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L E Eguiarte

Publications and source records attributed to L E Eguiarte.

10 recordsLinked to original sources
PubMed2003

Genetic differentiation in the Agave deserti (Agavaceae) complex of the Sonoran desert.

The Agave deserti complex, comprising A. deserti, A. cerulata and A. subsimplex, represents a group of species and subspecies with a near allopatric distribution and clear differences in morphology. Genetic differentiation and taxonomic status with respect to spatial distribution of 14 populations of the complex were analyzed in an effort to understand the evolution and speciation process within the genus. Allelic frequencies, levels of genetic variation, expected heterozygosity (H(S)), proportion of polymorphic loci (P), and genetic differentiation (theta and Nei's genetic distance) were estimated using 41 putative RAPD loci. All three species show high levels of genetic variation (H(S)=0.12-0.29, P=63.4-95.1), and low genetic differentiation between populations and species (theta populations=0.14+/-0.02 (SE); G(st)=0.11+/-0.02). Accordingly, gene flow among populations was estimated as high by three different methods (N(m)=2.91-6.14). Nei's genetic distances between the three species were low compared to the values obtained from other Agavaceae, and there was no clear correlation with taxonomic divisions. In a UPGMA analysis, A. subsimplex and A. cerulata formed exclusive monospecific clusters, whereas the A. deserti populations appear in more than one cluster together with other species. The results were consistent with a pattern of genetic isolation by distance.

Agave↗
PubMed2002

Does experimental evolution reflect patterns in natural populations? E. coli strains from long-term studies compared with wild isolates.

Our results show that experimental evolution mimics evolution in nature. In particular, only 1,000 generations of periodic recombination with immigrant genotypes is enough for linkage disequilibrium values in experimental populations to change from a maximum linkage value to a value similar to the one observed in wild strains of E. coli. Our analysis suggests an analogy between the recombination experiment and the evolutionary history of E. coli; the E. coli genome is a patchwork of genes laterally inserted in a common backbone, and the experimental E. coli chromosome is a patchwork where some sites are highly prone to recombination and others are very clonal. In addition, we propose a population model for wild E. coli where gene flow (recombination and migration) are an important source of genetic variation, and where certain hosts act as selective sieves; i.e., the host digestive system allows only certain strains to adhere and prosper as resident strains generating a particular microbiota in each host. Therefore we suggest that the strains from a wide range of wild hosts from different regions of the world may present an ecotypic structure where adaptation to the host may play an important role in the population structure.

Animals↗
PubMed2001

The interaction of protein structure, selection, and recombination on the evolution of the type-1 fimbrial major subunit (fimA) from Escherichia coli.

Fimbrial adhesins allow bacteria to interact with and attach to their environment. The bacteria possibly benefit from these interactions, but all external structures including adhesins also allow bacteria to be identified by other organisms. Thus adhesion molecules might be under multiple forms of selection including selection to constrain functional interactions or evolve novel epitopes to avoid recognition. We address these issues by studying genetic diversity in the Escherichia coli type-1 fimbrial major subunit, fimA. Overall, sequence diversity in fimA is high (pi = 0.07) relative to that in other E. coli genes. High diversity is a function of positive diversifying selection, as detected by d(N)/d(S) ratios higher than 1.0, and amino acid residuces subject to diversifying selection are nonrandomly clustered on the exterior surface of the peptide. In addition, McDonald and Kreitman tests suggest that there has been historical but not current directional selection at fimA between E. coli and Salmonella. Finally, some regions of the fimA peptide appear to be under strong structural constraint within E. coli, particularly the interior regions of the molecule that is involved in subunit to subunit interaction. Recombination also plays a major role contributing to E. coli fimA allelic variation and estimates of recombination (2N(e)c) and mutation (2N(e)mu) are about the same. Recombination may act to separate the diverse evolutionary forces in different regions of the fimA peptide.

Adhesins, Escherichia coli↗
PubMed2001

The elements of the locus of enterocyte effacement in human and wild mammal isolates of Escherichia coli: evolution by assemblage or disruption?

