Biomedical subjects
Publications and source records attributed to E A Kellogg.
The MYB-class proteins WEREWOLF and GLABRA1 are functionally interchangeable, even though one is normally expressed solely in roots and the other only in shoots. This shows that their different functions are the result of the modification of cis-regulatory sequences over evolutionary time. The two genes thus provide an example of morphological diversification created by gene duplication and changes in regulation.
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We sampled the 5' end of the granule-bound starch synthase gene (GBSSI or waxy) in Rosaceae, sequencing 108 clones from 18 species in 14 genera representing all four subfamilies (Amygdaloideae, Maloideae, Rosoideae, and Spiraeoideae), as well as four clones from Rhamnus catharticus (Rhamnaceae). This is the first phylogenetic study to use the 5' portion of this nuclear gene. Parsimony and maximum-likelihood analyses of 941 bases from seven complete and two partial exons demonstrate the presence of two loci (GBSSI-1 and GBSSI-2) in the Rosaceae. Southern hybridization analyses with locus-specific probes confirm that all four Rosaceae subfamilies have at least two GBSSI loci, even though only one locus has been reported in all previously studied diploid flowering plants. Phylogenetic analyses also identify four clades representing four loci in the Maloideae. Phylogenetic relationships inferred from GBSSI sequences are largely compatible with those from chloroplast (cpDNA: ndhF, rbcL) and nuclear ribosomal internal transcribed spacer (nrITS) DNA. Large clades are marked by significant intron variation: a long first intron plus no sixth intron in Maloideae GBSSI-1, a long fourth intron in Rosoideae GBSSI-1, and a GT to GC mutation in the 5' splice site of the fourth intron in all GBSSI-2 sequences. Our data do not support the long-held hypothesis that Maloideae originated from an ancient hybridization between amygdaloid and spiraeoid ancestors. Instead, Spiraeoideae genera (Kageneckia and Vauquelinia) are their closest relatives in all four GBSSI clades.
Phylogenetic analyses of partial phytochrome B (PHYB) nuclear DNA sequences provide unambiguous resolution of evolutionary relationships within Poaceae. Analysis of PHYB nucleotides from 51 taxa representing seven traditionally recognized subfamilies clearly distinguishes three early-diverging herbaceous "bambusoid" lineages. First and most basal are Anomochloa and Streptochaeta, second is Pharus, and third is Puelia. The remaining grasses occur in two principal, highly supported clades. The first comprises bambusoid, oryzoid, and pooid genera (the BOP clade); the second comprises panicoid, arundinoid, chloridoid, and centothecoid genera (the PACC clade). The PHYB phylogeny is the first nuclear gene tree to address comprehensively phylogenetic relationships among grasses. It corroborates several inferences made from chloroplast gene trees, including the PACC clade, and the basal position of the herbaceous bamboos Anomochloa, Streptochaeta, and Pharus. However, the clear resolution of the sister group relationship among bambusoids, oryzoids, and pooids in the PHYB tree is novel; the relationship is only weakly supported in ndhF trees and is nonexistent in rbcL and plastid restriction site trees. Nuclear PHYB data support Anomochlooideae, Pharoideae, Pooideae sensu lato, Oryzoideae, Panicoideae, and Chloridoideae, and concur in the polyphyly of both Arundinoideae and Bambusoideae.
Knotted-like (KNOX) proteins constitute a group of homeodomain proteins involved in pattern formation in developing tissues of angiosperms and other green plants. We conducted phylogenetic analyses of nucleotide and amino acid sequences of all known KNOX proteins in order to examine their evolution. Our analyses reveal two groups of KNOX proteins, classes I and II. Dicot and monocot sequences occur in both classes, indicating that the protein classes arose prior to the origin of the monocots. A conifer (Picea) sequence is nested within class I, suggesting that there are likely to be other copies of KNOX genes in this and other conifers. The orthology of several grass genes (including Zea Kn1, ZMKN1) is strongly supported by phylogenetic and synteny analyses. However, no compelling evidence supports the hypothesis of orthology previously proposed for several dicot genes and ZMKN1. Analysis of expression patterns suggests that the ancestral KNOX gene was expressed in all plant parts and that the propensity to be downregulated in roots and leaves evolved in the class I genes.
