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L R Maxson

Publications and source records attributed to L R Maxson.

18 recordsLinked to original sources
PubMed1996

Phylogenetic relationships among bufonoid frogs (Anura:Neobatrachia) inferred from mitochondrial DNA sequences.

Nucleotide sequences of portions of the mitochondrial 12S and 16S ribosomal RNA genes were used to extend a recent study of anuran phylogeny (Hay et al., Mol. Biol. Evol. 12: 928-937, 1995) and to further evaluate phylogenetic relationships within the Neobatrachia. An analysis of almost 900 nucleotides from each of 8 new representatives of the Dendrobatidae, Hylidae, Leptodactyolidae, and Myobatrachidae, plus 14 available members of the Neobatrachia provides support for 2 major lineages (Bufonoidea and Ranoidea) within this anuran suborder. The neotropical Bufonoidea and their derivatives are monophyletic. There is an interesting association of the 2 Australian myobatrachids with the South African Heleophrynidae, and the Sooglossidae is one of the basal bufonoid lineages. Within the New World bufonoid frogs, a monophyletic Dendrobatidae is strongly supported. An Australian hylid (Pelodryadinae) shows close affinity with the South American hylid Phyllomedusinae. A group composed of Hylinae (Hyla and Smilisca), Centrolenidae, Bufonidae, and the hylid Hemiphractinae, with the latter two clustered, was supported significantly. The addition of new taxa has more clearly defined some relationships within the suborder Neobatrachia and has indicated that the families Hylidae, Leptodactylidae, and Myobatrachidae may not be monophyletic.

Animals↗
PubMed1995

Higher-level snake phylogeny inferred from mitochondrial DNA sequences of 12S rRNA and 16S rRNA genes.

Portions of two mitochondrial genes (12S and 16S ribosomal RNA) were sequenced to determine the phylogenetic relationships among the major clades of snakes. Thirty-six species, representing nearly all extant families, were examined and compared with sequences of a tuatara and three families of lizards. Snakes were found to constitute a monophyletic group (confidence probability [CP] = 96%), with the scolecophidians (blind snakes) as the most basal lineages (CP = 99%). This finding supports the hypothesis that snakes underwent a subterranean period early in their evolution. Caenophidians (advanced snakes), excluding Acrochordus, were found to be monophyletic (CP = 99%). Among the caenophidians, viperids were monophyletic (CP = 98%) and formed the sister group to the elapids plus colubrids (CP = 94%). Within the viperids, two monophyletic groups were identified: true vipers (CP = 98%) and pit vipers plus Azemiops (CP = 99%). The elapids plus Atractaspis formed a monophyletic clade (CP = 99%). Within the paraphyletic Colubridae, the largely Holarctic Colubrinae was found to be a monophyletic assemblage (CP = 98%), and the Xenodontinae was found to be polyphyletic (CP = 91%). Monophyly of the henophidians (primitive snakes) was neither supported nor rejected because of the weak resolution of relationships among those taxa, except for the clustering of Calabaria with a uropeltid, Rhinophis (CP = 94%).

Animals↗
PubMed1995

Phylogenetic relationships of amphibian families inferred from DNA sequences of mitochondrial 12S and 16S ribosomal RNA genes.

Nucleotide sequence comparisons were used to investigate ordinal and familial relationships within the class Amphibia. Approximately 850 base pairs of the mitochondrial 16S ribosomal RNA (rRNA) gene from representatives of 28 of the 40 families of extant amphibians were sequenced. Phylogenetic analyses of these data together with published data of the 12S rRNA gene for the same families and both genes for three more taxa (approximately 1,300 base pairs total for 35 taxa) support the monophyly of each of the three amphibian orders: Anura (confidence value with the interior-branch test: P(c) = 99%), Caudata (P(c) = 100%), and Gymnophiona (P(c) = 99%). An analysis using the four-cluster method cannot discriminate significantly between all three possible unrooted trees involving the three orders of amphibians and an outgroup. Within the Anura, there is support for the monophyly of the two suborders: Neobatrachia (P(c) = 100%) and Archaeobatrachia (P(c) = 97%); the latter was believed to be paraphyletic on the basis of morphology. Within the Archaeobatrachia, the following pairs of taxa cluster: Pelobatidae + Pelodytidae (P(c) = 99%), Pipidae + Rhinophrynidae (P(c) = 99%), Ascaphus + Leiopelmatidae (P(c) = 89%), and Bombina + Discoglossidae (P(c) = 99%). The latter six taxa cluster (P(c) = 94%) such that Pelobatidae + Pelodytidae forms a basal lineage within the Archaeobatrachia. Three major lineages are distinguished within the Neobatrachia: the superfamily Bufonoidea sensu Duellman (P(c) = 86%), the superfamily Ranoidea sensu Lynch (P(c) = 99%), and the Sooglossidae. Basal within the Bufonoidea, Myobatrachidae + Heleophrynidae cluster at P(c) = 96%. The enigmatic Dendrobatidae clusters with the bufonoid families (P(c) = 92%) and is excluded from the ranoid families (P(c) = 99%). (ABSTRACT TRUNCATED AT 250 WORDS)

Amphibians↗
PubMed1992

Ancestry of unisexual salamanders.

