Hexaploid Taxa Research Articles

Allopolyploidy: An Underestimated Driver in Juniperus Evolution.

Allopolyploidy is considered as a principal driver that shaped angiosperms' evolution in terms of diversification and speciation. Despite the unexpected high frequency of polyploidy that was recently discovered in the coniferous genus Juniperus, little is known about the origin of these polyploid taxa. Here, we conducted the first study devoted to deciphering the origin of the only hexaploid taxon in Juniperus along with four of its closely related tetraploid taxa using AFLP markers with four primers combinations. Phylogenetic analysis revealed that the 10 studied species belong to 2 major clusters. J. foetidissima appeared to be more related to J. thurifera, J. sabina, and J. chinensis. The Bayesian clustering analysis showing a slight variation in genetic admixture between the studied populations of J. foetidissima, suggesting an allopolyploid origin of this species involving J. thurifera and J. sabina lineages, although a purely autopolyploidy origin of both J. thurifera and J. foetidissima cannot be ruled out. The admixed genetic pattern revealed for J. seravschanica showed that the tetraploid cytotypes of this species originated from allopolyploidy, whereas no clear evidence of hybridization in the origin of the tetraploid J. thurifera and J. chinensis was detected. This study provides first insights into the polyploidy origin of the Sabina section and highlights the potential implication of allopolyploidy in the evolution of the genus Juniperus. Further analyses are needed for a more in-depth understanding of the evolutionary scenarios that produced the observed genetic patterns.

Genome size, ploidy levels, and development of novel SSR primer to evaluate genetic diversity of Corylopsis Siebold & Zucc. germplasm collections

Corylopsis Seibold & Zucc. is a genus of flowering shrubs with few cultivars and little information on genome size, ploidy levels, and genetic relationships among taxa. The United States National Arboretum and collaborating gardens in North America maintain a diverse germplasm collection of Corylopsis including cultivated and wild-collected taxa. The objectives of this study were to determine genome size, ploidy, and genetic diversity in Corylopsis. Flow cytometry revealed a ploidy series of diploid, tetraploid, pentaploid, and hexaploid taxa in Corylopsis with 2C genome sizes ranging from 1.9 pg to 5.7 pg. Representative taxa were selected for root tip cytology, revealing diploid (C. platypetala Rehder & E.H. Wilson), tetraploid (C. gotoana Makino), pentaploid (unnamed hybrid selection), and hexaploid (C. willmottiae Rehder & E.H. Wilson) taxa. Seventeen genomic simple sequence repeat markers were developed and used to determine the genetic diversity of Corylopsis. These markers had an average of 7.4 alleles per locus with a range of three to 17 alleles. A dendrogram was generated by unweighted pair group method with arithmetic mean (UPGMA) cluster analysis using the Jaccard similarity coefficient to visualize genetic relationships and ploidy series within taxa. Information gained from this study will further breeding efforts and aid management of germplasm collections of Corylopsis.

Genetic similarities versus morphological resemblance: Unraveling a polyploid complex in a Mediterranean biodiversity hotspot

