Philippe Letourmy

Effects of remnant trees in fallows on diversity and structure of forest regrowth in a slash-and-burn agricultural system in southern Cameroon

Remnant trees in fields may facilitate regeneration in fallows by attracting seed dispersers and creating favourable sites for plant establishment. Ntumu shifting cultivators in Cameroon clearing rain forest spare some trees to provide shade for crops. This study examines their effects on regeneration by comparing vegetation in quadrats beneath and away from their crowns, in fallows of varying age (3-20 y). For fallows of all age classes, plant diversity was not significantly different between positions. Trees accounted for larger proportions of species and stems beneath remnant trees. Herbs, principally large monocots, accounted for larger proportions away from trees. Basal area was much higher in quadrats beneath trees. In all quadrats, the majority of individuals belonged to species with animal-dispersed seeds. These. accounted for a larger proportion of individuals beneath trees (75%) than away (64%). Wind-dispersed species accounted for a larger proportion of stems away from trees (23.6%) than beneath (11.7%). These data suggest that enhanced seed rain by attraction of perching animals, demonstrated in a companion study, influenced regeneration patterns. Effects on regeneration were similar beneath trees of all animal-dispersed (Pycnanthus angolensis, Myristicaceae) and a wind-dispersed (Triplochiton scleroxylon, Sterculiaceae) species. Remnant trees in fallows may facilitate succession by nucleation, resulting in more rapid restoration of forest and conservation of nutrients in plant biomass. (Resume dauteur)

Comparison of the genetic determinism of two key phenological traits, flowering and maturity dates, in three Prunus species: peach, apricot and sweet cherry.

The present study investigates the genetic determinism of flowering and maturity dates, two traits highly affected by global climate change. Flowering and maturity dates were evaluated on five progenies from three Prunus species, peach, apricot and sweet cherry, during 3–8 years. Quantitative trait locus (QTL) detection was performed separately for each year and also by integrating data from all years together. High heritability estimates were obtained for flowering and maturity dates. Several QTLs for flowering and maturity dates were highly stable, detected each year of evaluation, suggesting that they were not affected by climatic variations. For flowering date, major QTLs were detected on linkage groups (LG) 4 for apricot and sweet cherry and on LG6 for peach. QTLs were identified on LG2, LG3, LG4 and LG7 for the three species. For maturity date, a major QTL was detected on LG4 in the three species. Using the peach genome sequence data, candidate genes underlying the major QTLs on LG4 and LG6 were investigated and key genes were identified. Our results provide a basis for the identification of genes involved in flowering and maturity dates that could be used to develop cultivar ideotypes adapted to future climatic conditions.

Seed rain beneath remnant trees in a slash-and-burn agricultural system in southern Cameroon

On clearing fields, Ntumu farmers in southern Cameroon leave some large trees. The seed rain beneath 30 such remnant trees (12 species) was com- pared with that 10 m away from the edges of their crowns. Of a total of 39 765 seeds recorded in 90 seed traps over 2 y, 73.6% were of species different from the tree associated with each set of traps (foreign seed rain). Seed rain included 100 morphospecies, two-thirds of which possessed endozoochorous seeds. Seeds of the pioneer tree Musanga cecropioides accounted for 71.4% of total foreign seed rain; seeds of this and other animal-dispersed species accounted for 94.5% of the total. Seed rain was 25 times higher beneath remnant trees than 10 m away. Mean species richness of monthly seed rain was three times higher beneath remnant trees than 10 m away. Both fleshy-fruited and wind-dispersed species of remnant trees attracted seed-dispersing animals which greatly enhanced the seed rain; attraction thus did not depend solely on presence of fleshy fruits. Seed rain was lower when human activity in fields was intense and increased during the period of growth of the last crops, which were not usually weeded. Heavy seed rain just before fallow may contribute significantly to regeneration, as does the presence of remnant trees.

