Resynthetisierter Raps als Modell zur Untersuchung der fixierten Heterosis bei allopolyploiden Pflanzen

The spontaneous hybridisation of related species by combining their genomes (allopolyploidy) has played a prominent role in plant evolution. Many wild species as well as major field crops like wheat, oats, cotton, tobacco and rapeseed are allopolyploids. One explanation for the success of allopolyploids are favourable interactions between homoeologous genes of the parental genomes. These favourable interactions may also occur in homozygous allopolyploids and can therefore be called fixed heterosis. However, experimental data on fixed heterosis is not available, since the high performance of allopolyploids can also be attributed to an effect of polyploidy which is difficult to separate from the effect of fixed heterosis. Therefore this study was carried out (1) to develop a suitable plant material for the analysis of fixed heterosis and polyploidy by the resynthesis of Brassica napus (genome AACC) from its parental species Brassica rapa (AA) and Brassica oleracea (CC), (2) to quantify both effects and (3) to study the interrelation between fixed and classical heterosis.Eight homozygous lines of B. rapa (subspecies oleifera , pekinensis and trilocularis ) as well as eight homozygous lines of B. oleracea (subspecies alboglabra and botrytis ) were used to produce resynthesized B. napus . In total 61 of the 64 possible resynthesized lines have been produced. The success of interspecific hybridisation was significantly dependent on the maternal genotype ( B. rapa ) used for hybridization and averaged 0.46 vital embryos per pollinated bud.The same B. rapa and B. oleracea genotypes were used to produce autopolyploids in order to estimate the effect of higher ploidy level. Here five autotetraploid B. rapa and two autotetraploid B. oleracea lines have been obtained.Additionally intraspecific crosses between all B. rapa and B. oleracea genotypes as well as between eight resynthesized rapeseed genotypes were performed in a diallel manner in order to study the effect of classical heterosis within the three species. Hence a material was available which is completely balanced in the allelic contributions but differs in the amount of heterozygosity (homozygous vs. heterozygous), ploidy (diploid vs. tetraploid) and genomic diversity (autotetraploid vs. allotetraploid). All genotypes were grown in the greenhouse in order to measure the vegetative biomass production 20 and 26 days after sowing. At least nine replications with 18 plants per replication were sown. Morover the parental diploid lines were analysed with RAPD markers to estimate genetic distances. Here, 159 and 87 polymorphic bands were used to calculate genetic distances between the B. rapa and B. oleracea genotypes, respectively.The magnitude of classical heterosis for vegetative biomass production was only correlated with the genetic distance when merely crosses within the same subspecies of B. rapa and B. oleracea were regarded. In contrast there was no correlation when crosses between different subspecies or crosses between resynthesised lines were regarded. The allopolyploid B. napus genotypes showed considerable higher yields compared to their parental species. The magnitude of fixed heterosis averaged 50% relative to the parental mean and 8% relative to the better parent. In contrast the autotetraploid lines yielded from 1% to 64% less than the corresponding diploid lines. The mean midparent classical heterosis was 62% (33% relative to the better parent) in B. rapa , 90% (75%) in B. oleracea and 20% (4%) in B. napus . The results show that the higher performance of resynthesized rapeseed compared to its diploid parental species is based on fixed heterosis and can not be explained by an effect of the higher ploidy level. A prediction of the magnitude of fixed heterosis in the allopolyploid based on the magnitude of classical heterosis in the diploid parents seems to be not possible since fixed heterosis of allopolyploids was not correlated to the general combining ability for classical heterosis in the parental species. On the other hand it is shown that the allopolyploid B. napus , which already benefits from heterotic interactions via fixed heterosis, shows less classical heterosis than its diploid progenitors. Therefore in the present study the resynthesized B. napus was only superior to its parental species when homozygous genotypes were compared but not when heterozygous genotypes were compared. Consequently the main advantage of allopolyploidy in evolution might rather be a higher tolerance to inbreeding and the resulting homozygosity than a higher performance in general. This higher tolerance against inbreeding is thereby based on intergenomic heterozygosity and the resulting fixed heterosis. Thus fixed heterosis is a decisive factor for the establishment of allopolyploid species.