We investigated spikelet development in four distantly related species of the grass tribe Andropogoneae to determine whether spikelet development and the formation of unisexual florets are uniform throughout the tribe. We studied development in Bothriochloa bladhii, Coelorachis aurita, Heteropogon contortus, and Hyparrhenia hirta, and compared these with Panicum, a member of the sister tribe Paniceae. Many aspects of spikelet development in the species we have studied correlate with what is already known for Tripsacum and maize (both Andropogoneae), despite variation in how unisexual florets are distributed on the plant. The formation of unisexual spikelets is also uniform. All florets initiate both pistil and stamen primordia. In florets destined to be male, cell death occurs in the subepidermal layers of the gynoecium after the formation of a gynoecial ridge. In florets destined to be female, there is no apparent cell death in the stamens, but growth ceases after anther formation. The similarity in spikelet development and the formation of unisexual florets point to a common genetic mechanism for sex determination throughout the Andropogoneae and possibly the entire Panicoideae. Use of a cell death pathway to cause gynoecial abortion may be the basis of one morphological character that defines the subfamily.
The grass family includes some 10,000 species, and it encompasses tremendous morphological, physiological, ecological, and genetic diversity. The phylogeny of the family is becoming increasingly well understood. There were two major radiations of grasses, an early diversification leading to the subfamilies Pooideae, Bambusoideae, and Oryzoideae, and a later one leading to Panicoideae, Chloridoideae, Centothecoideae, and Arundinoideae. The phylogeny can be used to determine the direction of changes in genome arrangement and genome size.
Similarities among model systems can lead to generalizations about plants, but understanding the differences requires systematic data. Molecular phylogenetic analyses produce results similar to traditional classifications in the grasses (Poaceae), and relationships among the cereal crops are quite clear. Chloroplast-based phylogenies for the Solanaceae show that tomato is best considered as a species of Solanum, closely related to potatoes. Traditional classifications in the Brassicaceae are misleading with regard to true phylogenetic relationships and data are only now beginning to clarify the situation. Molecular data are also being used to revise our view of relationships among flowering plant families. Phylogenetic data are critical for interpreting hypotheses of the evolution of development.
Interest in the use of low-copy nuclear genes for phylogenetic analyses of plants has grown rapidly, because highly repetitive genes such as those commonly used are limited in number. Furthermore, because low-copy genes are subject to different evolutionary processes than are plastid genes or highly repetitive nuclear markers, they provide a valuable source of independent phylogenetic evidence. The gene for granule-bound starch synthase (GBSSI or waxy) exists in a single copy in nearly all plants examined so far. Our study of GBSSI had three parts: (1) Amino acid sequences were compared across a broad taxonomic range, including grasses, four dicotyledons, and the microbial homologs of GBSSI. Inferred structural information was used to aid in the alignment of these very divergent sequences. The informed alignments highlight amino acids that are conserved across all sequences, and demonstrate that structural motifs can be highly conserved in spite of marked divergence in amino acid sequence. (2) Maximum-likelihood (ML) analyses were used to examine exon sequence evolution throughout grasses. Differences in probabilities among substitution types and marked among-site rate variation contributed to the observed pattern of variation. Of the parameters examined in our set of likelihood models, the inclusion of among-site rate variation following a gamma distribution caused the greatest improvement in likelihood score. (3) We performed cladistic parsimony analyses of GBSSI sequences throughout grasses, within tribes, and within genera to examine the phylogenetic utility of the gene. Introns provide useful information among very closely related species, but quickly become difficult to align among more divergent taxa. Exons are variable enough to provide extensive resolution within the family, but with low bootstrap support. The combined results of amino acid sequence comparisons, maximum-likelihood analyses, and phylogenetic studies underscore factors that might affect phylogenetic reconstruction. In this case, accommodation of the variable rate of evolution among sites might be the first step in maximizing the phylogenetic utility of GBSSI.