Impact of Sugarcane yellow leaf virus on Sugarcane Yield and Juice Quality in Réunion Island

Sugarcane yellow leaf virus (SCYLV) was first detected in sugarcane of Réunion Island in 1997. A field experiment was undertaken to assess the potential impact of this virus on sugarcane production. The agronomic characteristics of SCYLV-infected plants were compared to those of virus-free plants of three sugarcane cultivars (R570, R577 and R579) which occupy more than 90% of the cultivated sugarcane area on Réunion Island. In the plant crop, significant losses in stalk weight (28%) and in sugar content (11%) were detected for cultivar R577, but not for either of the two other cultivars. In the first ratoon crop, yield reduction was detected for cultivar R577 (37%), but also for cultivar R579 (19%). Cultivar R577 also showed significant losses in sugar content (12%) due to reduced amount and quality of extracted cane juice. No yield reduction was found for cultivar R570, although stalk height and diameter were reduced in SCYLV-infected canes of this cultivar in the first ratoon crop. Leaf yellowing was observed at harvest of plant and ratoon crops when sugarcane was no longer irrigated, and 10–59% of symptomatic stalks could be attributed to the presence of SCYLV. The most severe yellowing symptoms were related to infection of sugarcane by the virus.

Variation in Infection Capacity and in Virulence Exists Between Genotypes of Sugarcane yellow leaf virus

Two experiments, one in Guadeloupe and one in Réunion Island, were performed to transmit different genotypes of Sugarcane yellow leaf virus (SCYLV) to eight sugarcane cultivars differing in resistance to infection by the virus and to yellow leaf. Transmission was attempted from SCYLV-infected sugarcane plants or leaves to healthy tissue-cultured plantlets grown in vitro and with the aphid vector Melanaphis sacchari. After inoculation and elimination of insects with an insecticide, plantlets were transferred to Montpellier, France and grown in a greenhouse. Plants were tested for presence of SCYLV by tissue-blot immunoassay and reverse-transcription polymerase chain reaction after 5 to 6 months of growth. SCYLV genotypes BRA-PER, CUB, and REU were detected in 47, 62, and 39% of plants inoculated with these genotypes in Guadeloupe, respectively. SCYLV genotypes BRA-PER and REU and a mixed infection of genotypes BRA-PER and REU were detected in 56, 33, and 42% of plants inoculated with these genotypes in Réunion Island, respectively. Genotypes BRA-PER and CUB could be transmitted to all eight sugarcane cultivars, but genotype REU could never be transmitted to resistant sugarcane cvs. H78-4153 and H78-3567. SCYLV genotype REU was transmitted successfully to sugarcane cv. R570 in Guadeloupe, but not in Réunion Island. Genotypes BRA-PER and CUB induced yellow leaf symptoms in susceptible or highly susceptible sugarcane cultivars, whereas genotype REU induced very few symptoms. SCYLV was not found in several symptomatic plants, suggesting an association of disease with undetectable populations of the virus or a nonviral cause. This is the first report of variation in infection capacity and in virulence of SCYLV.

Population dynamics of Xanthomonas albilineans in sugarcane plants as determined with an antibiotic-resistant mutant

Characteristics of a spontaneous mutant of Xanthomonas albilineans (GPE 5SR) resistant to streptomycin and rifampicin were not different from those of the wild parental strain (GPE 5) in cultural, biochemical, and immunological properties. The mutant strain was at least as virulent as the parental wild strain and was used as an epidemiological tool for the study of the population dynamics of X. albilineans. Populations of X. albilineans GPE 5SR and symptom expression of leaf scald were monitored in six sugarcane clones differing in resistance to the disease. The pathogen invaded leaves, stalks, and roots. The apex of the stalk including the apical meristem was invaded. Symptom expression and population size in leaves and shoot apices were correlated under field conditions

Utilization of Biological Insecticides for the Protection of Stored Yam Chips

The processing of yam into dehydrated chips is used extensively in certain areas of West Africa to stabilize the product. However, the chips are often severely attacked by borers, which can reduce whole stocks to powder within a few months. The level of protection provided by various biological products derived from neem (Azadirachta indica) and Crotalaria caricea was compared with that of a synthetic pesticide, Sofagrain™(1.5% deltamethrin + 0.5% pirimiphos-methyl), a reference chemical for stored products in Benin. Insect damage was assessed over a period of 10 months in the store room. Infestation was well controlled by all treatments for up to six months, after which only Sofagrain provided any protection. After 10 months, the chemical still provided a good level of protection, whereas the untreated control was severely attacked by the following insects: Dinoderus porcellus and D. bifoveolatus, Rhyzopertha dominica, Lasioderma serricorne, Palorus subdepressus and Tribolium castaneum. L. serricorne was the most abundant pest, followed by P. subdepressus and T. castaneum. Among the biological products used, the three neem-based treatments (oil, seed powder and leaf powder) gave the best level of protection, whilst Crotalaria seeds and periodic turning over of storage bags had little effect on reducing damage.

