Development of 17 microsatellite markers in the federally endangered species Liatris helleri (Asteraceae).

PREMISE: Microsatellite markers were developed in the federally endangered species Liatris helleri (Asteraceae) to evaluate species boundaries with closely related congeners within the genus. METHODS AND RESULTS: Using Illumina data, 17 primer pairs were developed in populations of L. helleri. The primers amplified motifs from tri- to hexanucleotide repeats with one to 17 alleles per locus. Primers were also tested for cross-amplification in L. aspera, L. microcephala, and L. pycnostachya. CONCLUSIONS: The developed primers for L. helleri serve as a novel genetic tool for future investigations in this genus, allowing for more explicit species delineation as well as population genetic analyses.

Fourteen polymorphic microsatellite markers for a widespread limestone endemic, Carex eburnea (Cyperaceae: Carex sect. Albae).

PREMISE OF THE STUDY: Microsatellite primers were developed for a widespread limestone endemic sedge, Carex eburnea, to facilitate investigation of the genetic diversity and phylogeography of this taxon and its closest relative, C. mckittrickensis. METHODS AND RESULTS: Forty-eight primer pairs were designed from Illumina sequence data and screened for suitability. Fourteen of these primer pairs were polymorphic and generated one to seven alleles per locus. Cross-species amplifications were conducted for all four members of Carex sect. Albae. CONCLUSIONS: These primer pairs can be used to assess the genetic diversity and population structure in future studies of C. eburnea and C. mckittrickensis, and likely in other members of Carex sect. Albae.

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 bioinformatic 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 pericentromeres and elsewhere in the genome.

Sixteen polymorphic microsatellite markers for a federally threatened species, Hexastylis naniflora (Aristolochiaceae), and co-occurring congeners.

PREMISE OF THE STUDY: Twenty microsatellite loci were developed for the federally threatened species Hexastylis naniflora (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 heterozygosity of 0.39. CONCLUSIONS: These 16 polymorphic primers for H. naniflora will be useful tools in species identification and quantifying genetic diversity within the genus.

Allopolyploidy, diversification, and the Miocene grassland expansion.

The role of polyploidy, particularly allopolyploidy, in plant diversification is a subject of debate. Whole-genome duplications precede the origins of many major clades (e.g., angiosperms, Brassicaceae, Poaceae), suggesting that polyploidy drives diversification. However, theoretical arguments and empirical studies suggest that polyploid lineages may actually have lower speciation 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 using 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 frequency 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.

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.

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.