In eastern North America there are populations of all-female salamanders that incorporate the nuclear genomes of two or three of four sympatric bisexual species. The hybrids can be diploid, triploid, tetraploid or pentaploid, and 18 different combinations have been reported. All hybrids require sperm from a sympatric male of one of the bisexual species to reproduce, but the sperm may or may not be incorporated in the egg. Some of the hybrids are believed to represent separate, clonal species, but little is known of the origin of this hybrid complex. Vertebrate mitochondrial DNA is inherited maternally, allowing identification of the female parent that gave rise to hybrid lineages. A portion of the cytochrome b gene was sequenced from diploid and triploid hybrids that represent combinations of all four species. Nearly all hybrids had a similar mitochondrial genome sequence, independent of nuclear genome composition and ploidy, and the sequence was distinct from that of any of the four bisexual species. The hybrids maintain a mitochondrial lineage that has evolved independently of their nuclear genome and represent the most ancient known unisexual vertebrate lineage.

Ambystoma↗
PubMed1992

Crocodilian evolution: insights from immunological data.

The quantitative immunological technique of microcomplement fixation was used to examine serum albumin evolution among members of the order Crocodylia. The cross-reactivity of the albumin antisera and antigens employed in this study had been examined previously using the qualitative technique of immunodiffusion. The phylogenetic conclusions derived from these two data sets are highly congruent, including support of the families Alligatoridae and Crocodylidae, with the placement of Gavialis as the sister taxon of Tomistoma. Both methods provide similar information on the relative amounts of amino acid sequence divergence between albumin molecules; however, the data obtained from microcomplement fixation comparisons are more discriminating than those derived from immunodiffusion. The estimated divergence times within the Crocodylia derived from the fossil record are examined in light of divergence times predicted by the microcomplement fixation-based albumin clock. The traditional phylogenetic placement of Gavialis outside the remaining extant crocodilians is inconsistent with all molecular data sets and we suggest that a careful reexamination of both the extant and the fossil morphological data is warranted.

Alligators and Crocodiles↗
PubMed1991

Phylogenetic relationships and biogeography of xantusiid lizards, inferred from mitochondrial DNA sequences.

Portions of two mitochondrial genes (12S ribosomal RNA and cytochrome b) were sequenced in seven species to examine phylogenetic relationships within the lizard family Xantusiidae. Phylogenies derived from these sequences (709 total bp) are concordant and indicate that the Cuban species Cricosaura typica is the sister group to all other xantusiids. The Middle American genus Lepidophyma is the closest relative of Xantusia, and X. riversiana (California Islands) the closest relative of X. vigilis (mainland). These findings are not in agreement either with the results of a recent morphological analysis that united Cricosaura and Lepidophyma as closest relatives or with past studies that have recognized X. riversiana as a separate genus. Levels of sequence divergence, as well as the age and affinities of some mainland fossil taxa, suggest that the origin of Cricosaura was associated with the tectonic evolution of the Greater Antilles in the late Cretaceous. These results further demonstrate that significant resolution of phylogenies can be obtained with relatively short DNA sequences and that these mitochondrial genes are concordant in their estimation of phylogeny.

Animals↗
PubMed1990

Tetrapod phylogeny inferred from 18S and 28S ribosomal RNA sequences and a review of the evidence for amniote relationships.

The 18S ribosomal RNAs of 21 tetrapods were sequenced and aligned with five published tetrapod sequences. When the coelacanth was used as an outgroup, Lissamphibia (living amphibians) and Amniota (amniotes) were found to be statistically significant monophyletic groups. Although little resolution was obtained among the lissamphibian taxa, the amniote sequences support a sister-group relationship between birds and mammals. Portions of the 28S ribosomal RNA (rRNA) molecule in 11 tetrapods also were sequenced, although the phylogenetic results were inconclusive. In contrast to previous studies, deletion or down-weighting of base-paired sites were found to have little effect on phylogenetic relationships. Molecular evidence for amniote relationships is reviewed, showing that three genes (beta-hemoglobin, myoglobin, and 18S rRNA) unambiguously support a bird-mammal relationship, compared with one gene (histone H2B) that favors a bird-crocodilian clade. Separate analyses of four other genes (alpha-crystallin A, alpha-hemoglobin, insulin, and 28S rRNA) and a combined analysis of all sequence data are inconclusive, in that different groups are defined in different analyses and none are strongly supported. It is suggested that until sequences become available from a broader array of taxa, the molecular evidence is best evaluated at the level of individual genes, with emphasis placed on those studies with the greatest number of taxa and sites. When this is done, a bird-mammal relationship is most strongly supported. When regarded in combination with the morphological evidence for this association, it must be considered at least as plausible as a bird-crocodilian relationship.

Animals↗
PubMed1986

Micro-complement fixation: a quantitative estimator of protein evolution.