Escherichia coli is an excellent model for studying the evolution of pathogenicity since within one species various genes can be found in pathogenic islands and plasmids causing a wide spectrum of virulence. A collection of 122 strains from different human and wild mammal hosts were analysed by PCR and Southern hybridization for the presence of a subset of the genes included in the LEE (locus of enterocyte effacement). In the PCR analysis, two markers (cesT/eae and espB genes) were found together in more strains (25.4%) than either were found alone. The cesT/eae gene was less frequently found alone (8.2%) than was the espB gene (15.6%). Four regions of the LEE were analysed in a subsample of 25 strains using Southern hybridization. The four regions were all present (44%), all absent (12%) or present in different combinations (44%) in a given strain. The flanking regions of the LEE showed the highest rate of hybridization (in 72% of the strains). The results indicate that the LEE is a dynamic genetic entity, both the complete gene cluster and the individual genes. The genes that comprise this locus seem to be horizontally acquired (or lost) in an independent way and may control other functions in non-pathogenic E. coli lineages. In this way, horizontal transfer may allow the gradual stepwise construction of gene cassettes facilitating coordinate regulation and expression of novel functions.

Adhesins, Bacterial↗
PubMed2000

Low levels of genetic variation within and high levels of genetic differentiation among populations of species of Abies from southern Mexico and Guatemala.

Populations of Abies in southern Mexico and Guatemala (A. flinckii, A. guatemalensis, A. hickeli, and A. religiosa) have a patchy distribution. This pattern is particularly clear in A. guatemalensis. Genetic diversity within populations, measured by average heterozygosity at 16 isozyme loci, is lower than the range reported for most conifers (mean H(o) ranging from 0.069 in A. guatemalensis to 0.113 in A. flinckii), while differentiation among populations is higher than that observed in most conifer species studied (θ = F(st) ranging from 0.073 in A. hickeli to 0.271 in A. flinckii). Estimated levels of gene flow are low (ranging from 0.672 in A. flinckii to 3.17 in A. hickeli). Populations in most cases had an excess of homozygosity over that expected under Hardy-Weinberg equilibrium, suggesting some inbreeding (F(is) ranging from 0.074 in A. flinckii to 0.235 in A. guatemalensis). A significant relationship between gene flow and geographic distance was observed in A. religiosa, but not in the other three taxa studied. The patterns of genetic variation appear to have been influenced by the distributions and histories of these species. Paleoclimatic evidence suggests that the ranges of these species retreated upwards during the Pleistocene glaciation and became fragmented during the warming period that followed. The populations could have passed through genetic bottlenecks that reduced genetic variation and led to interpopulation differentiation.

Journal Article↗
PubMed1999

Genetic structure of natural populations of Escherichia coli in wild hosts on different continents.

Current knowledge of genotypic and phenotypic diversity in the species Escherichia coli is based almost entirely on strains recovered from humans or zoo animals. In this study, we analyzed a collection of 202 strains obtained from 81 mammalian species representing 39 families and 14 orders in Australia and the Americas, as well as several reference strains; we also included a strain from a reptile and 10 from different families of birds collected in Mexico. The strains were characterized genotypically by multilocus enzyme electrophoresis (MLEE) and phenotypically by patterns of sugar utilization, antibiotic resistance, and plasmid profile. MLEE analysis yielded an estimated genetic diversity (H) of 0.682 for 11 loci. The observed genetic diversity in this sample is the greatest yet reported for E. coli. However, this genetic diversity is not randomly distributed; geographic effects and host taxonomic group accounted for most of the genetic differentiation. The genetic relationship among the strains showed that they are more associated by origin and host order than is expected by chance. In a dendrogram, the ancestral cluster includes primarily strains from Australia and ECOR strains from groups B and C. The most differentiated E. coli in our analysis are strains from Mexican carnivores and strains from humans, including those in the ECOR group A. The kinds and numbers of sugars utilized by the strains varied by host taxonomic group and country of origin. Strains isolated from bats were found to exploit the greatest range of sugars, while those from primates utilized the fewest. Toxins are more frequent in strains from rodents from both continents than in any other taxonomic group. Strains from Mexican wild mammals were, on average, as resistant to antibiotics as strains from humans in cities. On average, the Australian strains presented a lower antibiotic resistance than the Mexican strains. However, strains recovered from hosts in cities carried significantly more plasmids than did strains isolated from wild mammals. Previous studies have shown that natural populations of E. coli harbor an extensive genetic diversity that is organized in a limited number of clones. However, knowledge of this worldwide bacterium has been limited. Here, we suggest that the strains from a wide range of wild hosts from different regions of the world are organized in an ecotypic structure where adaptation to the host plays an important role in the population structure.