•The Balkan Peninsula is recognized as one of the hotspots of biodiversity in Europe. This area has shown since the Last Glacial Maximum appropriate conditions for species diversification and hybridization, which has led to the existence of numerous taxonomically unresolved entities. Here, we focus on the Western Balkans and explore the genetic structure and relationships among species belonging to the V. austriaca - V. orbiculata diploid-polyploid complex, including populations showing intermediate morphologies.•A combination of nuclear markers (microsatellites), plastid DNA regions (trnH-psbA, ycf6-psbM) and ploidy level estimations using flow cytometry are employed to assess the genetic structure and evolutionary dynamics of this polyploid complex. To reconstruct the evolutionary history, an approximate Bayesian computation approach is combined with projections of the species distribution models onto the climatic scenarios of the Mid-Holocene (6 ka BP) and Last Glacial Maximum (22 ka BP).•Four main groups were found: one well-established entity within the diploid level, V. dalmatica, a second diploid-tetraploid group which corresponds to V. orbiculata, a hexaploid cluster harboring V. austriaca subsp. jacquinii individuals, and an enigmatic tetraploid group. According to the molecular data obtained, this latter cluster represents an allopolyploid cryptic lineage −with V. orbiculata and V. dalmatica as putative parents− morphologically similar to V. orbiculata, but genetically more related to V. austriaca subsp. jacquinii. Veronica dalmatica and this “uncertain tetraploid” group are involved in the formation of the hexaploid taxon V. austriaca subsp. jacquinii, with the possibility of recent gene flow among different cytotypes.•The present study supports a scenario of diversification from a diploid common ancestor leading to two different but interrelated lineages. The first one would correspond with the diploid V. orbiculata plus tetraploid individuals of this species arising through allo- and autopolyploidization, and the second one would involve all ploidy levels with allopolyploidization being prevalent.

Phenotypic and Genetic Variation of an Interspecific Centaurium Hybrid (Gentianaceae) and Its Parental Species.

Interspecific hybridization is one of the major actuators of evolutionary changes in plants. As the result of allopolyploid hybridization, offspring may gain different ploidy levels in comparison to parental species, which can provide them instant reproductive isolation. Two tetraploid sister species, Centaurium erythraea and C. littorale, readily cross-fertilize, resulting in hybrids of various ploidy. In northern Serbia, two stable populations of a hexaploid taxon C. pannonicum have been documented. It has been proposed previously that this taxon emerged after an interspecific hybridization event between two tetraploid sister-species: C. erythraea and C. littorale subsp. compressum. The existing populations of the hybridogenic taxon, as well as neighboring populations of the two parental taxa were here characterized by both morphometrics and molecular markers (EST-SSR and trnL-F). Three leaf and two flower characteristics were found to be informative in delimitation of the parental taxa and in their discernment from hybrid individuals, the latter having intermediate values. Eight microsatellite markers were found to have good ability to distinguish studied taxa, placing C. pannonicum in closer relationship with C. erythraea. Conversely, trnL-F plastid marker nominated C. littorale subsp. compressum to be the donor of the C. pannonicum plastid DNA. Reproductive isolation of the hexaploid hybrid individuals from the parental species should be examined as the next logical step in describing the new species.

Chenopodium luteorubrum (Chenopodiaceae/Amaranthaceae sensu APG): a remarkable new species from the Russian Far East

Chenopodium luteorubrum is described as a new species of Chenopodiaceae (Amaranthaceae sensu APG) from coastal habitats along the Sea of Japan in the Russian Far East. It has been demonstrated that this allohexaploid species (2n = 6x = 54) combines in its genome the subgenomes ‘A’ (closely related to or inherited from C. bryoniifolium), ‘C’ (unknown or extinct species), and ‘D’ (C. acuminatum s. l.). That subgenome combination is unique; it differs from the combination in C. album s. str. and closely related hexaploid taxa, which have ‘B’ (derived from C. ficifolium/C. suecicum), ‘C’, and ‘D’ subgenomes. Here we describe and illustrate the new species and provide information on its evolution, ecology, morphology, karyology, and distribution. The new species is compared with morphologically somewhat similar taxa C. album s. str. (hexaploid), C. suecicum (diploid), C. betaceum s. l. (C. strictum auct., tetraploid), and C. acuminatum (diploid).

Genome skimming provides new insight into the relationships in Ludwigia section Macrocarpon, a polyploid complex.

