Genetic relationships in European and Asiatic Buxus species and their possibilities of interspecific hybridisation

To introgress resistance against Cylindrocladium into European Buxus, interspecific hybridisation would be an interesting approach. Therefore in a first step, the genetic relationships and diversity within the European and Asiatic Buxus species were analysed using AFLP, genome size analysis and chromosome counts. Based on these results, two major clusters could be defined. One genetic cluster contained B. sempervirens and B. balearica, European species, and B. colchica, an Asiatic species but with leaf morphology similar to B. sempervirens. Species in this cluster were characterised by a genome size between 1.38 and 1.69 pg 2C-1 and a chromosome number of 2n = 2x = 28 (diploid). Only four B. sempervirens cultivars within this cluster were triploid. A second cluster contained the Asiatic Buxus species B. microphylla, B. harlandii, B. hyrcana, B. myrica, B. henryi, B. bodinieri and B. wallichiana. Within this second genetic cluster three different ploidy levels could be observed. B. harlandii, B. hyrcana and nine B. microphylla cultivars were tetraploid (2n = 4x = 56) with a genome size of > 2.5 pg 2C-1. Fifteen other B. microphylla cultivars were triploid (2n = 3x = 42). The other Asiatic Buxus species, B. henryi, B. bodinieri and eight B. microphylla cultivars, were diploid with a genome size of ca. 1.5 pg 2C-1.
Using this information a breeding program was set up. Interspecific crosses were performed mostly between B. sempervirens, B. microphylla and B. harlandii. As a control, intraspecific crosses within B. sempervirens, B. microphylla and B. harlandii were made. Almost no prezygotic barriers were observed. Obtained seeds were germinated in the greenhouse. About 50% of the resulting F1 seedlings were proved to be hybrids after AFLP analysis.