Americas↗
PubMed1999

Why be a honeyless honey mesquite? Reproduction and mating system of nectarful and nectarless individuals.

Populations of Prosopis glandulosa var. torreyana in the Chihuahuan desert have a fixed dimorphic system of nectar production in which half the individuals produce nectar (are nectarful) and the other half are nectarless. We analyzed the impact of nectar production on different estimates of fitness, comparing nectarful against nectarless individuals in size, mating system, seed traits, and fruit set in a 1-ha scrubland. Of the reproductive individuals (358), 46% were nectarful and 54% were nectarless. Neither tree size nor flowering phenology differed between nectar morphs. Fixation indices (F) for both progeny (F = -0.2) and adults (F = -0.45) were negative, and high heterozygosities were found in adults and progeny (H = 0.45). No differences were found between nectar morphs for F, H, and single (t(s) = 1.1) and multilocus (t(m) = 1.03) outcrossing rates. Controlled pollinations showed differences between selfing and control treatments with no differences between nectar morphs. Nectarless individuals produced significantly more pollen grains than did nectar producers, but all other measured floral traits showed no differences. Nectarful trees were visited by pollinators 21 times more often and had a significantly higher overall fruit set than did nectarless trees. No differences between nectar morphs in seed mass or in percentage seed germination were found, but heavier seeds tended to have higher heterozygosities. Both morphs had similar success as females, but nectarless trees had ∼7% higher male function. We discuss three possible scenarios for the evolution of the fixed dimorphism in nectar production, two involving unstable phases (substitution of one morph by the other, and evolution towards dioecy) and one stable scenario (maintenance of the dimorphic system).

Journal Article↗
PubMed1999

Genetic diversity of the endangered endemic Agave victoriae-reginae (Agavaceae) in the Chihuahuan Desert.

Long-lived perennials are a species-rich, ecologically important component of the North American deserts, yet we know little about their genetic structure, information important for their conservation. Agave victoriae-reginae is an endemic of the Chihuahuan Desert of northern Mexico that is endangered by collection for the ornamental trade. We examined levels and patterns of variation at ten polymorphic allozyme loci in ten populations representing the range of the species. Levels of genetic variation (mean H(e)= 0.335) and differentiation (mean F(ST) = 0.236) were high. Phenetic clustering suggested the existence of at least three distinct groups of populations. If this pattern of variation is representative of other long-lived desert perennials, it may explain the species richness of this group and will pose a real challenge to gene conservation efforts.

Journal Article↗
PubMed1993

Phylogenetic analysis of rbcL sequences identifies Acorus calamus as the primal extant monocotyledon.

The identity of the oldest lineage of monocotyledons is a subject of debate. Alternative interpretations of morphological homologies are variously consistent with proposals that species of Alismatanae, Dioscoreales, or Melanthiales were the earliest descendants of the first monocotyledons. We present phylogenetic analyses based on DNA sequences of the plastid locus rbcL in which Acorus calamus, an herb with unspecialized floral features and of uncertain affinities, is supported as a member of the oldest extant lineage of monocotyledons. This conclusion is consistent with a substantial body of morphological, anatomical, and embryological evidence and offers an explanation for the failure to identify any close relationship between Acorus and other genera.

Chloroplasts↗