Morphological, cytological and molecular analyses of a synthetic hexaploid derived from an interspecific hybrid between Gossypium hirsutum and Gossypium anomalum

Gossypium anomalum represents an inestimable source of genes that could potentially be transferred into the gene pool of cultivated cotton. To resolve interspecific hybrid sterility problems, we previously treated triploid hybrids derived from a cross between Gossypium hirsutum and G. anomalum with 0.15% colchicine and obtained a putative fertile hexaploid. In this study, we performed morphological, molecular and cytological analyses to assess the hybridity and doubled status of putative interspecific hybrid plants. Most of the morphological characteristics of the putative hexaploid plants were intermediate between G. hirsutum and G.anomalum. Analysis of mitotic metaphase plates revealed 78 chromosomes, confirming the doubled hybrid status of the hexaploid. Genome-wide molecular analysis with different genome-derived SSR markers revealed a high level of polymorphism(96.6%) between G. hirsutum and G. anomalum. The marker transferability rate from other species to G. anomalum was as high as 98.0%. The high percentage of polymorphic markers with additive banding profiles in the hexaploid indicates the hybridity of the hexaploid on a genome-wide level. A-genome-derived markers were more powerful for distinguishing the genomic differences between G. hirsutum and G. anomalum than D-genome-derived markers. This study demonstrates the hybridity and chromosomally doubled status of the(G. anomalum × G. hirsutum)2hexaploid using morphological, cytological and molecular marker methods. The informative SSR markers screened in the study will be useful marker resources for tracking the flow of G. anomalum genetic material among progenies that may be produced by future backcrosses to G. hirsutum.

Genome sequence of Gossypium anomalum facilitates interspecific introgression breeding

Crop wild relatives are an important reservoir of natural biodiversity. However, incorporating wild geneticdiversity into breeding programs is often hampered by reproductive barriers and a lack of accurate genomicinformation. We assembled a high-quality, accurately centromere-anchored genome of Gossypium anomalum, a stress-tolerant wild cotton species. We provided a strategy to discover and transfer agronomicallyvaluable genes from wild diploid species to tetraploid cotton cultivars. With a (Gossypium hirsutum 3 G.anomalum)2 hexaploid as a bridge parent, we developed a set of 74 diploid chromosome segment substitution lines (CSSLs) of the wild cotton species G. anomalum in the G. hirsutum background. This set of CSSLsincluded 70 homozygous substitutions and four heterozygous substitutions, and it collectively containedabout 72.22% of the G. anomalum genome. Twenty-four quantitative trait loci associated with plant height,yield, and fiber qualities were detected on 15 substitution segments. Integrating the reference genome withagronomic trait evaluation of the CSSLs enabled location and cloning of two G. anomalum genes thatencode peroxiredoxin and putative callose synthase 8, respectively, conferring drought tolerance andimproving fiber strength. We have demonstrated the power of a high-quality wild-species reference genomefor identifying agronomically valuable alleles to facilitate interspecific introgression breeding in crops.