Interpreting relationships within groups containing polyploids, which are frequent in angiosperms, can be greatly assisted by genomic techniques. In this study, we used a genome-skimming approach to investigate the evolutionary relationships and origins of polyploids in the monophyletic group, Ludwigia section Macrocarpon (Onagraceae), which includes diploid, tetraploid, and hexaploid taxa. We sampled all known taxa and ploidy levels in the section and conducted shotgun sequencing. We assembled plastomes, mitochondrial sequences, and completed nuclear ribosomal regions, reconstructed phylogenies, and conducted comparative genomic analyses for plastomes to gain insights into the relationships among studied taxa. Within the section, results showed that the South American diploid taxa L. bonariensis and L. lagunae were closely related. We reported the first chromosome count (2n = 4× = 32) for L. neograndiflora, which is closely related to the two South American diploid taxa, although its exact origin remains unclear. The samples of the widespread, polyploid taxon L. octovalvis do not form a monophyletic group. Both tetraploid and hexaploid L. octovalvis lineages have originated more than once. At least one tetraploid in the L. octovalvis lineage may have been involved in the origins of hexaploids. One or more extinct/unsampled intermediate tetraploids in the L. octovalvis lineages had also likely been involved in the origins of hexaploids. Genome skimming provided important insights into the complex evolutionary relationships within sect. Macrocarpon, but additional sampling and data from single-copy nuclear regions are necessary to further elucidate the origins of the polyploids in this section.

Molecular cytogenetic characterization of Triticum timopheevii chromosomes provides new insight on genome evolution of T. zhukovskyi

Triticum timopheevii (2n = 4x = 28, GGAtAt) is a tetraploid wheat formerly cultivated in western Georgia. The natural allopolyploid Triticum zhukovskyi is a hexaploid taxon originated from hybridization of T. timopheevii with cultivated einkorn T. monococcum (2n = 2x = 14, AmAm). Karyotypically T. timopheevii and T. zhukovskyi differ from other tetraploid and hexaploid wheats and were assigned to the section Timopheevii of the genus Triticum L. Triticum timopheevii and T. zhukovskyi are resistant to many fungal diseases and therefore could potentially be utilized for wheat improvement. We were aiming to precisely identify all T. timopheevii chromosomes and to trace the evolution of T. zhukovskyi. For this, we developed a set of molecular cytogenetic landmarks based on eleven DNA probes. Each chromosome can now be characterized by two to eight probes. The pTa-535 sequence allows the identification of all At-genome chromosomes, whereas G-genome and some At-genome chromosomes can be identified using (GAA/CTT) n and pSc119.2 probes. The probes pAesp_SAT86, pAs1, Spelt-1, Spelt-52 and 5S and 45S rDNA can be applied as additional markers to discriminate particular chromosomes or chromosomal regions. The distribution of (GAA/CTT) n , pTa-535 and pSc119.2 DNA probes on T. timopheevii chromosomes is distinct from other tetraploid wheats and can therefore be used to track individual chromosomes in introgression programs. Our study confirms the origin of T. zhukovskyi from hybridization of T. timopheevii with T. monococcum; however, we show that the emergence was accompanied by changes involving mostly At-genome chromosomes. This may be due to the presence of two closely related A-genomes in the T. zhukovskyi karyotype.

Molecular cytogenetic identification and phenotypic description of a new synthetic amphiploid, Triticum timococcum (AtAtGGAmAm).

A recently developed synthetic amphiploid, Triticum timococcum Kost., nom. nud. (2n = 6x = 42, AtAtGGAmAm) is described in the present study. This hexaploid taxon was developed by colchicine treatment in Martonvásár from the hybrid of a selected accession of Triticum timopheevii Zhuk. (2n = 4x = 28, AtAtGG) and a prebred semi-dwarf line of Triticum monococcum L. (2n = 2x = 14, AmAm). A detailed cytomolecular examination was carried out using the sequential multicolour fluorescence and genomic in situ hybridization techniques (FISH and mcGISH). It was proved that T. timococcum has 42 chromosomes originating from its parents. The chromosomes of the A genomes of T. monococcum and T. timopheevii could be distinguished in the amphiploid using FISH. The successful discrimination of the chromosomes was supported by the karyotypes of the three genomes and the successful optimization of the mcGISH technique for the A and G chromosomes achieved in the present study. A phenotypic evaluation was also carried out under natural and artificial growing conditions in 2012 and 2013. Based on the results, T. timococcum has intermediate characteristics in terms of spike (spikelet) shape and plant height, while it is similar to the female parent, T. timopheevii regarding pubescence. Like its parents, T. timococcum showed outstanding resistance to the main fungal diseases of wheat. T. timococcum headed later and developed longer and looser spikes, fewer tillers and only a third as many seeds than its parents. The third generation of T. timococcum was able to develop an acceptable number of seeds, even taking into account the reduced germination ability in the field.

