Testing Local Host Adaptation and Phenotypic Plasticity in a Herbivore When Alternative Related Host Plants Occur Sympatrically

Host race formation in phytophagous insects can be an early stage of adaptive speciation. However, the evolution of phenotypic plasticity in host use is another possible outcome. Using a reciprocal transplant experiment we tested the hypothesis of local adaptation in the aphid Brevicoryne brassicae. Aphid genotypes derived from two sympatric host plants, Brassica oleracea and B. campestris, were assessed in order to measure the extent of phenotypic plasticity in morphological and life history traits in relation to the host plants. We obtained an index of phenotypic plasticity for each genotype. Morphological variation of aphids was summarized by principal components analysis. Significant effects of recipient host on morphological variation and life history traits (establishment, age at first reproduction, number of nymphs, and intrinsic growth rate) were detected. We did not detected genotype × host plant interaction; in general the genotypes developed better on B. campestris, independent of the host plant species from which they were collected. Therefore, there was no evidence to suggest local adaptation. Regarding plasticity, significant differences among genotypes in the index of plasticity were detected. Furthermore, significant selection on PC1 (general aphid body size) on B. campestris, and on PC1 and PC2 (body length relative to body size) on B. oleracea was detected. The elevation of the reaction norm of PC1 and the slope of the reaction norm for PC2 (i.e., plasticity) were under directional selection. Thus, host plant species constitute distinct selective environments for B. brassicae. Aphid genotypes expressed different phenotypes in response to the host plant with low or nil fitness costs. Phenotypic plasticity and gene flow limits natural selection for host specialization promoting the maintenance of genetic variation in host exploitation.     

Morphological,ecological and biological variations in the mustard aphid,Lipaphis pseudobrassicae (Kaltenbach) (Hemiptera: Aphididae) from different host plants

Genetic and morphological differentiation of insect populations in relation to the use of different host plants is an important phenomenon that leads to ecological specialization. In this study, we describe variations in morphology, and in ecological and biological parameters of Lipaphis pseudobrassicae (Kaltenbach) clones associated with three host species of Cruciferae, Brassica juncea (L.) var. rai sarson Czern and Cross (brown mustard), Brassica campestris L. var. sarson Prain (yellow mustard), and Rorippa indica (L.) Hiern (wild herb). This study was aimed at obtaining evidence regarding phenotypic differentiation induced by, or associated with, the use of distinct host species. Ten morphological characters, 4 growth parameters and 8 biological functions were investigated in wingless aphids collected from plants of the three host species. Aphids from B. campestris and B. juncea clones were bigger in size, heavier in weight and showed higher growth rates and fecundity than the clones from R. indica. Between the two crop plants, clones from B. juncea showed significantly higher growth rates than the clones from B. campestris. Transfer of L. pseudobrassicae populations from B. campestris to B. juncea and R. indica and vice versa resulted in poor performance. Results indicate that the average phenotype of L. pseudobrassicae individuals inhabiting different host plant species differs as a consequence of the contrasting feeding environments the host species provide.     

Effect of cultivars with varying levels of resistance to aphids on development time,sex ratio,size and longevity of the parasitoid <Emphasis Type="Italic">Aphidius colemani</Emphasis>

The influence of cultivars of common cabbage, Brassica oleracea var. capitata with varying levels of resistance to Brevicoryne brassicae (L.) and Myzus persicae (Sulzer) on key biological characteristics of Aphidius colemani (Viereck) was investigated under laboratory conditions. The total development time for female parasitoids reared on M. persicae did not differ significantly between Minicole (green-leaved, partially resistant with antibiosis factors for B. brassicae) and Derby Day (green-leaved, susceptible to both aphid species); but development was significantly faster (ca 10%) on Ruby Ball (red-leaved, partially resistant with antixenosis factors for B. brassicae). Total development time for females reared on B. brassicae was slightly shorter on Ruby Ball than on Minicole. Males reared on M. persicae developed into adults significantly faster (ca 10%) on Ruby Ball than on Minicole. However, when B. brassicae was the host, no significant variations in development time were observed. Sex ratios, size and longevity of both male and female parasitoids on either host were not significantly influenced by cultivar. The results are discussed in relation to the compatible utilisation of host-plant resistance and biological control in the integrated management of aphids.     

