A bstract.A combination o f biological control and host-plant resistance is needed to control greenhouse whitefly, Trialeurodes vapo rariorum (Westwood). The high level o f susceptibility o f several host plants to whitefly, based on their performance on these plants, is well documented. These studies only provide information on the overall host-plant acceptance by whiteflies. Here, w e use a method that allows an examination o f the different tissue layers in the overall acceptance. The effects o f plant tissue factors on whitefly probing profiles were monitored using the electrical penetration graph (EPG) method. The EPGs o f whitefly originating from a culture on glasshouse cucumber, were recorded for 8 hours on sweet pepper, tomato, gerbera and cucumber plants produced in a glasshouse. On sweet pepper the graphs showed that whitefly made many short probes, had long xylem phases, short phloem phases, and the shortest duration o f first probes. An opposite probing profile was found on cucumber: longer probes, shorter xylem phases, fewer phloem phases but o f longer duration, and longer first probes. The values o f these parameters for gerbera and tomato were intermediate. Whiteflies encountered the greatest stimulation or the least resistance in the tissues o f cucumber, and the least stimulation or the greatest resistance in the tissues o f sweet pepper. Rejection o f host plants probably occurred before the phloem tissue was reached, as the probes prior to a whitefly leaving a host plant were so short that the stylets cannot have reached the phloem. But phloem factors also determine host-plant rejection, as phloem probing on sweet pepper -a poor host plant -was much shorter than on the other host plants. Resistance factors seem, therefore, to be located both in the epidermis/mesophyll and in the phloem. We hypothesize that the factors encountered by whitefly in the different tissue layers during probing contribute to the acceptance or rejection o f a host plant. Based on the performance o f whitefly on these plants, which is also reflected in the values of the EPG parameters, the order o f acceptance ranked from high to low is cucumber > tomato = gerbera > sweet pepper. INTRODUCTIONThe greenhouse whitefly, Trialeurodes vaporariorum (Westwood), is a serious pest of vegetables and ornamen tals growing in greenhouses and outside throughout the world (van Lenteren et al., 1992). This insect attacks more than 860 plant species from 469 genera and 121 families (Xu, 1994). Whitefly cause (a) direct damage by phloem sap ingestion, (b) indirect damage by transmitting plant viruses and other diseases (Cohen, 1990) and (c) decreased photosynthesis as a result of sooty moulds that develops on the honeydew excreted by the insect (Vet et al., 1980). One way of controlling whiteflies is through host-plant resistance . Complete host-plant resistance to insects has been found, and is employed against several insect pests, but is seldom per manent. Partial resistance against whitefly has been found in several plant species, ...
Genetic diversity and biogeography of RTB 291 291Genetic diversity and biogeography of red turpentine beetle Dendroctonus valens in its native and invasive regions Abstract Sequences of 479 bp region of the mitochondrial COI gene were applied to detect population genetic diversity and structure of Dendroctonus valens populations. By comparing the genetic diversity between native and invasive populations, it was shown that the genetic diversity of Chinese populations was obviously lower than that of native populations with both indices of haplotype diversity and Nei's genetic diversity, suggesting genetic bottleneck occurred in the invasive process of D. valens, and was then followed by a relatively quick population buildup. According to phylogenetic analyses of haplotypes, we suggested that the origin of the Chinese population was from California, USA. Phylogenetic and network analysis of native populations of D. valens revealed strong genetic structure at two distinct spatial and temporal scales in North America. The main cause resulting in current biogeographic pattern was supposedly due to recycled glacial events. Meanwhile, a cryptic species might exist in the Mexican and Guatemalan populations.
