Walter Bleeker

Hybridization and Rorippa austriaca (Brassicaceae) invasion in Germany.

Introgressive hybridization between the invasive Rorippa austriaca and the native R. sylvestris in Germany has been studied using chloroplast DNA (trnL intron) and amplified fragment length polymorphism. Three hybrid zones between the invasive and native species were located in the Ruhr Valley (Mülheim) and at the River Main near Würzburg (Randersacker, Winterhausen). In each hybrid zone hybridization was indicated by additivity of region‐specific amplified fragment length polymorphism markers proving independent hybridization events. The hybrids were either morphologically intermediate (R. × armoracioides) or were close to R. sylvestris. The trnL intron of R. austriaca is characterized by a species‐specific deletion. This diagnostic chloroplast marker of R. austriaca was detected in three individuals of R. sylvestris providing evidence for introgression of the invasive chloroplast into the native species. Bidirectional introgression of R. austriaca markers into R. sylvestris and of R. sylvestris markers into R. austriaca was detected in the amplified fragment length polymorphism analysis. Some of the invasive R. austriaca populations showed high within‐population variation. A possible association among introgression, within‐population variation and invasion success is discussed. The morphologically intermediate hybrid R. × armoracioides is currently spreading in northern Germany. It forms large populations without its parent species R. austriaca and R. sylvestris. It is concluded that hybridization between invasive R. austriaca and native R. sylvestris may lead to the evolution of a new invasive species R. × armoracioides.

Molecular systematics ofCardamine and allied genera (Brassicaceae): Its and non-coding chloroplast DNA

Representatives of the generaCardamine, Dentaria, Nasturtium, Rorippa andArmoracia (Brassicaceae) were analyzed to elucidate their phylogenetic relationships based on nuclear (ITS) and non-coding chloroplast (cp) DNA sequences.Dentaria seems to be polyphyletic. The two studiedDentaria species group with differentCardamine clades, and it is argued thatD. bulbifera is an allopolyploid originating from a hybridization between aCardamine and aDentaria species. In the ITS tree,Nasturtium andRorippa form well supported clades but their relationship toCardamine andArmoracia remains unresolved. In the cpDNA tree,Nasturtium groups together withCardamine. Hybridization events apparently played a role in the evolution ofNasturtium. TheCardamine/Nasturtium clade is separated from a clade placingRorippa andArmoracia together.Armoracia is closely related toRorippa. Analyses of the 19Cardamine species studied revealed three main groupings, a northern hemispheric and two southern hemispheric groups. Within the northern hemisphere taxa theC. pratensis complex forms a well supported clade which seems to be closely related toC. amara, C. raphanifolia andC. flexuosa. The positions ofC. hirsuta andC. impatiens are uncertain. The two southern hemisphere clades consist of New Guinean species and south-eastern Australian/Tasmanian and subantarctic species, respectively. They may reflect migration routes from the northern to the southern hemisphere, but further studies are necessary to fully understand the evolution of the bihemispheric distribution pattern ofCardamine.

Evolutionary Processes Associated with Biological Invasions in the Brassicaceae

This review highlights evolutionary processes in the Brassicaceae which cause, accompany or are a consequence of biological invasions. Case studies in Capsella and Diplotaxis address the genetics of colonising species. The greatest colonising ability in Capsella bursa-pastoris is associated with polyploidy, predominant selfing, and high genetic diversity. Success of colonial populations seems to be due to the introduction of preadapted genotypes. Colonising species in Diplotaxis reveal contrasting evolutionary patterns. Genetic attributes in D. muralis include annuality, polyploidy, and predominant selfing. Very different from Capsella, D. muralis is nearly devoid of genetic diversity as revealed by molecular markers. In contrast to D. muralis, the colonising D. tenuifolia is perennial, diploid, self-incompatible, and displays high genetic diversity. Hybrid speciation, establishment of the hybrids in man-made habitats, stabilisation of their reproductive system, and reproductive isolation from the parent populations were analysed in Cardamine. The Nasturtium example highlights the importance of hybridisation for the evolution of invasiveness. The last case study concentrates on the evolutionary consequences of hybridisation between native and invading Rorippa species. Introgressive hybridisation between the invasive Rorippa austriaca and native Rorippa species is common and widespread in central Europe, and interspecific gene transfer has led to the formation of a new invasive genotype. Each successful invasion presents new aspects and sound case studies are needed in order to understand the ecology and evolution of the colonisation process and to enable us to assess the evolutionary consequences of biological invasions.

