**3. Differences between sweet sorghum and grain sorghum**

Sweet sorghum plant produces sugars which can be directly fermented, together with its ability to produce high biomass volumes under adverse conditions, this crop is consider ideal for the generation of bioethanol of first and second generation. Also, its cultivation is suitable for marginal lands, avoiding competence for land with other food crops [11**–**13]. However, the genetics underlying these traits have been barely studied. The genetic differences between sweet and grain sorghum consist on a series of variations in the sequence and alterations of the genetic structure. The variations at sequence level are usually identified by single-nucleotide polymorphisms (SNPs), association sequences, genetic diversity and domestication [14**–**16].

*S. bicolor* has three subspecies: arundinaceum, bicolor y drummondii. All the feasible varieties belong to bicolor, which is divided into 5 races: bicolor, caudatum, durra, guinea y kafir. Sweet sorghum differs phenotypically from grain sorghum in several aspects: juicy stem rich in sugars, higher plant length, higher biomass production and less amount of grains in the spike [11]. These differences cannot be only explained by the genetic variation among these two varieties, which suggest DNA methylation and other epigenetic mechanisms are key factors to describe them [17].


**Table 1.**

*Main differences between grain and sweet sorghum.*

*Ekefre* et al*., [22]. Genotypic differences data obtained from Jiang* et al*., [17].*

Sweet sorghum has been found on different races [18], which challenges its origin, selection and genetics. This also suggest high genetic variability between sweet and grain sorghum, which could be exploited for genetic improvement of sweet sorghum. Currently the BTx623 grain sorghum genome sequence is available [10], which provides a genomic base for comparative studies of the genome. Regardless this achievement, it is still difficult to access the information related to the hidden variability among genomes of the same species. Zheng et al., [19] studied the resequencing of the two sweet and one grain sorghum genomes, with the aim of identify polymorphism patterns of the sequences and structural variations, using BTx623 as a reference genome. This study allowed the identification of great differences in the number of SNPs, indels, copy number variations and structural variations (SV) among these genomes. The comparison of this genetic variation defined potential genomic regions and metabolic pathways associated to sweet sorghum and traits such as sugar production. **Table 1** presents phenotypic and genotypic differences between grain and sweet sorghum.
