Genetic Diversity Of A Parasitic Weed, Striga Hermonthica, On Sorghum And Pearl Millet In Mali

Eleven populations of witchweed, Striga her-monthica, were collected in four regions of Mali and investigated with 12 microsatellite markers. Extensive genetic diversity was observed, with most plants heterozy-gous for most markers. Allelic diversity was broadly distributed across populations with little genetic differenti-ation and large amounts of gene flow. Nearby fields of pearl millet and sorghum were found to have indistinguishable witchweed populations. Some population structure was apparent, but did not correlate with the local environment or host genotype, suggesting that seed transportation or other human-driven variables act to differentiate central Malian S. hermonthica populations from southern Malian populations

Allopolyploidy, Diversification And The Miocene Grassland Expansion

The role of polyploidy, particularly allopolyploidy, in plant di-versification is a subject of debate. Whole-genome duplications precede the origins of many major clades (e.g., angiosperms, Brassicaceae, Poaceae), suggesting that polyploidy drives diversi-fication. However, theoretical arguments and empirical studies suggest that polyploid lineages may actually have lower specia-tion rates and higher extinction rates than diploid lineages. We focus here on the grass tribe Andropogoneae, an economically and ecologically important group of C4 species with a high frequency of polyploids. A phylogeny was constructed for ca. 10% of the species of the clade, based on sequences of four concatenated low-copy nuclear loci. Genetic allopolyploidy was documented us-ing the characteristic pattern of double-labeled gene trees. At least 32% of the species sampled are the result of genetic allopolyploidy and result from 28 distinct tetraploidy events plus an additional six hexaploidy events. This number is a minimum, and the actual fre-quency could be considerably higher. The parental genomes of most Andropogoneae polyploids diverged in the Late Miocene coincident with the expansion of the major C4 grasslands that dominate the earth today. The well-documented whole-genome duplication in Zea mays ssp. mays occurred after the divergence of Zea and Sorghum. We find no evidence that polyploidization is followed by an increase in net diversification rate; nonetheless, allopolyploidy itself is a major mode of speciation

Uneven Chromosome Contraction And Expansion In The Maize Genome

Maize (Zea mays or corn), both a major food source and an important cytogenetic model, evolved from a tetraploid that arose about 4.8 million years ago (Mya). As a result, maize has extensive duplicated regions within its genome. We have sequenced the two copies of one such region, generating 7.8 Mb of sequence spanning 17.4 cM of the short arm of chromosome 1 and 6.6 Mb (25.6 cM) from the long arm of chromosome 9. Rice, which did not undergo a similar whole genome duplication event, has only one orthologous region (4.9 Mb) on the short arm of chromosome 3, and can be used as reference for the maize homoeologous regions. Alignment of the three regions allowed identification of syntenic blocks, and indicated that the maize regions have undergone differential contraction in genic and intergenic regions and expansion by the insertion of retrotransposable elements. Approximately 9% of the predicted genes in each duplicated region are completely missing in the rice genome, and almost 20% have moved to other genomic locations. Predicted genes within these regions tend to be larger in maize than in rice, primarily because of the presence of predicted genes in maize with larger introns. Interestingly, the general gene methylation patterns in the maize homoeologous regions do not appear to have changed with contraction or expansion of their chromosomes. In addition, no differences in methylation of single genes and tandemly repeated gene copies have been detected. These results, therefore, provide new insights into the diploidization of polyploid species

Methylation-Sensitive Linking Libraries Enhance Gene-Enriched Sequencing Of Complex Genomes And Map DNA Methylation Domains