Tri-trophic effects of inter- and intra-population variation in defence chemistry of wild cabbage (<Emphasis Type="Italic">Brassica oleracea</Emphasis>)

The effect of direct chemical defences in plants on the performance of insect herbivores and their natural enemies has received increasing attention over the past 10 years. However, much less is known about the scale at which this variation is generated and maintained, both within and across populations of the same plant species. This study compares growth and development of the large cabbage butterfly, Pieris brassicae, and its gregarious pupal parasitoid, Pteromalus puparum, on three wild populations [Kimmeridge (KIM), Old Harry (OH) and Winspit (WIN)] and two cultivars [Stonehead (ST), and Cyrus (CYR)] of cabbage, Brassica oleracea. The wild populations originate from the coast of Dorset, UK, but grow in close proximity with one another. Insect performance and chemical profiles were made from every plant used in the experiment. Foliar glucosinolates (GS) concentrations were highest in the wild plants in rank order WIN > OH > KIM, with lower levels found in the cultivars. Caterpillar-damaged leaves in the wild cabbages also had higher GS levels than undamaged leaves. Pupal mass in P. brassicae varied significantly among populations of B. oleracea. Moreover, development time in the host and parasitoid were correlated, even though these stages are temporally separated. Parasitoid adult dry mass closely approximated the development of its host. Multivariate statistics revealed a correlation between pupal mass and development time of P. brassicae and foliar GS chemistry, of which levels of neoglucobrassicin appeared to be the most important. Our results show that there is considerable variation in quantitative aspects of defensive chemistry in wild cabbage plants that is maintained at very small spatial scales in nature. Moreover, the performance of the herbivore and its parasitoid were both affected by differences in plant quality.     

Interactions among three trophic levels: the influence of host plant on performance of Pieris brassicae and its parasitoid, Cotesia glomerata

The relative suitability of four host plants was determined both for unparasitized Pieris brassicae L. caterpillars and for Cotesia glomerata (L.) developing in P. brassicae. For unparasitized P. brassicae, growth rate and pupal weight were highest on Brussels sprouts and Swedish turnip, intermediate on rape, and lowest on nasturtium. In contrast, C. glomerata larval developmental rate and adult longevity were greatest for wasps from P. brassicae reared on nasturtium.On all four plants, the host-parasitoid complex attained a lower final weight than unparasitized P. brassicae, and it is argued that this difference is due primarily to reduced consumption by parasitized P. brassicae. Among parasitized caterpillars, however, complex weight was positively correlated with clutch size, suggesting that C. glomerata larvae are able to partially counteract the effect of parasitization per se on host consumption.The host plants of P. brassicae appear to face an evolutionary dilemma: in order to increase the total mass of parasitoids produced, they must suffer greater loss of foliage. This trade-off, if common in nature, may represent a formidable constraint on coevolution between host plants and parasitoids.     

Status and Perspectives of Clubroot Resistance Breeding in Crucifer Crops

Clubroot disease is a major threat to crops belonging to the Brassicaceae. It is controlled most effectively by the use of resistant cultivars. Plasmodiophora brassicae, the causal agent, shows a wide variation for pathogenicity, which can be displayed by using differential host sets. Except for Brassica juncea and B. carinata, resistant accessions can be found in all major crops. Most resistance sources are race-specific, despite some race-independent resistant accessions which can be found in B. oleracea. European field isolates from P. brassicae display great variation and show a tendency to overcome different resistance sources from either B. rapa or B. oleracea. At present, resistance genes from stubble turnips (B. rapa) are most effective and most widely used in resistance breeding of different Brassica crops. Resistance to P. brassicae from turnips was introduced into Chinese cabbage, oilseed rape, and B. oleracea. Although most turnips carry more than one resistance gene, the resistant cultivars from other crops received primarily a single, dominant resistance gene having a race-specific effect. Populations of P. brassicae that are compatible against most of the used resistance sources have been present in certain European areas for many decades. Such pathogen populations appeared in Japanese Chinese cabbage crops only a few years after the introduction of resistant cultivars. As the spread of virulent P. brassicae pathotypes seems to be slow, resistant cultivars are still a very effective method of control in many cropping areas. Mapping studies have revealed the presence of several clubroot-resistance genes in the Brassica A and C genomes; most of these genes are showing race specificity. Only in B. oleracea was one broad-spectrum locus detected. Two loci from the A genome confer resistance to more than one pathotype, but not to all isolates. Progress made in the determination of resistance loci should be used to formulate and introduce an improved differential set. Future efforts for breeding P. brassicae resistance will focus on durability by broadening the genetic basis of clubroot resistance by using either natural variation or transgenic strategies.     

