Thure P. Hauser

Fitness of backcross and F2 hybrids between weedy Brassica rapa and oilseed rape (B. napus)

With the cultivation of genetically modified crops, transgenes may spread by introgression from crops into weedy and wild populations of related species. The likelihood of this depends in part on the fitness of first and later generation hybrids. We here present results on the fitness of F2 and backcross hybrids between oilseed rape (Brassica napus) and weedy B. rapa. Two populations of B. rapa, two varieties of B. napus, and their F1 hybrids were used for controlled crosses, and seed development, survival in the field, pollen viability, pod- and seed-set were estimated for the offspring. Offspring from F2 and backcrosses had a reduced fitness relative to their parents for most of the fitness components and for a combined estimate of fitness, with F2 offspring suffering the lowest fitness. Despite their lower fitness on average, some of the hybrids were as fit as the parents. Significant fitness differences were detected between backcross and F2 offspring from different B. rapa populations, B. napus varieties, and parental plants. Our results suggest that introgression of transgenes from oilseed rape to B. rapa will be slowed down, but not hindered, by the low fitness of second generation hybrids.

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

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.

DROUGHT STRESS AND INBREEDING DEPRESSION IN LYCHNIS FLOS-CUCULI (CARYOPHYLLACEAE)

Interactions between drought stress and inbreeding depression were studied in Lychnis flos‐cuculi. Four inbreeding levels (F = 0, 0.25, 0.50 and 0.75), and three watering treatments were used. Performance was scored for germination rate and proportion, survival, plant size, proportion of plants flowering, flowering date, stem height, number of flowers, flower size, anther weight, fruiting proportion and number of capsules. Multiplicative fitness values were estimated from these traits. Inbreeding affected most of the traits studied, and a severe inbreeding depression was found for the combined fitness estimates. The higher inbreeding depression found here relative to the same family groups in a former experiment may reflect greater dominance and suppression in the present experiment at higher density.

Inbreeding depression and mating-distance dependent offspring fitness in large and small populations of Lychnis flos -cuculi ( Caryophyllaceae)

The spatial structure of four Lychnis flos‐cuculi populations, varying in size and degree of isolation, was studied by comparing the fitness of offspring resulting from self‐pollination and pollinations by neighbouring plants, plants within the same population, and plants from other populations. Selfed offspring had the lowest fitness of the four offspring groups. No significant difference was found between the performance of offspring from pollinations by neighbouring plants and offspring pollinated by plants further apart but within the same population. A lower fitness of offspring from pollinations between neighbours would be expected if these matings, on average, yielded inbred offspring which suffered from inbreeding depression. These results imply that either a tight neighbourhood structuring is not present, or that the inbreeding depression for offspring by neighbours is too low to detect, although these are inbred. Crossings between populations produced offspring with a significantly higher fitness than offspring sired within populations. There were no significant differences in response to inbreeding among the populations, and differences in mean fitness among populations had no clear relation to the population size or degree of isolation. A reduced fitness of small populations due to inbreeding depression or a less severe response to experimental inbreeding due to purging of deleterious alleles is therefore not supported by our results.

Frequency-dependent fitness of hybrids between oilseed rape (Brassica napus) and weedy B. rapa (Brassicaceae)

Fitness of interspecific hybrids is sometimes high relative to their parents, despite the conventional belief that they are mostly unfit. F(1) hybrids between oilseed rape (Brassica napus) and weedy B. rapa can be significantly more fit than their weedy parents under some conditions; however, under other conditions they are less fit. To understand the reasons, we measured the seed production of B. napus, B. rapa, and different generations of hybrid plants at three different densities and in mixtures of different frequencies (including pure stands). Brassica napus, B. rapa, and backcross plants (F(1) ♀ × B. rapa) produced many more seeds per plant in pure plots than in mixtures and more seeds in plots when each was present at high frequency. The opposite was true for F(1) plants that produced many more seeds than B. rapa in mixtures, but fewer in pure stands. Both vegetative and reproductive interactions may be responsible for these effects. Our results show that the fitness of both parents and hybrids is strongly frequency-dependent and that the likelihood of introgression of genes between the species thus may depend on the numbers and densities of parents and their various hybrid offspring in the population.

UDP-Glycosyltransferases from the UGT73C Subfamily in Barbarea vulgaris Catalyze Sapogenin 3-O-Glucosylation in Saponin-Mediated Insect Resistance

Triterpenoid saponins are bioactive metabolites that have evolved recurrently in plants, presumably for defense. Their biosynthesis is poorly understood, as is the relationship between bioactivity and structure. Barbarea vulgaris is the only crucifer known to produce saponins. Hederagenin and oleanolic acid cellobioside make some B. vulgaris plants resistant to important insect pests, while other, susceptible plants produce different saponins. Resistance could be caused by glucosylation of the sapogenins. We identified four family 1 glycosyltransferases (UGTs) that catalyze 3-O-glucosylation of the sapogenins oleanolic acid and hederagenin. Among these, UGT73C10 and UGT73C11 show highest activity, substrate specificity and regiospecificity, and are under positive selection, while UGT73C12 and UGT73C13 show lower substrate specificity and regiospecificity and are under purifying selection. The expression of UGT73C10 and UGT73C11 in different B. vulgaris organs correlates with saponin abundance. Monoglucosylated hederagenin and oleanolic acid were produced in vitro and tested for effects on P. nemorum. 3-O-β-d-Glc hederagenin strongly deterred feeding, while 3-O-β-d-Glc oleanolic acid only had a minor effect, showing that hydroxylation of C23 is important for resistance to this herbivore. The closest homolog in Arabidopsis thaliana, UGT73C5, only showed weak activity toward sapogenins. This indicates that UGT73C10 and UGT73C11 have neofunctionalized to specifically glucosylate sapogenins at the C3 position and demonstrates that C3 monoglucosylation activates resistance. As the UGTs from both the resistant and susceptible types of B. vulgaris glucosylate sapogenins and are not located in the known quantitative trait loci for resistance, the difference between the susceptible and resistant plant types is determined at an earlier stage in saponin biosynthesis.