Homeodomain proteins are transcription factors that play a critical role in early development in eukaryotes. These proteins previously have been classified into numerous subgroups whose phylogenetic relationships are unclear. Our phylogenetic analysis of representative eukaryotic sequences suggests that there are two major groups of homeodomain proteins, each containing sequences from angiosperms, metazoa, and fungi. This result, based on parsimony and neighbor-joining analyses of primary amino acid sequences, was supported by two additional features of the proteins. The two protein groups are distinguished by an insertion/deletion in the homeodomain, between helices I and II. In addition, an amphipathic alpha-helical secondary structure in the region N terminal of the homeodomain is shared by angiosperm and metazoan sequences in one group. These results support the hypothesis that there was at least one duplication of homeobox genes before the origin of angiosperms, fungi, and metazoa. This duplication, in turn, suggests that these proteins had diverse functions early in the evolution of eukaryotes. The shared secondary structure in angiosperm and metazoan sequences points to an ancient conserved functional domain.
The gene teosinte branched 1 controls major differences in architecture between cultivated maize and its wild ancestor. The differences correlate with different amounts of gene product.
Oxygen is an important environmental regulator for the transcription of several genes in Saccharomyces cerevisiae, but it is not yet clear how this yeast or other eukaryotes actually sense oxygen. To begin to address this we have examined the effects of oxygen concentration on the expression of several nuclear genes (CYC1, CYC7, COX4, COX5a, COX5b, COX6, COX7, COX8, and COX9) for proteins of the terminal portion of the respiratory chain. COX5b and CYC7 are hypoxic genes; the rest are aerobic genes. We have found that the level of expression of these genes is determined by oxygen concentration per se and not merely the presence or absence of oxygen and that each of these genes has a low oxygen threshold (0. 5-1 microM O2) for expression. For some aerobic genes (COX4, COX5a, COX7, COX8, and COX9) there is a gradual decline in expression between 200 microM O2 (air) and their oxygen threshold. Below this threshold expression drops precipitously. For others (COX5a and CYC1) the level of expression is nearly constant between 200 microM O2 and their threshold and then drops off. The hypoxic genes COX5b and CYC7 are not expressed until the oxygen concentration is below 0.5 microM O2. These studies have also revealed that COX5a and CYC1, the genes for the aerobic isoforms of cytochrome c oxidase subunit V and cytochrome c, and COX5b and CYC7, the genes for the hypoxic isoforms of cytochrome c oxidase subunit V and cytochrome c, are coexpressed at a variety of oxygen concentrations and switch on or off at extremely low oxygen concentrations. By shifting cells from one oxygen concentration to another we have found that aerobic genes are induced faster than hypoxic genes and that transcripts from both types of gene are turned over quickly. These findings have important implications for cytochrome c oxidase function and biogenesis and for models of oxygen sensing in yeast.
Forty-eight representatives of 12 tribes attributed to the subfamily Pooideae s.l. of grasses (Monocots) have been studied by sequencing the more variable 3' end of the chloroplast ndhF gene. Six representatives from 5 different tribes of Poaceae (Oryzeae, Streptogyneae, Bambuseae, Arundineae, Phareae) and from Joinvilleaceae were used as outgroups. Phylogenetic analyses of the data render a monophyletic Pooideae clade and provide an evolutionary hypothesis for all its tribes (Poeae, Aveneae, Bromeae, Triticeae, Brachypodieae, Meliceae, Stipeae, Lygeae, Nardeae, Diarrheneae, Brachyelytreae, Phaenospermatae). The subfamily is composed of two subsets of tribes, a basal group that includes six old lineages or satellite tribes, in which Brachyelytreae is the first diverging branch followed by the Lygeae/Nardeae clade, and a group of six more recently evolved tribes (Pooideae s.s.), in which the divergence of Diarrheneae antedates that of Brachypodieae, sister group to the "core pooids." The character changes found within the sequenced nucleotide positions of the ndhF gene also have proven to be informative at lower hierarchical levels (genus and species) for some tribes.