The Response Specificity of <Emphasis Type="Italic">Trichogramma</Emphasis> Egg Parasitoids towards Infochemicals during Host Location

Parasitoids are confronted with many different infochemicals of their hosts and food plants during host selection. Here, we investigated the effect of kairomones from the adult host Pieris brassicae and of cues present on Brussels sprout plants infested by P. brassicae eggs on the behavioral response of the egg parasitoid Trichogramma evanescens. Additionally, we tested whether the parasitoid’s acceptance of P. brassicae eggs changes with different host ages. The wasps did not discriminate between olfactory cues from mated and virgin females or between mated females and males of P. brassicae. T. evanescens randomly climbed on the butterflies, showing a phoretic behavior without any preference for a certain sex. The parasitoid was arrested on leaf parts next to 1-day-old host egg masses. This arrestment might be due to cues deposited during oviposition. The wasps parasitized host eggs up to 3 days old equally well. Our results were compared with former studies on responses by T .brassicae showing that T. evanescens makes less use of infochemicals from P. brassicae than T. brassicae.     

Phenotypic plasticity of Brevicoryne brassicae in responses to nutritional quality of two related host plants

Abstract 1. The host species used by a herbivorous insect may impose different selective pressures promoting host race formation, yet the presence of plasticity can potentially constrain host race formation. 2. The goal of this study is to determine if there is phenotypic plasticity in life history traits of Brevicoryne brassicae in response to host and nutritional quality of two host species, Brassica oleraceae and Brassica campestris, and to what extent there are genetic differences among genotypes in plasticity. 3. Plants of B. oleraceae and B. campestris were fertilised with three different nitrogen doses (with nutritive solutions of 50, 200 and 400 ppm of soluble nitrogen) to produce plants with different nutritional qualities. Eight clones of B. brassicae were reared on those plants, and days to reproduction (DTR), number of nymphs, and fitness (r_m) were recorded. 4. A significant genotype × host interaction was detected in days to reproduction. Genotype × nitrogen interaction (plasticity) was detected in the number of nymphs when aphids were raised on B. campestris. Aphids showed plasticity in DTR and marginal plasticity in r_m in reaction to the varying nitrogen content of B. oleraceae. 5. The phenotypic plasticity to fine‐scale variation of host (nutritional quality) documented here may be an important source of phenotypic variation and may potentially constrain host race formation.     

Food plant and herbivore host species affect the outcome of intrinsic competition among parasitoid larvae

1. In nature, several parasitoid species often exploit the same stages of a common herbivore host species and are able to coexist despite competitive interactions amongst them. Less is known about the direct effects of resource quality on intrinsic interactions between immature parasitoid stages. The present study is based on the hypothesis that variation in the quality or type of plant resources on which the parasitoids indirectly develop may be complementary and thus facilitate niche segregation favouring different parasitoids in intrinsic competition under different dietary regimes. 2. The present study investigated whether two herbivore species, the cabbage butterflies Pieris brassicae and Pieris rapae (Pieridae), and the quality of two important food plants, Brassica oleracea and Brassica nigra (Brassicaceae), affect the outcome of intrinsic competition between their primary larval endoparasitoids, the gregarious Cotesia glomerata (Braconidae) and the solitary Hyposoter ebeninus (Ichneumonidae). 3. Hyposoter ebeninus is generally an intrinsically superior competitor over C. glomerata. However, C. glomerata survived more antagonistic encounters with H. ebeninus when both developed in P. brassicae rather than in P. rapae caterpillars, and while its host was feeding on B. nigra rather than B. oleracea. Moreover, H. ebeninus benefitted from competition by its higher survival in multiparasitised hosts. 4. These results show that both plant and herbivore species mediate the battleground on which competitive interactions between parasitoids are played out and may affect the outcomes of these interactions in ways that enable parasitoids to segregate their niches. This in turn may promote coexistence among parasitoid species that are associated with the same herbivore host.     