Species-genomic relationships among the tribasic diploid and polyploid Carthamus taxa based on physical mapping of active and inactive 18S–5.8S–26S and 5S ribosomal RNA gene families, and the two tandemly repeated DNA sequences

In the genus Carthamus (2n=20, 22, 24, 44, 64; x=10, 11, 12), most of the homologues within and between the chromosome complements are difficult to be identified. In the present work, we used fluorescent in situ hybridisation (FISH) to determine the chromosome distribution of the two rRNA gene families, and the two isolated repeated DNA sequences in the 14 Carthamus taxa. The distinctive variability in the distribution, number and signal intensity of hybridisation sites for 18S–26S and 5S rDNA loci could generally distinguish the 14 Carthamus taxa. Active 18S–26S rDNA sites were generally associated with NOR loci on the nucleolar chromosomes. The two A genome taxa, C. glaucus ssp. anatolicus and C. boissieri with 2n=20, and the two botanical varieties of B genome C. tinctorius (2n=24) had diagnostic FISH patterns. The present results support the origin of C. tinctorius from C. palaestinus. FISH patterns of C. arborescens vis-à-vis the other taxa indicate a clear division of Carthamus taxa into two distinct lineages. Comparative distribution and intensity pattern of 18S–26S rDNA sites could distinguish each of the tetraploid and hexaploid taxa. The present results indicate that C. boissieri (2n=20) is one of the genome donors for C. lanatus and C. lanatus ssp. lanatus (2n=44), and C. lanatus is one of the progenitors for the hexaploid (2n=64) taxa. The association of pCtKpnI-2 repeated sequence with rRNA gene cluster (orphon) in 2–10 nucleolar and non-nucleolar chromosomes and the consistent occurrence of pCtKpnI-1 repeated sequence at the subtelomeric region in all the taxa analysed indicate some functional role of these sequences.

Polyploidy associated with altered and broader ecological niches in the Claytonia perfoliata (Portulacaceae) species complex

Polyploids are often hypothesized to have distinct and broader niches than their diploid progenitors. Differences in geographic distributions between diploid and polyploids are frequently used to infer niche differentiation and increased breadth, but they are seldom used to test these hypotheses explicitly. Niche overlap and breadth were compared for diploids, tetraploids, and hexaploids of three taxa in the Claytonia perfoliata complex (C. parviflora, C. perfoliata, and C. rubra) with the use of species distribution models. Resampling and randomization approaches were used to test hypotheses of niche differentiation, breadth, and conservatism. Niche differentiation was detected between polyploid and diploid cytotypes assigned to the same taxon (e.g., C. parviflora 2× vs. 4×) but not between hexaploids and tetraploids within a taxon (e.g., C. parviflora 4× vs. 6×). Individual hexaploid cytotypes had broader ecological niches than individual diploid cytotypes. However, as a group the three hexaploid taxa did not exceed the combined niche breadth of the three diploids, suggesting that polyploidy does not result in transgressive niche breadth for this group. Niche overlap was lowest among diploids and was highest among the three hexaploid cytotypes, consistent with introgression associated with polyploidy resulting in greater ecological similarity. Although cytotypes possessed nonidentical niches, after accounting for environmental differences among ranges, cytotypes were more similar than expected, suggesting niche conservatism and similar responses to environmental characteristics. These results suggest that polyploids occupy distinct and broader niches relative to diploids but that cytotypes also share fundamentally similar responses to environmental variation across ploidy levels.