Biogeography and Phylogeny of Cardamine (Brassicaceae)1

Abstract The biogeography and phylogeny of Cardamine L. were inferred based on sequences of the nuclear ribosomal ITS regions and the plastid trnL intron and trnL-F spacer regions. This genus is one of the largest and polyploid-rich genera of the Brassicaceae and has its center of diversity in Eurasia. Species were included from all populated continents, representing all sections except two monotypic ones. The results support a hypothesis of recent and rapid speciation in the genus. The traditional sectional classification was not supported. We found evidence for several extremely long-distance dispersal events. Colonization of the Southern Hemisphere and the Arctic has occurred repeatedly; we identified at least three phylogenetically distinct Arctic lineages, two distinct Oceanian lineages, and four distinct South American lineages. Polyploidization has occurred independently many times during the evolution of Cardamine. Recent divergence combined with widespread polyploidization offer an explanation for the complex taxonomy of the genus.

Evolution of hybrid taxa in Nasturtium R.Br. (Brassicaceae).

Nasturtium officinale (2n=4x=32),N. microphyllum (2n=8x=64) andN. ×sterile (2n=6x=48) have been investigated by isozyme analyses to study evolutionary processes withinNasturtium. Four additional species assumed to be involved in the formation of the octoploidN. microphyllum (Rorippa amphibia, R. sylvestris, R. palustris, andCardamine amara) were also examined. A total of 641 individuals were analyzed for six isozyme systems (alcohol dehydrogenase, aspartate aminotransferase, glutamate dehydrogenase, leucine aminopeptidase, malate dehydrogenase, and phosphoglucoisomerase). Eleven gene complexes coding for 43 allozymes were detected. Fifteen alleles were observed inN. officinale, twelve of them being fixed. All alleles fixed inN. officinale were also present and fixed inN. microphyllum. Seven additional fixed alleles were observed inN. microphyllum. The presence of these seven alleles inRorippa taxa provide evidence for an allopolyploid origin ofN. microphyllum withN. officinale and aRorippa taxon involved.C. amara is not a parent species ofN. microphyllum. N. ×sterile showed a fixed banding pattern which was identical to that ofN. microphyllum. It is argued thatN. ×sterile is a hybrid betweenN. officinale andN. microphyllum. Human impact has played a major role in the evolution ofN. ×sterile. The formation and persistence of the hybrid were influenced by introducingN. officinale into the natural distribution area ofN. microphyllum and by creating ditches and ponds where due to its vegetative capabilitiesN.×sterile could establish.

Genetic differentiation and reproductive isolation of a naturally occurring floral homeotic mutant within a wild‐type population of Capsella bursa‐pastoris (Brassicaceae)

Apart from the common floral architecture in Brassicaceae, variation in flower morphology occurs in several genera within the family and is considered to affect speciation processes. We analysed genetic differentiation and flowering time variation of two floral variants of Capsella bursa‐pastoris, the Spe variant and the wild‐type, which occur sympatrically in a vineyard in southwest Germany. The Spe variant is characterized by an additional whorl of stamens instead of petals and was formerly classified as an independent taxon ‘Capsella apetala’ Opiz. Amplified fragment length polymorphism and allozyme analysis revealed a substantial genetic differentiation of the two floral variants and a higher genetic variation within the wild‐type subpopulation compared with the Spe subpopulation. The low genetic variation in the mutant provided evidence of a recent local origin or recent introduction. Flowering time analysis indicated that, within the analysed population, the Spe variant flowers significantly later than the wild‐type (P < 0.001). We conclude that the evolution and persistence of Spe within a wild‐type population is facilitated by high selfing rates and been enhanced by a shift in flowering phenology. Hence, our data provide substantial evidence that the Spe phenotype has established itself as an isolated entity within a wild‐type population and may thus serve as a model for the analysis of the evolutionary significance of homeotic mutants in wild populations.

Interspecific hybridization in Rorippa (Brassicaceae): Patterns and processes

Abstract The current knowledge on natural hybridization between European lowland Rorippa species is reviewed. Morphological, cytological and molecular markers provide substantial evidence that four Rorippa species (R. amphibia, R. austriaca, R. palustris and R. sylvestris) are involved in hybridization processes. The main factors promoting initial hybridization events between Rorippa species in Europe are (1) natural and anthropogenic habitat disturbance, (2) the invasion of non‐native R. austriaca into western central Europe and (3) the self‐incompatibility system of R. amphibia, R. austriaca and R. sylvestris coupled with clonal growth. Outcomes of hybridization between European Rorippa species show a broad range from incidental hybrids through the formation of hybrid swarms to the evolution of new hybrid taxa. Rorippa × armoracioides is certainly the most successful Rorippa hybrid. It results from hybridization between R. austriaca and R. sylvestris and is known from many localities in the native range of R. austriaca and in regions invaded by R. austriaca. Within this system hybrid zones of different age provide the rare opportunity to analyse different stages of hybrid formation and hybrid speciation in natural populations.