First investigation into the genetic control of meiosis in sugarcane.

The sugarcane (Saccharum spp.) genome is one of the most complex of all. Modern varieties are highly polyploid and aneuploid as a result of hybridization between Saccharum officinarum and S. spontaneum. Little research has been done on meiotic control in polyploid species, with the exception of the wheat Ph1 locus harboring the ZIP4 gene (TaZIP4-B2) which promotes pairing between homologous chromosomes while suppressing crossover between homeologs. In sugarcane, despite its interspecific origin, bivalent association is favored, and multivalents, if any, are resolved at the end of prophase I. Thus, our aim herein was to investigate the purported genetic control of meiosis in the parental species and in sugarcane itself. We investigated the ZIP4 gene and immunolocalized meiotic proteins, namely synaptonemal complex proteins Zyp1 and Asy1. The sugarcane ZIP4 gene is located on chromosome 2 and expressed more abundantly in flowers, a similar profile to that found for TaZIP4-B2. ZIP4 expression is higher in S. spontaneum a neoautopolyploid, with lower expression in S. officinarum, a stable octoploid species. The sugarcane Zip4 protein contains a TPR domain, essential for scaffolding. Its 3D structure was also predicted, and it was found to be very similar to that of TaZIP4-B2, reflecting their functional relatedness. Immunolocalization of the Asy1 and Zyp1 proteins revealed that S. officinarum completes synapsis. However, in S. spontaneum and SP80-3280 (a modern variety), no nuclei with complete synapsis were observed. Importantly, our results have implications for sugarcane cytogenetics, genetic mapping, and genomics.

Unveiling the predominance of Saccharum spontaneum alleles for resistance to orange rust in sugarcane using genome-wide association.

KEY MESSAGE: Six QTLs of resistance to sugarcane orange rust were identified in modern interspecific hybrids by GWAS. For five of them, the resistance alleles originated from S. spontaneum. Altogether, they efficiently predict disease resistance. Sugarcane orange rust (SOR) is a threatening emerging disease in many sugarcane industries worldwide. Improving the genetic resistance of commercial cultivars remains the most promising solution to control this disease. In this study, an association panel of 568 modern interspecific sugarcane hybrids (Saccharum officinarum x S. spontaneum) from Réunion's breeding program was evaluated for its resistance to SOR under natural conditions of infection. Two genome-wide association studies (GWAS) were conducted between disease reactions and 183,842 single nucleotide polymorphism (SNP) markers obtained by targeted genotyping-by-sequencing. Five resistance quantitative trait loci (QTLs), named Oru1, Oru2, Oru3, Oru4 and Oru5, were identified using a single-locus GWAS (SL-GWAS). These five QTLs all originated from the species S. spontaneum. A multi-locus GWAS (ML-GWAS) uncovered an additional but less significant resistance QTL named Oru6, which originated from S. officinarum. All six QTLs had a moderate to major phenotypic effect on disease resistance. Prediction accuracy estimated with linear regression models based on each of the five QTLs identified by SL-GWAS was between 0.16-0.41. Altogether, these five QTLs provided a relatively high prediction accuracy of 0.60. In comparison, accuracies obtained with six genome-wide prediction models (i.e., GBLUP, Bayes-A, Bayes-B, Bayes-C, Bayesian Lasso and RKHS) reached only 0.65. The good prediction accuracy of disease resistance provided by the QTLs and the predominant S. spontaneum origin of their resistance alleles pave the way for effective marker-assisted breeding strategies.