Phylogenetic relationships among the polyploid and diploidAegilopsspecies inferred from the nuclear 5S rDNA sequences (Poaceae: Triticeae)

Phylogenetic inferences of the polyploid Aegilops taxa were drawn based upon the analysis of 909 nuclear 5S rDNA sequences obtained from 15 Aegilops polyploid taxa (531 sequences new to this paper) and 378 sequences from our previous study on the diploid taxa. The 531 sequences can be split into two orthologous groups (unit classes), the long AE1 and short AE1 previously identified in the diploid set. An examination of the relationships between unit classes and their associated haplomes suggests that U haplome sequences found in Ae. umbellulata are the closest to the T sequences found in Amblyopyrum muticum and that sequences of the polyploid species expected to be the M type found in Ae. comos are more similar to the T haplome sequences, except in the three hexaploids Ae. glumiaristata, Ae. juvenalis, and Ae. vavilovii and the tetraploid Ae. crassa where they are found to be similar to the M haplome sequences. These three hexaploid taxa likely originated from the tetraploid Ae. crassa (DM), while the closest taxon to the fourth hexaploid, Ae. recta, is the tetraploid Ae. neglecta (UM). Based upon the distribution of the unit classes, several reticulate phylogenies depicting evolutionary relationships among diploid, tetraploid, and hexaploid taxa were constructed; however, none of these widely used methods could depict the expected reticulate relationship as previously drawn from cytogenetic analyses in this group of allopolyploid species. These results suggest that evolutionary relationships derived from models based upon the assumption of bifurcating species require careful interpretation when these same models are applied to species with reticulate evolution.

Nitrogen and Phosphorus Efficiency in Wild and Cultivated Species of Wheat

ABSTRACT The wild and primitive species of Aegilops and Triticum genera are potentially important and not fully recognized sources of a novel variation for wheat breeding. However, less is known on their adaptation to macronutrient shortages. Nutrition- and genotype-dependent variation in the nitrogen (N) and phosphorus (P) efficiency was evaluated among these and cultivated accessions of the Aegilops and Triticum genera. In a factorial experiment, plants were grown in 9 dm3 soil-pots under high and reduced N-P-potassium (K) fertilization. The limited nutrition caused a decrease in uptake efficiency and nutrient concentrations in grains. However, essential increases in the efficiency of N and P utilization in grain mass formation were noticed under the nutrients shortage. The examined accessions exhibited a broad genotypic variation in the efficiency of N and P use and the response to limited fertilization. This variation was also dependent upon ploidy levels, identity and interactions of genomes present. Significant genotype by nutrition interactions were noticed for the uptake efficiencies, nutrient harvest indices and utilization efficiency ratios. The cultivated and primitive, polyploid Triticum species were the most efficient in N and P utilization. The diploid Aegilops and Triticum accessions were unable to use N and P efficiently and only Ae. tauschii accessions exhibited a moderate tolerance to the reduced nutrition. In turn, the wild and primitive tetraploid and hexaploid taxa usually combined high uptake efficiency with enhanced tolerance to nutrients shortage. The results suggest that the tetraploid T. carthlicum, T. dicoccoides, and T. timopheevii and the hexaploid T. sphaerococcum are the most promising taxa for wheat breeding.

Genetic divergence among morphotypes of Lake Tana (Ethiopia) barbs

The genus Barbus is a paraphyletic assemblage that includes three different ploidy levels: diploid, tetraploid and hexaploid. ‘B.’intermedins, which inhabits Lake Tana (Ethiopia), is a hexaploid taxon that forms a ‘species flock’ consisting of at least 14 morphotypes. Thirty-one presumptive allozymic loci were analysed in a large sample of ‘B.’ intermedius. No diagnostic loci were observed between the morphotypes, but substantial differences in allele frequencies were found. One morphotype, Acute, differs significantly from the others. The presence of this genetic differentiation within the lake strongly suggests that the morphotypes are diverging lineages. Because of some parallel characteristics with cichlid species flocks, we put forward the hypothesis that the morphotypes of barbs in Lake Tana represent a species flock that originated via sympatric speciation with more rapid morphological than molecular differentiation. If the differentiation and the reproductive isolation are real, it is probable that the morphotypes already are or will soon reach the status of species.

