Evolution of South African brambles (Rubus L.) – new insights from molecular markers

Abstract

Background: South African brambles (Rubus L., Rosaceae) represent a complex group of six native species and at least 12 introduced taxa with different ploidy levels and varying tendencies to hybridisation. The role of hybridisation, introgression and apomixis in the ongoing evolution has been hypothesised based on morphological observations, but it has not been rigorously studied to date, and nor has the phylogeny of the group. Objectives and methods: This paper aims to reveal the evolutionary patterns and mechanisms in South African brambles by employing three types of molecular markers: plastid and nuclear ribosomal DNA sequences, and nuclear microsatellites. Results: The data confirmed the tetraploid R. thaumasius A.Beek and diploid R. ludwigii Eckl. & Zeyh. as distinct native species, while the other four native species are shown to be closely related and likely derived from three ancestors. Conclusion: Ancient hybridisation and limited gene flow between regions (particularly between winter- and summer-rainfall zones) appear to be the main drivers of current patterns in the tetraploid R. pinnatus Willd. and hexaploid R. rigidus Sm. Current hybridisation is also likely, although rare. The mechanism of ‘octoploid bridge’ is proposed, which overcomes the ploidy reproduction barrier between R. pinnatus (or other tetraploids) and R. rigidus. No gene flow was detected between native and alien taxa, but clonal duplications were discovered in the R. bergii × pinnatus hybrid, which implies the possibility of apomictic spread of homoploid hybrids formed between native and introduced brambles and the potential for a new invasion. On the other hand, heteroploid hybrids (R. bergii × rigidus) are formed recurrently and spread only vegetatively.