The quantitative immunological technique of micro-complement fixation (MC'F) has been routinely used during the past decade to assess evolutionary relationships among living vertebrate species. The large data base that has been generated, along with the excellent correlations between immunologically measured genetic distances and paleontologically derived estimates of divergence times, have formed the basis for the albumin molecular clock. Immunological distance (ID) involves a logarithmic transformation of experimentally measured antibody concentrations. The justification for this transformation has rested entirely on empirical correlations. Consequently, several other transformations have been proposed as giving better fits to particular data sets. We derive, from first principles, the relationship between ID and the amino acid sequence replacements (AAR) between compared albumins. ID is shown to be a linear estimator of AAR. This ID-AAR relationship is based on a proposed process of antibody assortment and exclusion. We present experimental data confirming that such an antibody assortment-exclusion process occurs in MC'F. This process can explain both the high sensitivity and the quantitative phylogenetic nature of the MC'F assay. The assortment-exclusion process also predicts a divergence limit beyond which MC'F data no longer provide robust phylogenetic data.

Biological Evolution↗
PubMed1984

Molecular probes of phylogeny and biogeography in toads of the widespread genus Bufo.

Genetic relationships among 25 species of Central and South American Bufo and among representative North, Central, and South American, Asian, and African Bufo were probed, using the quantitative immunological technique of microcomplement fixation (MC'F) which indicated a clear separation of North, Central, and South American lineages of Bufo. The South American lineage likely diverged from the Central and North American lineages in the Eocene; the latter two lineages diverged later, probably in the mid-Oligocene. Some species groups of South American toads, defined on the basis of traditional morphological studies, are genetically quite similar within groups, whereas others are genetically divergent. The amount of albumin evolution does not appear to parallel the amount of karyotypic, morphological, ecological, or behavioral evolution documented. Comparisons suggest that the African lineages separated from the American and Asian lineages in the late Cretaceous, corresponding to the time of the final separation of Gondwanaland, the southern supercontinent including the modern continents of South America, Africa, Australia, Antarctica, and India. The Asian lineages diverged from the lineage giving rise to all of the American species in the early Paleocene.

Animals↗
PubMed1974

Convergent morphological evolution detected by studying proteins of tree frogs in the Hyla eximia species group.

Protein studies have uncovered an apparent case of convergent evolution among North American tree frogs. The species Hyla eximia and Hyla regilla are so similar in external morphology that the "wrightorum" subspecies is assigned by some authorities to H. eximia and by others to H. regilla. Yet microcomplement fixation experiments show that "wrightorum" albumin, though virtually indistinguishable from authentic H. eximia albumin, differs as much from H. regilla albumin as from albumins of species outside the genus Hyla, such as Acris crepitans. The morphological resemblance of "wrightorum" to H. regilla is thus probably due to convergence.

Amino Acid Sequence↗
PubMed1974

Two types of molecular evolution. Evidence from studies of interspecific hybridization.

To assess the significance of macromolecular sequence differences among species, we compared the serum albumins of 81 pairs of vertebrate species capable of producing viable hybrids. Micro-complement fixation experiments showed that the average difference between the albumins within such pairs was only 3 immunological distance units for placental mammals (31 pairs), but 36 units for frogs (50 pairs). Albumin immunological distance is strongly correlated with other measures of genetic distance, including those made with DNA annealing techniques. It therefore seems likely that mammalian species pairs capable of hybridization are far more similar at the macromolecular sequence level than is the case for most hybridizable frogs. We think the most likely explanation for the marked molecular restriction on hybridization among mammals is that the ratio of regulatory evolution to protein evolution is higher for mammals than for frogs. Mammals may have experienced unusually rapid regulatory evolution; indeed, this could be the factor responsible for their unusually rapid anatomical evolution.

Amino Acid Sequence↗
PubMed1974

The importance of gene rearrangement in evolution: evidence from studies on rates of chromosomal, protein, and anatomical evolution.

We have compared the relative rates of protein evolution and chromosomal evolution in frogs and mammals. The average rate of change in chromosome number has been about 20 times faster in mammals than in frogs. Whereas it takes only 3.5 million years, on the average, for a pair of mammal species to develop a difference in chromosome number, the corresponding period for frogs is 70 million years. In contrast, the rate of protein evolution in mammals has been roughly equal to that in frogs. The rapid rate of gene rearrangement in mammals parallels both their rapid anatomical evolution and their rapid evolutionary loss of the potential for interspecific hybridization. Thus, gene rearrangements may be more important than point mutations as sources for evolutionary changes in anatomy and way of life.

Adaptation, Biological↗
PubMed1971

Albumin evolution in frogs: a test of the evolutionary clock hypothesis.

Frogs are an ancient group compared to placental mammals. Yet, although there are about as many species of frogs as there are of mammals, zoologists consider that frogs have undergone only limited morphological divergence, while placental mammals have diversified greatly in morphology and way of life. The serum albumins of numerous frog species were compared by the quantitative microcomplement fixation technique. Frogs that are morphologically similar enough to merit taxonomic distinction at only the species level often exhibit differences in the serological properties of their albumins larger than those usually seen between mammals placed in distinct families or suborders. Thus, there seems to be a contrast between albumin evolution and evolution at the organismal level. The large differences between albumins among frogs can be explained by the hypothesis that albumin evolution has proceeded at the same rate in frogs as in mammals.

Acrylamides↗