Gene flow and spatio-temporal genetic variation among sympatric populations of Tetranychus kanzawai (Acari: Tetranychidae) occurring on different host plants, as estimated by microsatellite gene diversity

We investigated spatio-temporal genetic variation in allele frequency and estimated gene flow among sympatric populations of Tetranychus kanzawai on different host plants by the use of microsatellite markers. In the analysis of spatial genetic variation, no isolation by distance was detected among the populations. Gene flow between populations on Hydrangea macrophylla and those on other host plants was relatively restricted, whereas the populations on Akebia quinata and Clerodendrum trichotomum were almost panmictic. Our study on temporal genetic variation showed (1) that population differentiation was slightly reduced during the period from April to May owing to frequent gene flow among populations; and (2) that population differentiation was greatly enhanced from May to October because of bottleneck effects. Genetic differentiation among T. kanzawai populations was caused by the effect of host plants rather than by the effect of geographic distance among populations, suggesting possibility of sympatric host race formation in this species.This revised version was published online in May 2005 with a corrected cover date.     

Temporal consistency in herbivore responses to glucosinolate polymorphism in populations of wild cabbage (Brassica oleracea)

Natural populations of wild cabbage (Brassica oleracea) show significant qualitative diversity in heritable aliphatic glucosinolates, a class of secondary metabolites involved in defence against herbivore attack. One candidate mechanism for the maintenance of this diversity is that differential responses among herbivore species result in a net fitness balance across plant chemotypes. Such top-down differential selection would be promoted by consistent responses of herbivores to glucosinolates, temporal variation in herbivore abundance, and fitness impacts of herbivore attack on plants varying in glucosinolate profile. A 1-year survey across 12 wild cabbage populations demonstrated differential responses of herbivores to glucosinolates. We extended this survey to investigate the temporal consistency of these responses, and the extent of variation in abundance of key herbivores. Within plant populations, the aphid Brevicoryne brassicae consistently preferred plants producing the glucosinolate progoitrin. Among populations, increasing frequencies of sinigrin production correlated positively with herbivory by whitefly Aleyrodes proletella and negatively with herbivory by snails. Two Pieris butterfly species showed no consistent response to glucosinolates among years. Rates of herbivory varied significantly among years within populations, but the frequency of herbivory at the population scale varied only for B. brassicae. B. brassicae emerges as a strong candidate herbivore to impose differential selection on glucosinolates, as it satisfies the key assumptions of consistent preferences and heterogeneity in abundance. We show that variation in plant secondary metabolites structures the local herbivore community and that, for some key species, this structuring is consistent over time. We discuss the implications of these patterns for the maintenance of diversity in plant defence chemistry.     

‘Myrosin cells’ are not a prerequisite for aphid feeding on oilseed rape (Brassica napus) but affect host plant preferences