Genetic divergence among morphotypes of Lake Tana (Ethiopia) barbs

The genusBarbusis a paraphyletic assemblage that includes three different ploidy levels: diploid, tetraploid and hexaploid.«B.» intermedius, which inhabits Lake Tana (Ethiopia), is a hexaploid taxon that forms a «species flock» consisting of at least 14 morphotypes. Thirty-one presumptive allozymic loci were analysed in a large sample of«B.» intermedius. No diagnostic loci were observed between the morphotypes, but substantial differences in allele frequencies were found. One morphotype, Acute, differs significantly from the others. The presence of this genetic differentiation within the lake strongly suggests that the morphotypes are diverging lineages. Because of some parallel characteristics with cichlid species flocks, we put forward the hypothesis that the morphotypes of barbs in Lake Tana represent a species flock that originated via sympatric speciation with more rapid morphological than molecular differentiation. If the differentiation and the reproductive isolation are real, it is probable that the morphotypes already are or will soon reach the status of species.

III‐Cytogenetic and molecular genetic data

Cyprinid fishes assigned to the genus Barbus constitute a poly‐phyletic assemblage that includes at least three different ploidy levels: diploid, tetraploid and hexaploid. ’B. ‘ intermedius, inhabiting Lake Tana (Ethiopia), is a hexaploid taxon that forms a ‘species flock’ consisting of at least 14 morphotypes. The phenomenon is described in this review with reference to currently available cytogenetic and molecular genetic data. Cytogenetic data show that ’B. ‘ intermedius is hexaploid both within and outside the Lake Tana basin. No chromosome banding analyses are currently available. Data based exclusively on conventionally Giemsa‐stained chromosomes suggest interpopulation variability. This should be confirmed using chromosome banding techniques. Major Histocompatibility Complex genes sequencing gave the first indication of genetic isolation of morphotypes. Allozymes have been analysed in a large sample of ’B.’ intermedius. No diagnostic loci were observed in the ’B.’ intermedius morphotypes, but more or less important differences in allele frequencies were found. Only one morphotype, Acute, differs significantly from the others. The presence of this genetically differentiated morphotype in the lake strongly suggests that the morphotypes are diverging lineages. Because of some parallel characteristics with cichlid species flocks, we put forward the hypothesis that the barb morphotypes in Lake Tana represent a species flock that originated via sympatric speciation with more rapid morphological than cytogenetic and molecular differentiation. If the differentiation and the reproductive isolation (at least partial) are real, it is probable that the morphotypes already are and/or soon will reach the status of species.

Typification of several names of theCerastium alpinumgroup(Caryophyllaceae)

Boscaiu. M., Marhold, K. & Ehrendorfer, F.: Typification of several names of the Cerastium alpinum group (Caryophyllaceae). — Willdenowia 27: 39–45. 1997. — ISSN 0511-9618.The names Cerastium eriophorum, C. villosum, C. lanatum, C. alpinum subsp. babiogorense and C. transsylvanicum are typified. C. eriophorum (= C. villosum) is the correct name for the diploid populations of the C. alpinum group from the Balkan mountains, Carpathians and Eastern Alps. C. lanatum is only applicable to densely hairy forms of the tetraploid C. alpinum s. str., and C. alpinum subsp. babiogorense is synonymous to this latter name. C. transsylvanicum is the correct name for the hexaploid taxon of the C. alpinum group, endemic to the Carpathians.

