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

BACKGROUND: Amaranth (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. RESULTS: Utilizing 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. CONCLUSIONS: The 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.

Simple sequence repeat marker development and genetic mapping in quinoa (Chenopodium quinoa Willd.).

Quinoa is a regionally important grain crop in the Andean region of South America. Recently quinoa has gained international attention for its high nutritional value and tolerances of extreme abiotic stresses. DNA markers and linkage maps are important tools for germplasm conservation and crop improvement programmes. Here we report the development of 216 new polymorphic SSR (simple sequence repeats) markers from libraries enriched for GA, CAA and AAT repeats, as well as 6 SSR markers developed from bacterial artificial chromosome-end sequences (BES-SSRs). Heterozygosity (H) values of the SSR markers ranges from 0.12 to 0.90, with an average value of 0.57. A linkage map was constructed for a newly developed recombinant inbred lines (RIL) population using these SSR markers. Additional markers, including amplified fragment length polymorphisms (AFLPs), two 11S seed storage protein loci, and the nucleolar organizing region (NOR), were also placed on the linkage map. The linkage map presented here is the first SSR-based map in quinoa and contains 275 markers, including 200 SSR. The map consists of 38 linkage groups (LGs) covering 913 cM. Segregation distortion was observed in the mapping population for several marker loci, indicating possible chromosomal regions associated with selection or gametophytic lethality. As this map is based primarily on simple and easily-transferable SSR markers, it will be particularly valuable for research in laboratories in Andean regions of South America.

A genetic linkage map of quinoa ( Chenopodium quinoa) based on AFLP, RAPD, and SSR markers.

Quinoa ( Chenopodium quinoa Willd.) is an important seed crop for human consumption in the Andean region of South America. It is the primary staple in areas too arid or saline for the major cereal crops. The objective of this project was to build the first genetic linkage map of quinoa. Selection of the mapping population was based on a preliminary genetic similarity analysis of four potential mapping parents. Breeding lines 'Ku-2' and '0654', a Chilean lowland type and a Peruvian Altiplano type, respectively, showed a low similarity coefficient of 0.31 and were selected to form an F(2) mapping population. The genetic map is based on 80 F(2) individuals from this population and consists of 230 amplified length polymorphism (AFLP), 19 simple-sequence repeat (SSR), and six randomly amplified polymorphic DNA markers. The map spans 1,020 cM and contains 35 linkage groups with an average marker density of 4.0 cM per marker. Clustering of AFLP markers was not observed. Additionally, we report the primer sequences and map locations for 19 SSR markers that will be valuable tools for future quinoa genome analysis. This map provides a key starting point for genetic dissection of agronomically important characteristics of quinoa, including seed saponin content, grain yield, maturity, and resistance to disease, frost, and drought. Current efforts are geared towards the generation of more than 200 mapped SSR markers and the development of several recombinant-inbred mapping populations.