Margaret F. Smith

Phylogenetic Relationships and the Radiation of Sigmodontine Rodents in South America: Evidence from Cytochrome b

Phylogenetic relationships among South American sigmodontine rodents were examined based on the complete sequence for the mitochondrial cytochrome b gene [1140 base pairs (bp)] for 66 species and between 759 and 1140 bp for an additional 19 species. Thirty-eight South American genera were represented, coming from eight of nine tribes. Outgroups included the North American murid rodents Peromyscus, Reithrodontomys, Scotinomys, and Neotoma, the Old World murine rodents Mus and Rattus, and the geomyoid genera Thomomys, Geomys, Dipodomys, and Perognathus as the most distant outgroup. The South American sigmodontines were supported as a monophyletic lineage. Within this radiation several clear-cut suprageneric groupings were identified. Many of the currently recognized tribal groupings of genera were found fairly consistently, although not always with high levels of bootstrap support. The various tribes could not be linked hierarchically with any confidence. In addition, several genera stand out as unique entities, without any apparent close relatives. The overall pattern suggests a rapid radiation of the sigmodontines in South America, followed by differentiation at the tribal and generic levels.

MTDNA phylogeny of andean mice : a test of diversification across ecological gradients

Parapatric speciation across sharp ecological gradients is an alternative to the more usual allopatric model as both a general explanation of organismal diversification and as an explicit statement regarding differentiation of tropical forest biotas (Endler, 1977, 1982a, 1982b). The relevance of this model to species of small mammals distributed across a 3,000 meter gradient on the eastern flank of the Peruvian Andes is examined here by phylogenetic analysis of comparative mitochondrial DNA sequences, relying on both freshly collected samples and extracts taken from skins preserved in museum collections. These analyses falsify the applicability of the gradient model in this case. Moreover, the phylogenetic approach employed here provides an explicit test of the feasibility of the gradient model for any other taxon, either plant or animal, of the lowland Amazonian forest.

The efficiency of intestinal calcium absorption is increased in late pregnancy but not in established lactation

SummaryThe fractional absorption of calcium (FA-Ca) was measured using a dual non-radioactive Ca isotope technique in 26 control women, 49 women in the last triimester (36 weeks) of pregnancy and 31 of these women in established (20 weeks) lactation. The ratio of the two non-radioactive Ca isotopes was measured, by high precision thermal ionisation mass spectrometry, in urine 12–24 hours after administration and was used to calculate Fa-Ca. This is the first study to clearly show that FA-Ca is significantly elevated in late pregnancy but not in established lactation, when compared with control women.

Phylogenetic evidence of mitochondrial DNA introgression among pocket gophers in New Mexico (family Geomyidae)

Mitochondrial DNA (mtDNA) variation in the cytochrome b gene was determined for two divergent taxa of pocket gophers, Thomomys bottae actuosus and T. b. ruidosae. These two taxa hybridize in a narrow contact zone, but introgression of nuclear markers such as allozymes or chromosomes does not extend much beyond the hybrid zone (Patton et al. 1979). We found that despite their distinctness, the two subspecies shared very similar mtDNA haplotypes. By a comparison of phylogenetic histories derived from nuclear markers (allozymes) and from mtDNA haplotypes sampled in different populations of T. bottae from New Mexico, we show that apparent similarity is due to an introgression of T. b. ruidosae mtDNA into T. b. actuosus nuclear background. Evidence of introgression is not limited to the present‐day contact zone between these two taxa, but extends at least 75 km away from it. The actuosus haplotype coexists along with the ruidosae mtDNA in the Gallinas Mts., which are inhabited by otherwise pure T. b. actuosus, while further north only typical actuosus haplotypes were detected. Of several potential mechanisms which could lead to such a geographical pattern of variation, we argue that a combination of range shifts due to climatic fluctuations, and genetic drift are most likely. Horizontal gene transfers due to hybridization are historical events which seem rather common among pocket gophers. Although they can be identified with careful phylogenetic study using independent data sets, the potential for misinterpreting a gene tree as an organismal tree is great in this and other groups of animals.

HYBRID ZONES IN THOMOMYS BOTTAE POCKET GOPHERS: GENETIC, PHENETIC, AND ECOLOGIC CONCORDANCE PATTERNS

In a recent summary of geographic variability of both chromosomal and genic systems in Thomomys bottae pocket gophers, Patton and Yang (1977) presented data depicting extensive degrees of interpopulation genetic divergence. In several instances, adjacent or near adjacent pairs of populations were seen to differ by lOIS fixed chromosomal rearrangements and to share less than 80% in overall allelic similarity in structural genes as measured by starch gel electrophoresis. Despite the enormity of observed interpopulation divergence (in many cases greater than that found between species of other mammals), available data suggested that these geographic segments retained the ability to interbreed. Evidence for this conclusion came from studies by other workers suggesting intergradation between several of the differentiated units based on the morphological intermediacy of some specimens. In no study, however, had the gross morphological indications of intergradation been fully supported by genetic analyses. The purpose of the present report is to provide a concomitant comparison of intergradation from a genetic, morphologic, and ecologic perspective between two of the most strongly differentiated, geographically adjacent units of T. bottae that were recognized by Patton and Yang (1977). Pocket gophers of the conifer forest zones of the White and Sacramento mountains of south central New Mexico, described as T. b . ruidosae by Hall (1932), are characterized by moderate size, dark coloration, and a karyotype composed of nearly all biarmed autosomes (2n = 76). This form meets T. b. actuosus Kelson Revised December 16, 1978