SYSTEMATIC INFERENCES FROM VARIATION IN ISOZYME PROFILES OF ARCTIC AND ALPINE CESPITOSE FESTUCA (POACEAE)

Variation in isozyme patterns was used to assess species boundaries in North American arctic and alpine representatives of the Festuca ovina L. complex. Isozyme profiles, in combination with chromosome number, delimit four discrete entities within the complex: F. brevissima Jurtzev (diploid); F. aggr. auriculata Drobov (diploid); F. brachyphylla Schultes (hexaploid); and tetraploid populations corresponding in morphology to F. baffinensis Polunin (arctic Canada) and F. minutiflora Rydberg (alpine United States). Although no fixed difference was detected between isozyme profiles of the latter two taxa, they are morphologically distinct. Thus variation in isozymes, morphology, and chromosome number delimits five taxa within the F. ovina complex in North America. Some alleles observed in the polyploid taxa were not detected among the diploids, and some observed in F. brachyphylla, the hexaploid taxon, were not detected in either the diploid or the tetraploid species. One possible explanation for these occurrences is that the North American polyploids originated in Eurasia, where many other potential diploid and tetraploid progenitors occur.

Meiotic studies ofElymus nutans andE. jacquemontii (Poaceae, Triticeae) and their hybrids withPseudoroegneria spicata and seventeenElymus species

Hybridizations ofElymus nutans andE. jacquemontii were carried out with one species ofPseudoroegneria (“S” genome), and 20Elymus species, each containing either of the “SH”, “SY”, “SYH”, or “SYW” genomes. Chromosome configurations were analysed at metaphase I of the two target taxa and their interspecific hybrids. It is concluded that (i)E. nutans is an allohexaploid containing the “SYH” genomes, andE. jacquemontii is an allotetraploid having the “SY” genomes; (ii) the genomic affinity is associated with the geographic distance between the species studied; (iii) minor genomic structural rearrangements have occurred within the hexaploid taxon ofE. nutans.

Entwicklungszyklus und Stoffproduktion von Populationen der Sippen Veronica hederifolia L. ssp. hederifolia und V. hederifolia L. ssp. lucorum (Klett et Richter) Hartl

Summary Investigations of two populations of V. hederifolia ssp. hederifolia and V. hederifolia ssp. lucorum revealed significant differences concerning determining parameters of their life-cycle, productivity and biomass allocation. The hexaploid taxon ssp. hederifolia, which is preferably distributed in agrestal weed communities, exhibits a higher degree of phenotypic plasticity within the life-cycle and a higher ability for dry matter production compared with the other (tetraploid) taxon, ssp. lucorum, which is found in forests and hedgerows. These characters are supposed to be responsible for the successful invasion of ssp. hederifolia into ecosystems with a higher degree of human impact and intensification, such as the agro-ecosystems, permitting ssp. hederifolia to play an important role in winterannual weed communities. The niche-differentiation presumably took place within an evolutionary process of unknown length. The degree of phenotypic or genotypic plasticity causing the observed ecological differences between both taxa could not be separated under our experimental conditions and needs further investigation.

Cytotaxonomy and breeding system of the genusBiscutella (Cruciferae)

Chromosome counts were determined for 46 populations ofBiscutella representing 28 taxa. The genus was found to contain diploid taxa with 2n = 12, 16 and 18, tetraploid taxa with 2n = 36 and hexaploid taxa having 2n = 54.B. laevigata L. s. l. consists of diploid and tetraploid populations which are poorly differentiated morphologically. TetraploidB. laevigata s. l. and hexaploidB. variegataBoiss. & Reuter (s. l.) are characterized by chromosomal instability. The variation in chromosome numbers and the occurrence of polyploidy is discussed in relation to the taxonomy of the genus. An investigation of the breeding system showed that most of the annual species were self-compatible and partly inbreeding and most of the perennial species self-incompatible and, therefore, outbreeding, while one annual species,B. cichoriifoliaLoisel., showed both systems.