The enzyme myrosinase (EC 3.2.3.1.147) is present in specialised myrosin cells and forms part of the glucosinolate–myrosinase system, also known as ‘the mustard oil bomb’, which has an important role in the defence system of cruciferous plants against insect pests. Transgenic Brassica napus MINELESS have been produced by transgenic ablation of myrosin cells. This prompted us to investigate the importance of myrosin cells in plant–aphid interactions. In order to study this, we challenged transgenic MINELESS and wild‐type cultivar Westar seedlings with the aphids Brevicoryne brassicae (a specialist) and Myzus persicae (a generalist). Our study included aphid free‐choice and aphid fecundity experiments. Data from these experiments showed that B. brassicae prefers wild‐type seedlings and M. persicae prefers MINELESS. B. brassicae and M. persicae showed significant variation in establishment on plants regardless of whether they were wild type or MINELESS and also differed significantly in affecting plant parts. Myrosinase activity in MINELESS control seedlings was 83.6% lower than the wild‐type control seedlings. Infestation with either of the two aphid species induced myrosinase levels in both wild‐type and MINELESS seedlings. Infestation with M. persicae reduced the concentration of most glucosinolates while B. brassicae had the opposite effect. B. brassicae enhanced the formation of glucosinolate hydrolysis products both in wild‐type and MINELESS seedlings. However, M. persicae decreased All ITC but increased 3,4ETBut NIT in wild‐type seedlings. Taken together, the investigation shows that the presence of myrosin cells affects the preference of generalist and specialist aphid species for Brassica napus plants.     

Significance of habitat type for the genetic population structure of <Emphasis Type="Italic">Panonychus citri</Emphasis> (Acari: Tetranychidae)

Restricted migration and habitat fragmentation promote genetic differentiation between populations: Because most of the hosts of Panonychus citri are woody plants, mainly citrus trees that are usually planted at intervals of several metres, this mite likely faces more risks (e.g., starvation) by dispersing between host plants, compared to other spider mite species that infest both herbaceous and woody plants, such as Tetranychus urticae. Such a limited gene flow between patches (host plants) can lead to differentiation of populations even within a small area. Therefore, we hypothesize that P. citri populations are genetically differentiated not only between distant populations but also within small areas, such as within a grove. To test this hypothesis, we investigated the divergence of P. citri populations in Japanese citrus groves according to a hierarchical arrangement of geographical distance, ranging from distant populations (10 groves distributed throughout different areas in two major Japanese islands; this level of analysis is referred to as ‘geographic’) to local populations (different trees in a specific grove; ‘local’). Three molecular markers were used an esterase locus, one microsatellite and a point mutation in the mitochondrial cytochrome oxidase subunit I. At a local level acaricide susceptibility tests were also performed using two acaricides: fenpyroximate (25 ppm) and etoxazole (3.33 ppm). At a broad geographic level the gene diversity decreased with decreasing area size and distance between populations. By contrast, at the local level, populations maintained a significant level of variation between trees within groves, and the divergence within groves was higher than between groves. Whereas no statistical difference of the mortalities was detected among groves for the two acaricides tested, the difference was statistically significant among trees within groves in fenpyroximate (ANOVA, p < 0.025) and marginal in etoxazole (0.1 < p < 0.05). We concluded that P. citri populations maintain a higher level of variation between trees (or patches of trees) within groves than between groves at the local level, though the gene diversity tended to be smaller with decreasing distance between populations at the geographical level. Results are discussed in relation to the dispersal behaviour of spider mites.     

Influence of water stress on the glucosinolate profile of Brassica oleracea var. italica and the performance of Brevicoryne brassicae and Myzus persicae

Drought stress alters the chemical composition of plants, which can influence their tolerance to insect herbivory. To evaluate plant chemical responses to drought stress, broccoli, Brassica oleracea L. var. italica Plenck (Brassicaceae), was grown under well‐watered, drought, and water‐logged conditions. The glucosinolate (GS) levels and the performance of two aphid species, the specialist Brevicoryne brassicae (L.) and the generalist Myzus persicae (Sulzer) (both Hemiptera: Aphididae), in relation to water stress conditions were studied. High Performance Liquid Chromatography analysis showed that water stress changed the levels of GS in broccoli plants. Plants grown for 2 weeks under drought stress were significantly smaller and showed decreased levels of total GS when compared with GS contents of well‐watered plants, whereas water‐logged conditions led to a slight increase in the GS contents. A substantial decrease in indolyl GS was detected in water‐deficient plants, whereas aliphatic GS decreased slightly. Analysis of sugar levels in phloem sap of broccoli plants revealed that plants under water‐logged conditions contained the highest amounts of sugars followed by drought‐stressed and well‐watered plants. The two aphid species responded differently to water stress‐induced changes in their host plants. Significantly larger populations of M. persicae were recorded on plants with a limited water supply than on plants grown under well‐watered or water‐logged conditions. Brevicoryne brassicae was less affected by water stress, and similar population sizes were found on plants that were subject to different treatments. Analysis of covariance showed a significant effect of the plants’ water condition but no significant effect of GS content on the performance of M. persicae. However, the specialist B. brassicae remained unaffected by changes induced under water stress conditions.     

