCKET GOPHER, PAPPOGEOMYS ALCORNI

Abstract Pappogeomys alcorni is a peripherally isolated species that currently is designated as a rare species in Mexico. There are only 4 known museum specimens, all collected between the years 1950 and 1966, and recent attempts have failed to locate individuals in the wild. Although only 1 new specimen (skin only) has been collected since the original systematic work on P. alcorni, new analytical methods permit reexamination of existing specimens. DNA was isolated from a 48-year-old museum skin to investigate the phylogenetic relationship of this rare taxon to other species of Pappogeomys and Cratogeomys. Phylogenetic analysis based on 424 base pairs of the mitochondrial cytochrome-b gene and morphometric analysis of 101 individuals of Pappogeomys indicate that P. alcorni is a geographically disjunct subspecies of P. bulleri rather than a monotypic species.

Cophylogeny on a Fine Scale: Geomydoecus Chewing Lice and Their Pocket Gopher Hosts, Pappogeomys bulleri

Abstract: 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.

Pocket gophers and chewing lice: a test of the maternal transmission hypothesis

The life‐history traits of pocket gophers and their chewing lice suggest that there is little opportunity for transmission of parasites among pocket gophers, with the exception of transmission from mother to offspring. Herein, we test the hypothesis that lice are transmitted maternally by using an indirect approach that compares the distribution of louse populations to the distribution of mitochondrial DNA haplotypes in the pocket gophers. Comparison of the chewing louse distributions to the distribution of mtDNA haplotypes for the gophers revealed no significant concordance, and thus falsifies the maternal transmission hypothesis.

Redescription of the pocket gopher Thomomys atrovarius from the Pacific coast of mainland Mexico

Abstract Thomomys atrovarius is redescribed to include the smooth-toothed pocket gophers that inhabit dry, thornscrub vegetation along the Pacific versant of the Sierra Madre Occidental of Mexico from northern Sinaloa into western Durango, northwestern Jalisco, and western Nayarit. Molecular analyses of mitochondrial and nuclear DNA sequences (including historical samples from museum skins) show high levels of genetic differentiation between T. atrovarius and T. umbrinus of the Sierra Madre Occidental (15.4% cytochrome-b divergence) and Mexican Central Plateau (16.9% divergence). Roughly coincident morphometric and genetic gaps divide T. atrovarius into 2 subspecies, T. a. parviceps in the north and T. a. atrovarius in the south, with probable intergradation in between. Most specimens of T. atrovarius, especially those of the southern subspecies, are distinguished easily from specimens of T. bottae and T. umbrinus on the basis of fur texture, and an analysis of cranial morphometrics shows little overlap between T. atrovarius and other Thomomys clades in Mexico. An analysis of niche parameters shows significantly different climate envelopes for T. atrovarius compared with other species of Thomomys, and a biogeographical review suggests that T. atrovarius has ancestral affinities to the south of its current distribution. A synonymy of T. atrovarius and a key to the currently recognized species of Thomomys in mainland Mexico are provided.

Phylogeographic Variation in Two Central American Pocket Gophers (Orthogeomys)

Restriction-site data for mitochondrial DNA were used to test the isolation-by-distance model of genetic variation in two species of Central American pocket gophers, Orthogeomys cherriei and O. underwoodi . Maximum-likelihood and parsimony analyses yielded trees with topologies concordant with predictions based on physiography. Relationships within O. cherriei reveal probable effects of a partial geographic barrier in the form of the Cordillera de Tilaran in northcentral Costa Rica. The phylogenetic tree for O. underwoodi branches in a ladder-like fashion from south to north suggesting the absence of obstructions to gene flow. Results of Mantel tests indicated that variation of mtDNA within O. cherriei is not consistent with a hypothesis of isolation-by-distance, whereas variation within O. underwoodi shows a statistically significant association with geographic distance. Estimates of intraspecific sequence divergence indicate that the present distributions of these two species resulted from climatic shifts in the Pleistocene.

Evolutionary Relationships of Pocket Gophers of the Genus Pappogeomys (Rodentia: Geomyidae)

Abstract Once encompassing as many as 9 species of pocket gophers spread across most of the Mexican Altiplano, the genus Pappogeomys is now restricted to a single species, P. bulleri, occupying the mountains, tablelands, and coastal plains near the western end of the Trans-Mexican Volcanic Belt in west-central Mexico. Herein, we review the taxonomic history of Pappogeomys and examine relationships among populations of P. bulleri from throughout the geographic range of the species based on analyses of nonpreferentially stained karyotypes and mitochondrial and nuclear sequence data. Results of these analyses are concordant and reveal 3 major clades of P. bulleri that are separated by major physiographic features of the region, including the Sierra Madre del Sur and drainages of the Río Grande de Santiago, Río Ameca, Río Ayuquila, and Río Armería. We reduce the number of subspecies of P. bulleri from 9 to 5 valid forms and provide a revised synonymy of the species.