Fertilization fitness and relation to chromosome number in interspecific progeny between Brassica napus and B-rapa:: a comparative study using natural and resynthesized B-napus

The variation of the chromosome number in interspecific progeny is difficult to predict. We defined the terms "fertilization fitness" and "relative fitness" as the parameters of competitive ability of the gametes and attempted to use them in a study on the variation of the chromosome number in the progeny between Brassica napus and B. rapa. Fertilization fitness was defined as the ratio of the actual frequency of the gametes with a specific chromosome number to the theoretical frequency based on binomial distribution. Relative fitness was defined as the ratio of fertilization fitness of the gametes with a specific chromosome number to fertilization fitness of the gametes with n = 10. A resynthesized accession and a natural accession of B. napus were used in this experiment to produce sesquidiploids AAC-S and AAC-N, respectively. A comparative study on these two sesquidiploid populations was carried out to reveal differences in the pattern of variation in the chromosome number. Fitness analysis showed that gamete competition was conspicuous and closely related to the chromosome number. Gamete competition occurred predominantly during the embryo developmental stage. In both sesquidiploid populations, fertilization fitness decreased abruptly when the chromosome number varied in the range from n = 10 to n = 12 or in the range from n = 19 to n = 17. However, in the range from n = 13 to n = 16, the effect of the chromosome number on fertilization fitness was more pronounced in the AAC-N population than in the AAC-S one. The AAC-N population showed stronger tendency in keeping its gametes toward euploid number. Fertilization fitness and relative fitness was found to be efficient parameters for the study of variation of the chromosome number in interspecific hybridization. The use of the Relative fitness parameter may enable to predict the frequency of a specific chromosome number in interspecific progenies.