A comparative study of the egg-laying behaviour and larval development of Pieris rapae L. and P. brassicae L. on the same host plants

Summary Pieris rapae and P. brassicae feed on the same host plants and have synchronized seasons. P. brassicae, whose larvae are twice the size of P. rapae, lays eggs in clusters of 40–100 eggs whereas P. rapae lays single eggs. In this paper we examine how egg clustering may be advantageous for P. brassicae. The larval development of each species was studied, and found not to differ significantly. P. brassicae larvae were observed to migrate from their host plant after defoliating it. A comparison of the efficiency of host plant utilization by the two pierid species was undertaken by measuring the effect of larval feeding on the growth of their host plants (kale and brussel sprouts). The results show that egg clustering is advantageous for larval fitness in terms of host resource exploitation, and we suggest that P. brassicae is adapted for ovipositing on clumped vegetation, while P. rapae is selected for exploiting isolated plants.     

Analysis of the Brassica oleracea genome by the generation of B. campestris-oleracea chromosome addition lines: characterization by isozymes and rDNA genes

Summary This study aimed at generating chromosome addition lines and disclosing genome specific markers in Brassica. These stocks will be used to study genome evolution in Brassica oleracea L., B. campestris L. and the derived amphidiploid species B. napus L. B. campestris-oleracea monosomic and disomic chromosome addition plants were generated by crossing and backcrossing the natural amphidiploid B. napus to the diploid parental species B. campestris. The pollen viability of the derived sesquidiploid and hyperploid ranged from 63% to 88%, while the monosomic and disomic addition plants had an average pollen fertility of 94% and 91%, respectively. The addition lines were genetically characterized by genome specific markers. The isozymes for 6PGD, LAP, PGI and PGM, and rDNA Eco RI restriction fragments were found to possess the desired genome specificity. Duplicated loci for several of these markers were observed in B. campestris and B. oleracea, supporting the hypothesis that these diploid species are actually secondary polyploids. A total of eight monosomic and eight disomic addition plants were identified and characterized on the basis of these markers. Another 51 plants remained uncharacterized due to the lack of additional markers. rDNA genes were found to be distributed in more than one chromosome, differing in its restriction sites. Intergenomic recombination for some of the markers was detected at frequencies between 6% and 20%, revealing the feasibility of intergenomic gene transfer.     

Comparison of photosynthetic activity of <Emphasis Type="Italic">Orychophragmus violaceus</Emphasis> and oil-seed rape

Orychophragmus violaceus, Brassica campestris cv. Chuanyou No.8, and Brassica juncea cv. Luzhousileng diurnal changes of net photosynthetic rate (P _N) and activities of carbonic anhydrase (CA) of leaves were studied. One uni-modal curve occurred at the diurnal changes of P _N in O. violaceus, but bimodal curves were found in B. campestris and B. juncea. Thus photosynthetic midday depression was not found in O. violaceus but in both Brassica species. Midday depression of P _N in O. violaceus was not related to high temperature or low humidity at midday but to the activity of CA.     

