Genetic diversity in Australian canola and implications for crop breeding for changing future environments

Australian canola breeders have successfully improved blackleg resistance, seed oil and meal quality and local adaptation of oilseed rape (Brassica napus) from 1970 to 2000 in five breeding cycles, averaging 6 years/cycle, in a closed population. The 18 ancestral varieties in 1970 included 16 B. napus from Canada, Europe and Asia and 2 B. juncea varieties. Introductions with improved seed quality were generally very susceptible to blackleg under Australian conditions. Of the varieties released from 1995 to 2002, 11 ancestral varieties contributed 98.7% of the pedigree composition, and 2 ancestors (Canadian low erucic spring variety Zephyr, and Polish low glucosinolate spring variety Bronowski) were present in the pedigrees of every variety. Approximately half of the ancestral contribution was from Asian B. napus or B. juncea, and half from European or Canadian B. napus. Assuming an effective population size of 11, the inbreeding coefficient of the population at the end of the fifth cycle was 0.21, which represents 21% cumulative loss of alleles through random genetic drift. The coefficients of ancestry among four varieties released from 1996 to 2002 ranged from 0.127 to 0.371. While the original parents were very diverse, the population is showing signs of loss of genetic diversity that will impact on future breeding progress. There has been a slow decay in polygenic blackleg resistance (average −0.15 resistance units per year) which, so far, has been countered by a net genetic improvement rate of approximately +0.13 resistance units per year over 30 years. New genetic diversity should be introduced to maintain a positive net improvement rate in blackleg resistance, or for adaptation to low rainfall environments. New technologies such as doubled haploidy may improve the efficiency of selection for earliness and other polygenic characters, and accelerate cycles of selection. Major challenges exist for Australian canola breeders to introgress new genetic diversity and increase effective population size, while retaining genetic gains made over the past 30 years. Similar challenges face most crop breeders who are attempting to improve their crop in the face of changing future environments.