Genome affinity in Rubus (Rosaceae) inferred from meiotic chromosome pairing of sixteen wild and cultivated bramble resources

Abstract

To illustrate the mode of polyploid origin in Rubus, meiotic processes of pollen mother cells and pollen viability in sixteen wild and cultivated bramble resources with various ploidy levels were investigated in this study. In diploids, formation of seven bivalents was the most predominant meiotic association at metaphase I, except for wild R. ellipticus var. obcordatus and red raspberry cultivars (R. idaeus) displaying a few univalents, but in black raspberry cultivar (R. occidentalis) both univalent and multivalents were recorded. Meiotic pairing in tetraploids R. lambertianus var. glaber, blackberry hybrids ‘Shawnee’ and ‘Arapaho’ recorded 13.64, 7.41 and 12.33 mean bivalents per cell, respectively. Various chromosome configurations were observed in hexaploid blackberry ‘Ollalie’ with mean of 12.80 bivalents and 3.85 quadrivalents. Besides predominant bivalents, univalents and multivalents were accounted in significant proportions among heptaploid blackberry cultivars. Abnormal chromosomal behaviours including chromatid bridges, lagging chromosomes, unequal segregation, micronuclei, and polyads were also frequently observed. The frequency of meiotic irregularities significantly increased with increasing ploidy levels, which has influence on the percentage of sterile pollen grains. Based on meiotic pairing and abnormalities, wild R. lambertianus var. glaber and blackberry ‘Arapaho’ were supposed to be well-established allotetraploids through diploidization process; blackberry ‘Shawnee’ and ‘Ollalie’ to be segmental allopolyploids with certain variations, while ‘Black Butte’, ‘Boysen’ and ‘Kotata’ to be alloheptaploids via intra-specific hybridization accumulating stable chromosomal variations. In addition, structural variations such as chromosomal inversion and translocation play an important role in the evolutionary history of Rubus.