Fitness of F1 hybrids between weedy Brassica rapa and oilseed rape (B. napus)

Abstract

As part of an ongoing study of the hybridization biology of cultivated oilseed rape (Brassica napus) and weedy B. rapa, we studied the fitness of hybrids between three weedy B. rapa populations and three varieties of B. napus. Reciprocal pollinations were performed, and the resulting offspring were scored for seed development, survival in the field, pod-and seed-set. Seeds from heterospecific crosses developed within pods in lower proportions than seeds from conspecific crosses. Hybrid offspring survived in the field as frequently as conspecific offspring, and produced many more pods that contained fewer seeds. Combining the fitness components into a multiplicative estimate, we found the hybrids to be intermediate to their parents, and significantly more fit than B. rapa. Significant genotypic differences were detected between offspring produced by different parental plants, populations and varieties for some of the fitness components scored. Our results on hybrid fitness are discussed with respect to the possibility that transgenes in oilseed rape may introgress spontaneously into weedy B. rapa.

Male fitness of oilseed rape (Brassica napus), weedy B. rapa and their F1 hybrids when pollinating B. rapa seeds

“…Surprisingly, F 1 plants sometimes set many more seeds than B. rapa (Hauser et al, 1998b) but much fewer in other cases (Mikkelsen, 1996;Jørgensen et al, 1996). We argued (Hauser et al, 1998b) that different frequencies of parents and hybrids, and thereby different compositions of the pollen cloud, might be one explanation for the discrepant results.…”

“…Surprisingly, F 1 plants sometimes set many more seeds than B. rapa (Hauser et al, 1998b) but much fewer in other cases (Mikkelsen, 1996;Jørgensen et al, 1996). We argued (Hauser et al, 1998b) that different frequencies of parents and hybrids, and thereby different compositions of the pollen cloud, might be one explanation for the discrepant results. If F 1 and backcross zygotes are preferentially aborting within B. rapa pods, whereas backcross zygotes are preferentially surviving within F 1 pods, then the seed set of B. rapa, B. napus, and F 1 hybrids should vary with their relative frequencies in the population.…”

“…To account for the effect of competition and preferential abortion, we calculated a 'competition factor' by scaling the survival of F 1 zygotes (relative to B. rapa) within mixed pods (0.48; Hauser et al, 1997) to their relative survival in single-donor pods (0.74; Hauser et al, 1998a; 'competition factor' = 0.65). The survival of backcross zygotes in mixed pods was then estimated by multiplying their survival in pure pods ((relative to B. rapa) = 0.62; Hauser et al, 1998b) with the competition factor: u 3·· = 0.62 × 0.65 = 0.40.…”

“…We therefore used a conservative estimate based on the survival of backcross zygotes within pods of single parentage (from Hauser et al, 1998b), but scaled down by a 'competition factor' (see Methods). We here assume that competition with B. rapa zygotes has the same relative impact on F 1 and backcross zygotes.…”

Hybrids between cultivated and wild carrots in natural populations in Denmark

“…In the case of hybridization between polyploid cultivars and diploid relatives, chromosomal imbalances may add to a low fitness of hybrids (Hauser et al, 1998a). However, hybrids between cultivated and wild plants may sometimes exhibit improved fitness relative to their parents (Ellstrand, 2003), due to inheritance of beneficial crop alleles (Campbell et al, 2006), heterosis (Hauser et al, 1998b), and novel combinations of genes and traits of the parents (Hauser et al, 1998a;Hooftman et al, 2005). Transgression of traits from the two parents is thought to be a major mechanism for creating evolutionary novelties in hybrids (Lexer et al, 2003).…”