Diversification in the genus Akodon (Rodentia : Sigmodontinae) in southeastern south America : Mitochondrial dna sequence analysis

Abstract Phylogenetic relationships of 6 species of the genus Akodon have been determined by mtDNA sequence analysis of the cytochrome-b gene, with the majority of specimens also identified by karyotype. We used maximum parsimony and distance analyses of 54 sequences to assess phylogenetic relationships and minimum spanning trees to compare genetic and geographic relationships among populations. The genus Bolomys was used as the outgroup. Akodon cursor, A. aff. cursor, and A. montensis, 3 morphologically cryptic but karyotypically different species of the cursor complex, each formed reciprocally monophyletic lineages. Twenty-one haplotypes were present in 23 individuals of A. cursor, and each of the 16 specimens of A. montensis had a unique haplotype. Within species, populations of A. cursor (19 localities) showed high variation with little geographic pattern. Distant populations (∼750 km apart, differing by 12 nucleotides) exhibited similar levels of differentiation as did geographically close populations (∼150 km apart, differing by 17 nucleotides). In contrast, A. montensis (12 localities) was structured geographically and exhibited a pattern consistent with an isolation-by-distance model of differentiation. Among other species in southeastern Brazil, A. mystax was related most closely to the cursor species complex, A. lindberghi was somewhat more distant, and A. serrensis was quite distinct.

Macrogeographic Patterns of Genetic Differentiation in the Pocket Gopher Thomomys Umbrinus

Electromorphic and chromosomal variation is analyzed in 26 populations of Tho- momys umbrinus sampled from throughout the range of the species. Interpopulation levels of genic differentiation are extreme, generally exceeding values measured between conspecific populations of most animals or plants. Two principal groups of T. umbrinus are recognized based on chromosomal evidence, one with 2n = 76 chromosomes and the other with 2n = 78. Further, the 2n = 78 group (but not the 2n = 76 group) is bisected into geographic subgroups with respect to chromosome morphology and heterochromatin position. The kind and degree of chromosomal differentiation observed suggests that the three groups may be reproductively incompatible. Allozymic evidence corroborates the above groupings, and an analysis of patterns of allele sharing suggests the absence of gene flow among the groups. A cladistic analysis of electro- morphic data indicates that the two 2n = 78 groups may be independently derived from the 2n = 76 lineage. The combined evidence supports the hypothesis that T. umbrinus is actually a composite of at least three biological species and confirms the observation that speciation in the genus Thomomys is unrelated to the level of genic differentiation between populations. (Evolu- tionary genetics; geographic variation; evolutionary concordance; pocket gophers; cladistic anal- ysis; paraphyly; speciation.)

Geographic Variation in Genic and Morphological Characters in Peromyscus californicus

Allelic variation was examined at 31 structural gene loci in 13 populations of Peromyscus californicus . Northern populations of P. californicus differ sharply from southern populations in allelic frequency at four of 17 polymorphic loci. Heterozygosity levels are high in the zone of contact between the northern and southern units, and also high in southern populations. A multivariate analysis of variation in morphological characters in 17 populations distinguishes two groups of populations similar to the grouping based on variation at electrophoretic loci. Northern populations are assigned to the subspecies Peromyscus californicus californicus ; southern populations are assigned to the subspecies Peromyscus californicus insignis . The role of changes in the topography of coastal California and changing vegetation and climatic patterns in contributing to differentiation within this species is considered.

Relationships of Pocket Gopher (Thomomys bottae) Populations of the Lower Colorado River

Two major genically differentiated geographic segments of Thomomys bottae meet along the lower Colorado River in California and Arizona. Maximum divergence recorded between adjacent populations is as high as 35% of the genome as measured by electrophoretic techniques. There is no detectable gene flow between north-south arranged subspecies on either bank of the river due to physiographic barriers. Indirect gene flow through adjoining populations peripheral to the river in the Salton Basin of California and along the Gila River of Arizona is apparent. Populations on opposite banks are remarkably similar genically, suggesting recency of common origin or occasional movement of individuals across the river. The genic pattern of population relatedness is distinct from that indicated by morphology. The genic data are interpreted as more likely reflecting the history of the populations in question whereas the morphology responds more directly to environmentally mediated selection regimes.

Molecular evidence for mating asymmetry and female choice in a pocket gopher (Thomomys) hybrid zone

This paper presents circumstantial evidence that the mating system of the North American pocket gophers (Rodentia: Geomyidae) is a promiscuous one, with female choice at its base. A molecular marker (a length variant in the mitochondrial Control region [D‐loop]) is used to show mating asymmetry in a hybrid zone between the species Thomomys bottae and Thomomys townsendii in north‐eastern California. All hybrids result from a bottae mother ×townsendii father cross. Because of significant differences in body size and resulting burrow diameter, bottae females must have actively sought their respective townsendii mates for the asymmetry in mating to have occurred, signalling female choice in these subterranean mammals that are otherwise characterized by exclusive‐use territories, skewed adult sex ratio in favour of females, and high variance in male reproductive success.