5S RNAs form part of the ribosome in most organisms. In some, e.g., prokaryotes and some fungi, the genes are part of the ribosomal operon, but in most eukaryotes they are in tandem arrays of hundreds to thousands of copies separate from the main ribosomal array. 5S RNA genes can be aligned across kingdoms. We were therefore surprised to find that, for 28 diploid species of the wheat tribe (Triticeae), nucleotide diversity within an array is up to 6.2% in the genes, not significantly different from that of the nontranscribed spacers. Rates of concerted evolution must therefore be insufficient to homogenize the entire array. Between species, there are significantly fewer fixed differences in the gene than would be expected, given the high within-species variation. In contrast, the amount of variation between species in the spacer is the same as or greater than that within individuals. This leads to a paradox. High variation within an individual suggests that there is little selection on any particular gene within an array. But conservation of the gene across species implies that polymorphisms are periodically eliminated at a rate approximately equal to or greater than that of speciation. Levels of intraspecific polymorphism and interspecific divergence are thus decoupled. This implies that selective mechanisms exist to eliminate mutations in the gene without also affecting the spacer.
We designate a region of the alcohol dehydrogenase locus (Adh) of the weedy crucifer, Arabidopsis thaliana, as "hypervariable" on the basis of a comparison of sequences from ecotypes Columbia and Landsberg. We found eight synonymous and two replacement mutations in the first 262 nucleotides of exon 4, and an additional two mutations in the contiguous region of intron 3. The rest of the sequence (2611 bp) has just three mutations, all of them confined to noncoding regions. Our survey of the hypervariable region among 37 ecotypes of A. thaliana revealed two predominant haplotypes, corresponding to the Columbia and Landsberg sequences. We identified five additional haplotypes and 4 additional segregating sites. The lack of haplotype diversity is presumably in part a function of low rates of recombination between haplotypes conferred by A. thaliana's tendency to self-fertilize. However, an analysis in 32 ecotypes of 12 genome-wide polymorphic markers distinguishing Columbia and Landsberg ecotypes indicated levels of outcrossing sufficient at least to erode linkage disequilibrium between dispersed markers. We discuss possible evolutionary explanations for the coupled observation of marked divergence within the hypervariable region and a lack of haplotype diversity among ecotypes. The sequence of the region for closely related species argues against the possibility that one allele is the product of introgression. We note (1) that several loss of function mutations (both naturally and chemically induced) map to the hypervariable region, and (2) the presence of two amino acid replacement polymorphisms, one of which causes the mobility difference between the two major classes of A. thaliana Adh electrophoretic alleles. We argue that protein polymorphism in such a functionally significant part of the molecule may be subject to balancing selection. The observed pattern of extensive divergence between the alleles is consistent with this explanation because balancing selection on a particular site maintains linked neutral polymorphisms at intermediate frequencies.
An exception to the generally conservative nature of plastid gene evolution is the gene coding for the beta" subunit of RNA polymerase, rpoC2. Previous work by others has shown that maize and rice have an insertion in the coding region of rpoC2, relative to spinach and tobacco. To assess the distribution of this extra coding sequence, we surveyed a broad phylogenetic sample comprising 55 species from 17 angiosperm families by using Southern hybridization. The extra coding sequence is restricted to the grasses (Poaceae). DNA sequence analysis of 11 species from all five subfamilies within the grass family demonstrates that the extra sequence in the coding region of rpoC2 is a repetitive array that exhibits more than a twofold increase in nucleotide substitution, as well as a large number of insertion/deletion events, relative to the adjacent flanking sequences. The structure of the array suggests that slipped-strand mispairing causes the repeated motifs and adds to the mechanisms through which the coding sequence of plastid genes are known to evolve. Phylogenetic analyses based on the sequence data from grass species support several relationships previously suggested by morphological work, but they are ambiguous about broad relationships within the family.
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