Single-molecule sequencing and Hi-C-based proximity-guided assembly of amaranth (Amaranthus hypochondriacus) chromosomes provide insights into genome evolution

Abstract

BackgroundAmaranth (Amaranthus hypochondriacus) was a food staple among the ancient civilizations of Central and South America that has recently received increased attention due to the high nutritional value of the seeds, with the potential to help alleviate malnutrition and food security concerns, particularly in arid and semiarid regions of the developing world. Here, we present a reference-quality assembly of the amaranth genome which will assist the agronomic development of the species.ResultsUtilizing single-molecule, real-time sequencing (Pacific Biosciences) and chromatin interaction mapping (Hi-C) to close assembly gaps and scaffold contigs, respectively, we improved our previously reported Illumina-based assembly to produce a chromosome-scale assembly with a scaffold N50 of 24.4 Mb. The 16 largest scaffolds contain 98% of the assembly and likely represent the haploid chromosomes (n = 16). To demonstrate the accuracy and utility of this approach, we produced physical and genetic maps and identified candidate genes for the betalain pigmentation pathway. The chromosome-scale assembly facilitated a genome-wide syntenic comparison of amaranth with other Amaranthaceae species, revealing chromosome loss and fusion events in amaranth that explain the reduction from the ancestral haploid chromosome number (n = 18) for a tetraploid member of the Amaranthaceae.ConclusionsThe assembly method reported here minimizes cost by relying primarily on short-read technology and is one of the first reported uses of in vivo Hi-C for assembly of a plant genome. Our analyses implicate chromosome loss and fusion as major evolutionary events in the 2n = 32 amaranths and clearly establish the homoeologous relationship among most of the subgenome chromosomes, which will facilitate future investigations of intragenomic changes that occurred post polyploidization.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-017-0412-4) contains supplementary material, which is available to authorized users.

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“…Lightfoot et al. () identified evidence of chromosome loss (the homoeolog of Ah5) and chromosome fusion (Ah1) in the amaranth genome that likely led to the observed base chromosome number reduction in the amaranths. Our comparison of cañahua to the amaranth genome identified 285 syntenic blocks with 13,200 colinear gene pairs.…”

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“…Recently, sequencing of complex crop genomes has been completed for multiple species, e.g. sugar beet, quinoa, grain amaranth, millet, and sorghum . These results greatly aid efforts to sequence weed genomes because some of these crops, e.g., grain amaranth, foxtail millet, quinoa, and perennial ryegrass, have weedy relatives (Fig.…”

“…sugar beet, quinoa, grain amaranth, millet, and sorghum. [27][28][29][30][31] These results greatly aid efforts to sequence weed genomes because some of these crops, e.g., grain amaranth, foxtail millet, quinoa, and perennial ryegrass, have weedy relatives ( Fig. 1).…”

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