Background: Many plant genomes are resistant to whole-genome assembly due to an abundance of repetitive sequence, leading to the development of gene-rich sequencing techniques. Two such techniques are hypomethylated partial restriction (HMPR) and methylation spanning linker libraries (MSLL). These libraries differ from other gene-rich datasets in having larger insert sizes, and the MSLL clones are designed to provide reads localized to "epigenetic boundaries" where methylation begins or ends.Results: A large-scale study in maize generated 40,299 HMPR sequences and 80,723 MSLL sequences, including MSLL clones exceeding 100 kb. The paired end reads of MSLL and HMPR clones were shown to be effective in linking existing gene-rich sequences into scaffolds. In addition, it was shown that the MSLL clones can be used for anchoring these scaffolds to a BAC-based physical map. The MSLL end reads effectively identified epigenetic boundaries, as indicated by their preferential alignment to regions upstream and downstream from annotated genes. The ability to precisely map long stretches of fully methylated DNA sequence is a unique outcome of MSLL analysis, and was also shown to provide evidence for errors in gene identification. MSLL clones were observed to be significantly more repeat-rich in their interiors than in their end reads, confirming the correlation between methylation and retroelement content. Both MSLL and HMPR reads were found to be substantially gene-enriched, with the SalI MSLL libraries being the most highly enriched (31% align to an EST contig), while the HMPR clones exhibited exceptional depletion of repetitive DNA (to ~11%). These two techniques were compared with other gene-enrichment methods, and shown to be complementary.Conclusion: MSLL technology provides an unparalleled approach for mapping the epigenetic status of repetitive blocks and for identifying sequences mis-identified as genes. Although the types and natures of epigenetic boundaries are barely understood at this time, MSLL technology flags both approximate boundaries and methylated genes that deserve additional investigation. MSLL and HMPR sequences provide a valuable resource for maize genome annotation, and are a uniquely valuable complement to any plant genome sequencing project. In order to make these results fully accessible to the community, a web display was developed that shows the alignment of MSLL, HMPR, and other gene-rich sequences to the BACs; this display is continually updated with the latest ESTs and BAC sequences

Sixteen Polymorphic Microsateliate Markers For A Federally Threatened Species, Hexastylis Naniflora (Aristolochiaceae), And Co-Occurring Congeners

Premise of study: Twenty mierosalellite loci were developed for the federally threatened species Hexastylis nanillor (Aristolochiaceae) to examine genetic diversity and to distinguish this species from co-occurring congeners, H. heterophylla and H. minor. Methods and Results: Next-generation sequencing approaches were used to identify microsatellite loci and design primers. One hundred fifty-two primer pairs were screened for repeatability, and 20 of these were further characterized for polymorphism. In H. naniflora, the number of alleles identified for polymorphic loci ranged from two to 23 (mean -8.8), with a mean heterozygosit y of 0.39. Conclusions: These 16 polymorphic primers for H. naniflora will be useful tools in species identification and quantifying genelic diversity within the genus

Testing A Vicariance Model To Explain Haplotype Distribution In The Psammophilic Scorpion Paruroctonus Utahensis

A model of vicariance speciation has been proposed to explain the proliferation of psammophilic (sand loving) species in several endemic genera of scorpions. The current distribution of Paruroctonus utahensis (Williams, 1968) populations on isolated sand dunes seems to fit this model; therefore, this species was examined to test the model for psammophilic population distribution and haplotype variation. A portion of the mitochondrial rDNA gene was sequenced from 44 individuals representing 7 populations of P. utahensis. Three individuals of Paruroctonus gracilior (Hoffmann, 1931) and 1 individual of Paruroctonus boquillas Sissom and Henson, 1998, were used for outgroup comparison. A maximum parsimony heuristic search indicated that the basal populations were in the south and derived populations occurred in the northeastern portion of the range. A nested clade analysis found a statistically significant correlation between haplotypes and their geographical distribution at several clade levels. The biological cause of this association was best explained by allopatric fragmentation. Our data supported the vicariance model of isolation by fragmentation of a larger habitat to explain the variation seen among populations of P. utahensis

Eleven Diverse Nuclear-Encoded Phylogenetic Markers For The Subfamily Panicoideae (Poaceae)

Premise of the study: Polyploidy is common in the grasses and low-copy nuclear loci are needed to further our understanding of phylogenetic relationships. Methods and Results: Genetic and genomic resources were combined to identify loci known to influence plant and inflorescence architecture. Degenerate primers were designed and tested to amplify regions of 11 nuclear-encoded loci across the panicoid grasses. Conclusions: The primers designed in this study amplify regions of a diverse set of genes within the panicoid grasses. Properly employed, these markers will allow the identification of allopolyploid taxa and their diploid progenitors