Male fitness of oilseed rape ( Brassica napus ), weedy B. rapa and their F 1 hybrids when pollinating B. rapa seeds

The likelihood that two species hybridise and backcross may depend strongly on environmental conditions, and possibly on competitive interactions between parents and hybrids. We studied the paternity of seeds produced by weedy Brassica rapa growing in mixtures with oilseed rape (B. napus) and their F1 hybrids at different frequencies and densities. Paternity was determined by the presence of a transgene, morphology, and AFLP markers. In addition, observations of flower and pollen production, and published data on pollen fertilisation success, zygote survival, and seed germination, allowed us to estimate an expected paternity. The frequency and density of B. napus, B. rapa, and F1 plants had a strong influence on flower, pollen, and seed production, and on the paternity of B. rapa seeds. Hybridisation and backcrossing mostly occurred at low densities and at high frequencies of B. napus and F1, respectively. F1 and backcross offspring were produced mainly by a few B. rapa mother plants. The observed hybridisation and backcrossing frequencies were much lower than expected from our compilation of fitness components. Our results show that the male fitness of B. rapa, B. napus, and F1 hybrids is strongly influenced by their local frequencies, and that male fitness of F1hybrids, when pollinating B. rapa seeds, is low even when their female fitness (seed set) is high.

Preferential exclusion of hybrids in mixed pollinations between oilseed rape (Brassica napus) and weedy B. campestris (Brassicaceae).

In most experimental hybridizations between oilseed rape (Brassica napus) and weedy B. campestris, either intra- or interspecific pollen has been applied to individual flowers. Under field conditions, however, stigmas will often receive a mixture of the two types of pollen, thereby allowing for competition between male gametophytes and/or seeds within pods. To test whether competition influences the success of hybridization, pollen from the two species was mixed in different proportions and applied to stigmas of both species. The resulting seeds were scored for paternity by isozyme and randomly amplified polymorphic DNA analysis. Using data on the proportion of fully developed seeds and the proportion of these seeds that were hybrids, a statistical model was constructed to estimate the fitness of conspecific and heterospecific pollen and the survival of conspecific and heterospecific zygotes to seeds. B. campestris pollen in B. napus styles had a significantly lower fitness than the conspecific pollen, whereas no difference between pollen types was found in B. campestris styles. Hybrid zygotes survived to significantly lower proportions than conspecific zygotes in both species, with the lowest survival of hybrid zygotes in B. napus pods. This is in contrast to the higher survival of hybrid seeds in B. napus than in B. campestris pods when pollinations are made with pure pollen. Altogether, the likelihood of a foreign pollen grain producing a seed was much lower on B. napus than on B. campestris. In addition, pods on B. napus developed to a lower extent the more heterospecific pollen was in the mix, whereas this had no effect on B. campestris.

Hybrids between cultivated and wild carrots in natural populations in Denmark

Many cultivated plant species are able to hybridize with related wild plants. However, it is not clear whether their hybrids are able to survive and reproduce outside managed fields, and if cultivar genes introgress into wild populations. In areas where wild carrots co-occur with carrot root-crops, pollen and seeds may flow from two different sources in the fields to the surrounding wild populations: from pure cultivar plants that occasionally flower, and from flowering ‘bolters’ that originate from hybridizations between wild (♂) and cultivated carrots (♀) in seed production fields in warmer regions of the world. To test whether hybrids are formed and survive in wild Danish populations, and whether prolonged hybridization has led to introgression of cultivar genes, we collected leaf material from adult individuals growing close to carrot fields and analysed their genotypic composition by AFLP. Four hybrids were identified among the 71 plants analysed, and these were most likely F2 or backcross individuals, sired by pollen from hybrid bolters. Wild populations close to fields were genetically somewhat more similar to cultivars than wild populations far from fields, suggesting that neutral or beneficial cultivar alleles can introgress into the wild gene pool. Despite generations of improvement and adaptation of cultivar carrots to highly managed field conditions, hybrids can thus sometimes survive in wild populations close to carrot fields, and their genes transfer to wild populations by introgression.

Germination, predation and dispersal of Acacia albida seeds

Zambia were found to be 13.1% for seeds collected early in the fruiting season and 8.9% in late season. There was no indication of reinfestations by bruchids. Seeds from dry cow dung were found to be intact (65%) or decayed (35%), suggesting that all bruchid-infested seeds were killed in the guts or in the cow dung. Untreated seeds from pods had very low germination (2%), but were almost all alive, as indicated by the high germination when scarified. Seeds from cow dung behaved similarly. Therefore, all seeds that were able to germinate at defecation had subsequently died in the dung. Seeds with visible bruchid exit holes had a high germination rate if collected early in the season, but were all dead later in the season. Pods kept under water did not sink for a week, and water-soaked seeds remained alive for up to 16 days, suggesting that water dispersal may locally play a role in the dispersal of A. albida seeds. Survival chances for cattle-dispersed and un-dispersed A. albida seeds were calculated from the proportions of intact and damaged seeds, death rates in cow dung, and germination rates obtained in this experiment, combined with estimates from other studies. Seed survival when dispersed by cattle seemed to be equal or lower than when