Genetic Divergence in the <Emphasis Type="Italic">cacophony</Emphasis> IVS6 Intron Among Five BrazilianPopulations of <Emphasis Type="Italic">Lutzomyia longipalpis</Emphasis>

Genes involved in the reproductive isolation are particularly useful as molecular markers in speciation studies. Lutzomyia longipalpis (Diptera: Psychodidae: Phlebotominae), a putative species complex, is a vector of visceral leishmaniasis in Latin America. We isolated from this species a fragment homologous to cacophony, a Drosophila gene that encodes features of the lovesong, an acoustic signal that is important in the sexual isolation of closely related species and known to vary considerably among L. longipalpis putative siblings species. Using an intron of the sandfly cacophony as a marker, we analyzed the molecular variation and sequence divergence among five populations of L. longipalpis from Brazil, three allopatric (Jacobina, Lapinha and Natal) and two putative sympatric sibling species from the locality of Sobral. A high level of polymorphism was found and analysis of the data indicates that very little gene flow is occurring among the populations of Jacobina, Lapinha, and Natal. A high level of differentiation was also observed between the two putative sympatric species of Sobral, one of which seems to be the same sibling species found in Natal, while the other is somewhat more related to Jacobina and Lapinha. However, the amount of estimated gene flow among the Sobral siblings is about seven times higher than the previously estimated for period, another lovesong gene, perhaps indicating that introgression might be affecting cacophony more than period. The results suggest that L. longipalpis is not a single species in Brazil, but it is yet not clear whether the different populations studied deserve species status rather than representing an incipient speciation process.     

Diploid Brassica napus somatic hybrids: characterization of nuclear and organellar DNA

Summary Five somatic hybrids between Brassica campestris and B. oleracea were obtained. Molecular, morphological and cytological information all suggest that the resynthesized B. napus plants were hybrids. All five plants were diploid (2n=38) and had mainly bivalents at meiosis. Seedset was low after selfing but normal after crossing with B. napus. Molecular proof of the hybrid nature of these plants was obtained by hybridization of a rDNA repeat to total DNA. Analysis of chloroplast DNA restriction patterns revealed that all hybrids had chloroplasts identical to the B. oleracea parent. The analysis of mitochondrial DNA indicated that three hybrids had restriction patterns identical to those of B. campestris, and the other two had restriction patterns similar to those of B. oleracea. The 11.3 kb plasmid present in mitochondria of the B. campestris parent was also found in mitochondria of all five hybrids. This suggests that the plasmid from a B. campestris type of mitochondria was transferred into mitochondria of a B. oleracea type.     

Antioxidant defense system in leaves of Indian mustard (<Emphasis Type="Italic">Brassica juncea</Emphasis>) and rape (<Emphasis Type="Italic">Brassica napus</Emphasis>) under cadmium stress

Plant species capable of hyper-accumulating heavy metals are of considerable interest for phytoremediation, and differ in their ability to accumulate metals from environment. Using two brassica species (Brassica juncea and Brassica napus), nutrient solution experiments were conducted to study variation in tolerance to cadmium (Cd) toxicity based on (1) lipid peroxidation and (2) changes in antioxidative defense system in leaves of both plants (i.e., superoxide dismutase (SOD EC 1.15.1.1), catalase (CAT EC 1.11.1.6), ascorbate peroxidase (APX EC 1.11.1.11), guaiacol peroxidase (GPX EC 1.11.1.7), glutathione reductase (GR EC 1.6.4.2), levels of phytochelatins (PCs), non-protein thiols (NP-SH), and glutathione. Plants were grown in nutrient solution under controlled environmental conditions, and subjected to increasing concentrations of Cd (0, 10, 25 and 50 μM) for 15 days. Results showed marked differences between both species. Brassica napus under Cd stress exhibited increased level of lipid peroxidation, as was evidenced by the increased malondialdehyde (MDA) content in leaves. However, in Brassica juncea treated plants, MDA content remained unchanged. In Brassica napus, with the exception of GPX, activity levels of some antioxidant enzymes involved in detoxification of reactive oxygen species (ROS), including SOD, CAT, GR, and APX, decreased drastically at high Cd concentrations. By contrast, in leaves of Brassica juncea treated plants, there was either only slight or no change in the activities of the antioxidative enzymes. Analysis of the profile of anionic isoenzymes of GPX revealed qualitative changes occurring during Cd exposure for both species. Moreover, levels of NP-SH and PCs, monitored as metal detoxifying responses, were much increased in leaves of Brassica juncea by increasing Cd supply, but did not change in Brassica napus. These results indicate that Brassica juncea plants possess the greater potential for Cd accumulation and tolerance than Brassica napus.