Allopolyploid origin in Rubus (Rosaceae) inferred from nuclear granule-bound starch synthase I (GBSSI) sequences

BACKGROUND: Polyploidy and hybridization are ubiquitous in Rubus L., a large and taxonomically challenging genus. Chinese Rubus are mainly concentrated into two major sections, the diploid Idaeobatus and the polyploid Malachobatus. However, it remains unclear to be auto- or allo- polyploid origin of polyploids in Rubus. We investigated the homoeologs and the structure of the GBSSI-1 (granule-bound starch synthase I) gene in 140 Rubus individuals representing 102 taxa in 17 (out of the total 24) subsections of 7 (total of 12) sections at different ploidy levels. RESULTS: Based on the gene structure and sequence divergence, we defined three gene variants, GBSSI-1a, GBSSI-1b, and GBSSI-1c. When compared with GBSSI-1a, both GBSSI-1b and GBSSI-1c have a shorter fourth intron, and GBSSI-1c had an additional deletion in the fifth intron. For diploids, either GBSSI-1a or GBSSI-1b was detected in 56 taxa consisting of 82 individuals from sect. Idaeobatus, while both alleles existed in R. pentagonus and R. peltatus. Both homoeologs GBSSI-1a and GBSSI-1b were identified in 39 taxa (48 individuals) of Malachobatus polyploids. They were also observed in two sect. Dalibardastrum taxa, in one sect. Chamaebatus taxon, and in three taxa from sect. Cylactis. Interestingly, all three homoeologs were observed in the three tetraploid taxa. Phylogenetic trees and networks suggested two clades (I and II), corresponding to GBSSI-1a, and GBSSI-1b/1c sequences, respectively. GBSSI-1 homoeologs from the same polyploid individual were resolved in different well-supported clades, and some of these homoelogs were more closely related to homoelogs in other species than they were to each other. This implied that the homoeologs of these polyploids were donated by different ancestral taxa, indicating their allopolyploid origin. Two kinds of diploids hybridized to form most allotetraploid species. The early-divergent diploid species with GBSSI-1a or -1b emerged before polyploid formation in the evolutionary history of Rubus. CONCLUSION: This study provided new insights into allopolyploid origin and evolution from diploid to polyploid within the genus Rubus at the molecular phylogenetic level, consistent with the taxonomic treatment by Yü et al. and Lu. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s12870-019-1915-7) contains supplementary material, which is available to authorized users.