Probing behaviour of Trialeurodes vaporariorum (Westwood) larvae was monitored using the DC electrical penetration graph (EPG) technique on the host plant cucumber. EPGs were recorded for 16 h, simultaneously with honeydew excretion using a ‘honeydew clock’. Three waveforms were distinguished: a pathway waveform (C), and two phloem waveforms, one with a high (H), and one with a low frequency (L) signal. The C waveform mainly occurred in the crawler stage of the 1st instar larvae. EPGs recorded from larvae during and after moulting indicated that the process involves stylet withdrawal; hence the stylets of each new instar need to penetrate again from the leaf surface to the phloem. All sessile stages, from L1 to pre‐pupa, spent almost their entire time in waveforms H and L. These waveforms alternated more frequently in the early instars than during the later ones, in which the H waveform became predominant. The H waveform was highly correlated with honeydew excretion and thus phloem sap ingestion. The L waveform was not related to honeydew excretion but EPGs indicated that the stylet tips remain in a sieve element during both waveforms. Periods of honeydew production demonstrated a delay of 30–40 min in relation to the onset and end of H and L waveforms. This delay is presumably related to the time needed for food passing through, or emptying of, the insect's gut. From the 1st instar to the pre‐pupa, the frequency of excreted honeydew droplets decreased but their size increased, causing a net increase of the excretion rate.
The presence of alien invasive species has serious negative impact on endemic biodiversity, especially on native species that occupy the same niche in the ecosystem. To study the influence of the alien invasive species Bursaphelenchus xylophilus on its native sister species B. mucronatus, the two nematode species were mix-cultured in a fungal mat and mix-inoculated into a susceptible host. By comparing the propagation parameters of both species under competitive and noncompetitive conditions it was shown that the propagation level of B. xylophilus was clearly higher than that of B. mucronatus under laboratory culture. Furthermore, the propagation capacity of B. xylophilus under competitive conditions was much higher than that under noncompetitive conditions, both in laboratory culture and with host inoculation. Bursaphelenchus xylophilus also excluded B. mucronatus when the two species were cultured as a mixture for a longer time. The relative abundance ratios of the two species in natural pinewoods were also determined by random sampling of dying pine trees from regions with different invasion histories. It was noted that with an increase in invasion years the distribution frequency of B. xylophilus increased while that of B. mucronatus decreased. Experimental tests verified our hypothesis that because of its high fecundity and strong competitive ability, the invasive species B. xylophilus out-competed the native species B. mucronatus and displaced it in natural ecosystems. The successful invasion of B. xylophilus is attributed to competitive displacement, which may be one of the ecological invasive mechanisms.
BackgroundMicroRNAs (miRNAs) are considered to be very important in regulating the growth, development, behavior and stress response in animals and plants in post-transcriptional gene regulation. Pinewood nematode, Bursaphelenchus xylophilus, is an important invasive plant parasitic nematode in Asia. To have a comprehensive knowledge about miRNAs of the nematode is necessary for further in-depth study on roles of miRNAs in the ecological adaptation of the invasive species.Methods and FindingsFive small RNA libraries were constructed and sequenced by Illumina/Solexa deep-sequencing technology. A total of 810 miRNA candidates (49 conserved and 761 novel) were predicted by a computational pipeline, of which 57 miRNAs (20 conserved and 37 novel) encoded by 53 miRNA precursors were identified by experimental methods. Ten novel miRNAs were considered to be species-specific miRNAs of B. xylophilus. Comparison of expression profiles of miRNAs in the five small RNA libraries showed that many miRNAs exhibited obviously different expression levels in the third-stage dispersal juvenile and at a cold-stressed status. Most of the miRNAs exhibited obviously down-regulated expression in the dispersal stage. But differences among the three geographic libraries were not prominent. A total of 979 genes were predicted to be targets of these authentic miRNAs. Among them, seven heat shock protein genes were targeted by 14 miRNAs, and six FMRFamide-like neuropeptides genes were targeted by 17 miRNAs. A real-time quantitative polymerase chain reaction was used to quantify the mRNA expression levels of target genes.ConclusionsBasing on the fact that a negative correlation existed between the expression profiles of miRNAs and the mRNA expression profiles of their target genes (hsp, flp) by comparing those of the nematodes at a cold stressed status and a normal status, we suggested that miRNAs might participate in ecological adaptation and behavior regulation of the nematode. This is the first description of miRNAs in plant parasitic nematodes. The results provide a useful resource for further in-depth study on molecular regulation and evolution of miRNAs in plant parasitic nematodes.
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