Young, Intact And Nested Retrotransposons Are Abundant In The Onion And Asparagus Genomes

Background and Aims Although monocotyledonous plants comprise one of the two major groups of angiosperms and include .65 000 species, comprehensive genome analysis has been focused mainly on the Poaceae (grass) family. Due to this bias, most of the conclusions that have been drawn for monocot genome evolution are based on grasses. It is not known whether these conclusions apply to many other monocots.† Methods To extend our understanding of genome evolution in the monocots, Asparagales genomic sequence data were acquired and the structural properties of asparagus and onion genomes were analysed. Speci?cally, several available onion and asparagus bacterial arti?cial chromosomes (BACs) with contig sizes .35 kb were annotated and analysed, with a particular focus on the characterization of long terminal repeat (LTR) retrotransposons.† Key Results The results reveal that LTR retrotransposons are the major components of the onion and garden aspara-gus genomes. These elements are mostly intact (i.e. with two LTRs), have mainly inserted within the past 6 million years and are piled up into nested structures. Analysis of shotgun genomic sequence data and the observation of two copies for some transposable elements (TEs) in annotated BACs indicates that some families have become particu-larly abundant, as high as 4–5 % (asparagus) or 3–4 % (onion) of the genome for the most abundant families, as also seen in large grass genomes such as wheat and maize.† Conclusions Although previous annotations of contiguous genomic sequences have suggested that LTR retrotran-sposons were highly fragmented in these two Asparagales genomes, the results presented here show that this was largely due to the methodology used. In contrast, this current work indicates an ensemble of genomic features similar to those observed in the Poaceae

Genomic Abundance Is Not Predictive Of Tandem Repeat Localization In Grass Genomes

Highly repetitive regions have historically posed a challenge when investigating sequence variation and content. High-throughput sequencing has enabled researchers to use whole-genome shotgun sequencing to estimate the abundance of repetitive sequence, and these methodologies have been recently applied to centromeres. Previous research has investigated variation in centromere repeats across eukaryotes, positing that the highest abundance tandem repeat in a genome is often the centromeric repeat. To test this assumption, we used shotgun sequencing and a bio-informatic pipeline to identify common tandem repeats across a number of grass species. We find that de novo assembly and subsequent abundance ranking of repeats can successfully identify tandem repeats with homology to known tandem repeats. Fluorescent in-situ hybridization shows that de novo assembly and ranking of repeats from non-model taxa identifies chromosome domains rich in tandem repeats both near peri-centromeres and elsewhere in the genome

The Dynamics Of LTR Retrotransposon Accumulation Across 25 Million Years Of Panicoid Grass Evolution

Sample sequence analysis was employed to investigate the repetitive DNAs that were most responsible for the evolved variation in genome content across seven panicoid grasses with 45-fold variation in genome size and different histories of polyploidy. In all cases, the most abundant repeats were LTR retrotransposons, but the particular families that had become dominant were found to be different in the Pennisetum, Saccharum, Sorghum and Zea lineages. One element family, Huck, has been very active in all of the studied species over the last few million years. This suggests the transmittal of an active or quiescent autonomous set of Huck elements to this lineage at the founding of the panicoids. Similarly, independent recent activity of Ji and Opie elements in Zea and of Leviathan elements in Sorghum and Saccharum species suggests that members of these families with exceptional activation potential were present in the genome(s) of the founders of these lineages. In a detailed analysis of the Zea lineage, the combined action of several families of LTR retrotransposons were observed to have approximately doubled the genome size of Zea luxurians relative to Zea mays and Zea diploperennis in just the last few million years. One of the LTR retrotransposon ampli?cation bursts in Zea may have been initiated by polyploidy, but the great majority of transposable element activations are not. Instead, the results suggest random activation of a few or many LTR retrotransposons families in particular lineages over evolutionary time, with some families especially prone to future activation and hyper-amplification