Hybrid performance in taro (Colocasia esculenta) in relation to genetic dissimilarity of parents

Taro (Colocasia esculenta) breeding, as other root crop breeding, is based on the production and evaluation of large numbers of hybrids. The selection of parents is based on their phenotypic value in the absence of information concerning general combining ability (GCA), specific combining ability (SCA), or genetic distances between varieties. By combining data from heritability trials and from genetic diversity studies conducted with AFLP and SSR markers, we aimed at studying the relationship between hybrid vigour and genetic dissimilarity between parents. The traits studied included number of suckers, corm weight, corm dimensions, and dry matter content. Correlation coefficients between hybrid gain and dissimilarity values were calculated. The prediction of hybrid performance based on the mid-parent value was compared to the prediction based on a modified expression that takes into account the genetic relationships between parents. Correlations were all but one positive but not statistically significant for all traits, with the exception of the number of suckers, when using SSR markers for dissimilarity calculations. Accordingly, the genetic dissimilarities in the prediction of hybrid performances did not increase the correlation between predicted and observed hybrid vigour values. However, large differences were observed among the residual means from the regression between predicted and observed values when using AFLP or SSR markers, mainly due to the much higher polymorphism revealed by the latter. Models need to be further adapted to the type of molecular marker used, since their ability to reveal different rates of polymorphism will have a direct incidence on the calculation of genetic dissimilarities between genotypes. Nevertheless, since SSR markers are more polymorphic and more informative than AFLP markers, they should be preferentially used for these studies. Low genetic dissimilarity of parents yielded weak heterosis effects and future studies need to be conducted by using a broader genetic base. This is the first study assessing the relationship of hybrid vigour with the genetic distances between parents, conducted on a tropical root crop.

Substrate Specificity-Conferring Regions of the Nonribosomal Peptide Synthetase Adenylation Domains Involved in Albicidin Pathotoxin Biosynthesis Are Highly Conserved within the Species Xanthomonas albilineans

ABSTRACT Albicidin is a pathotoxin produced by Xanthomonas albilineans, a xylem-invading pathogen that causes leaf scald disease of sugarcane. Albicidin is synthesized by a nonribosomal pathway via modular polyketide synthase and nonribosomal peptide synthetase (NRPS) megasynthases, and NRPS adenylation (A) domains are responsible for the recognition and activation of specific amino acid substrates. DNA fragments (0.5 kb) encoding the regions responsible for the substrate specificities of six albicidin NRPS A domains from 16 strains of X. albilineans representing the known diversity of this pathogen were amplified and sequenced. Polymorphism analysis of these DNA fragments at different levels (DNA, protein, and NRPS signature) showed that these pathogenicity loci were highly conserved. The conservation of these loci most likely reflects purifying selective pressure, as revealed by a comparison with the variability of nucleotide and amino acid sequences of two housekeeping genes (atpD and efp) of X. albilineans. Nevertheless, the 16 strains of X. albilineans were differentiated into several groups by a phylogenetic analysis of the nucleotide sequences corresponding to the NRPS A domains. One of these groups was representative of the genetic diversity previously found within the pathogen by random fragment length polymorphism and amplified fragment length polymorphism analyses. This group, which differed by three single synonymous nucleotide mutations, contained only four strains of X. albilineans that were all involved in outbreaks of sugarcane leaf scald. The amount of albicidin produced in vitro in agar and liquid media varied among the 16 strains of X. albilineans. However, no relationship among the amount of albicidin produced in vitro and the pathotypes and genetic diversity of the pathogen was found. The NRPS loci contributing to the synthesis of the primary structure of albicidin apparently are not involved in the observed pathogenicity differences among strains of X. albilineans.