Cotton is the most widely-used natural fiber and raw material for the textile industry in the world. As a fabric, cotton is very popular among people due to its remarkable breathability, comfort and warmth. At present, there are two cultivated allotetraploidspecies: G. Hirsutum(a.k.a upland cotton) and G. Barbadense(a.k.a extra-long staple cotton). Upland cottonproduces a higher fiber yield and is better able to survive harsh environments. Globally, about 90% of all cotton production is of cultivars derived from this species. Extra-long staple cotton produce long and silky fibers but it has a low yield. It can only be cultivated in some dry areas, such as Xinjiang, Egypt and Arizona, so it is extremely costly.
Researchers have put in constant efforts to combine the merits of these two species so that they can cultivate a cotton species which is not only high-yielding and adaptable but also able to produce longer, stronger and thinner fibers. This calls for a more insightful understanding of the genetic bases ofG. hirsutum and G. barbadensein terms of output, quality and adaptability.
Prof. ZHANG Tianzhen from the College of Agriculture and Biotechnology led a research into the global genetic and molecular bases underlying the evolutionary dynamics associated with the origin, speciation and diversification ofG.hirsutum and G.barbadense. Their research findings are published in the journal of Nature Genetics.
DNA sequencing is a crucial approach in this field. ZHANG Tianzhen et al. performed the de novo assemblies of the genomes of two cultivated allotetraploid cotton species by integrating data from Illumina PCR-free short-read sequencing, 10x Genomics sequencing, Hi-C, and optical and super-dense genetic maps.
By analyzing the two resulting high-quality genome assemblies, researchers determined that tissue- and/or developmental-stage specific expression and divergent neo- and subfunctionalization of homeologs between G. hirsutum and G. barbadense, which may have led to the variable adaptation responses of these two species after the formation of allotetraploid cotton.
“Put it simply, we find that the elongated fiber of G. barbadensecan be attributed to a longer post anthesis. Generally speaking, G. hirsutum ceases to grow at 15 days post anthesis (DPA) while G. barbadense stops growing at about 25 DPA. In our study, we discover that genes factor into this distinction,” said ZHANG Tianzhen.
G. hirsutum and G. barbadense also differ in adaptation to the environment. In previous studies, researchers reported that G. hirsutumis tolerant of both low and high temperatures. ZHANG Tianzhen et al. identified the genes which are insensitive to heat and cold stresses on the strength of gene mapping.
In G. hirsutum, the ethylene and abscisic acid (ABA) signaling pathways were activated in response to heat and cold stresses, which may well be intimately akin to the adaptability of G. hirsutum. Collinearity on chromosome A01 for G. hirsutum indicated that there did exist the Central Asian or Xinjiang-type extra-long staple cotton.
These findings help to elucidate the evolution of cotton genomes and their domestication history. The identification of the difference between G. hirsutum and G. barbadense at the genetic level offers genetic basis for the cultivation of a high-yielding, superior-fiber and adaptable species.