Genetic and sexual separation between insect resistant and susceptible Barbarea vulgaris plants in Denmark.

Co-evolution between herbivores and plants is believed to be one of the processes creating Earth's biodiversity. However, it is difficult to disentangle to what extent diversification is really driven by herbivores or by other historical-geographical processes like allopatric isolation. In the cruciferous plant Barbarea vulgaris, some Danish individuals are resistant to herbivory by flea beetles (Phyllotreta nemorum), whereas others are not. The flea beetles are, in parallel, either resistant or susceptible to the plants defenses. To understand the historical-evolutionary framework of these interactions, we tested how genetically divergent resistant and susceptible plants are, using microsatellite markers. To test whether they are reproductively fully compatible, resistant and susceptible plants were grown intermixed in an outdoor experiment, and the paternity of open-pollinated offspring was determined by analysis of molecular markers. Resistant and susceptible Danish plants were genetically strongly differentiated and produced significantly fewer hybrids than expected from random mating or nearest neighbour mating. Our results suggest that the two types belong to different evolutionary lineages that have been (partly) isolated at some time, during which genetic and reproductive divergence evolved. A parsimonious scenario could be that the two plant types were isolated in different refugia during the previous ice age, from which they migrated into and met in Denmark and possibly neighbouring regions. If so, resistance and susceptibility has for unknown reasons become associated with the different evolutionary lineages.

Genetic variation of inbreeding depression among floral and fitness traits in Silene nutans.

The magnitude and variation of inbreeding depression (ID) within populations is important for the evolution and maintenance of mixed mating systems. We studied ID and its genetic variation in a range of floral and fitness traits in a small and large population of the perennial herb Silene nutans, using controlled pollinations in a fully factorial North Carolina II design. Floral traits and early fitness traits, that is seed mass and germination rate, were not much affected by inbreeding (delta<0.2). In contrast, 'late' fitness traits and multiplicative fitness suffered severely from inbreeding (delta>0.4). Lack of genetic correlations indicated that ID in floral, early and late traits is genetically decoupled. There was a trend that the smaller population was less affected by ID than the large one, although the differences were not significant for most traits. Hence, evidence for purging of deleterious alleles remains inconclusive in this study. Genetic variation in ID among paternal families was statistically significant in most floral and all seed traits, but not in late fitness traits. However, some paternal families had delta<0.5, even in the multiplicative fitness measure that suffered most from ID (delta=0.74), suggesting that the mixed mating system of S. nutans might be evolutionary stable.

The temporal development in a hybridizing population of wild and cultivated chicory (Cichorium intybus L.).

Hybridization and its possible impacts is a subject of increased attention in connection with the risk of unintended gene flow from cultivated (including genetically modified) plants to wild relatives. Whether such gene flow by hybridization is likely to take place depends among other things on the persistence of the hybrids in a natural environment over time. To evaluate this, we studied an experimental hybridizing population of wild and cultivated chicories (Cichorium intybus) relative to a previous study on the same population 2 years earlier. We compared the genetic composition, morphology and fitness traits of plants from 2004 to the plants in the same plot in 2002. The majority of the plants in 2004 was more morphologically and genetically intermediate than in 2002. This indicates that no selection towards being wild-like or cultivar-like was present over the period of 2 years. Furthermore, no distinct fitness differences existed between the plants of 2004, probably due to most of the plants being intermediate. No hybridization barriers appeared to be present between wild and cultivated chicories beyond the F1 generation, since F2 hybrids and backcrosses were in abundance; in fact, hybrids of probably fourth or fifth generation were present. In conclusion, all results indicate that no barriers exist to the temporal persistence of chicory hybrids in a natural environment.

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 (male) and cultivated carrots (female) 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 F(2) 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.

Genealogy, morphology and fitness of spontaneous hybrids between wild and cultivated chicory (Cichorium intybus).

Crop species are known to hybridize spontaneously with wild relatives, but few studies have characterized the performance of hybrids at various genealogies, life stages and environments. A group of cultivar-like individuals and potential hybrids were observed in a roadside population of wild chicory plants in Denmark. Seeds were collected from all reproductive plants and grown in a common garden experiment, and their morphological and genetic compositions were analysed. Intermediate plants were identified as hybrids and comprised various backcross and F(n) combinations. A genotypic hybrid index (HI), spanning from wild-like to cultivar-like, was highly correlated to a morphological index. Plant survival, growth and reproduction were evaluated and compared to the genotypic HI. Overall, cultivar-like and intermediate plants grew larger than wild-like plants, flowered longer, and produced more flowers and seeds. The common garden included a nutrient gradient. At higher nutrient levels, intermediate and cultivar-like plants produced more flowers and seeds than wild-like plants, whereas this effect was less pronounced at lower nutrient levels. During winter, small rodents consumed roots of cultivar-like and intermediate plants preferentially. Thus, cultivated and wild chicory are able to hybridize spontaneously, producing hybrid offspring of several generations that may reproduce more effectively than their wild parent, but herbivory and poor environmental conditions may negatively affect their fitness.

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 F(1) 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 F(1) 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 F(1), respectively. F(1) 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 F(1) hybrids is strongly influenced by their local frequencies, and that male fitness of F(1)hybrids, when pollinating B. rapa seeds, is low even when their female fitness (seed set) is high.