Exploring selection signatures in the divergence and evolution of lipid droplet (LD) associated genes in major oilseed crops.

BACKGROUND: Oil bodies or lipid droplets (LDs) in the cytosol are the subcellular storage compartments of seeds and the sites of lipid metabolism providing energy to the germinating seeds. Major LD-associated proteins are lipoxygenases, phospholipaseD, oleosins, TAG-lipases, steroleosins, caleosins and SEIPINs; involved in facilitating germination and enhancing peroxidation resulting in off-flavours. However, how natural selection is balancing contradictory processes in lipid-rich seeds remains evasive. The present study was aimed at the prediction of selection signatures among orthologous clades in major oilseeds and the correlation of selection effect with gene expression. RESULTS: The LD-associated genes from the major oil-bearing crops were analyzed to predict natural selection signatures in phylogenetically close-knit ortholog clusters to understand adaptive evolution. Positive selection was the major force driving the evolution and diversification of orthologs in a lineage-specific manner. Significant positive selection effects were found in 94 genes particularly in oleosin and TAG-lipases, purifying with excess of non-synonymous substitution in 44 genes while 35 genes were neutral to selection effects. No significant selection impact was noticed in Brassicaceae as against LOX genes of oil palm. A heavy load of deleterious mutations affecting selection signatures was detected in T-lineage oleosins and LOX genes of Arachis hypogaea. The T-lineage oleosin genes were involved in mainly anther, tapetum and anther wall morphogenesis. In Ricinus communis and Sesamum indicum > 85% of PLD genes were under selection whereas selection pressures were low in Brassica juncea and Helianthus annuus. Steroleosin, caleosin and SEIPINs with large roles in lipid droplet organization expressed mostly in seeds and were under considerable positive selection pressures. Expression divergence was evident among paralogs and homeologs with one gene attaining functional superiority compared to the other. The LOX gene Glyma.13g347500 associated with off-flavor was not expressed during germination, rather its paralog Glyma.13g347600 showed expression in Glycine max. PLD-α genes were expressed on all the tissues except the seed,δ genes in seed and meristem while ß and γ genes expressed in the leaf. CONCLUSIONS: The genes involved in seed germination and lipid metabolism were under strong positive selection, although species differences were discernable. The present study identifies suitable candidate genes enhancing seed oil content and germination wherein directional selection can become more fruitful.

Response of Nagpur mandarin (Citrus reticulata Blanco) to high density planting systems.

High density planting system i.e. accommodating a higher number of plants than routine in a given area is an innovative agro-technology to increase yield and thereby early net returns. Due to conventional wide spacing plantation in Nagpur mandarin (Citrus reticulata Blanco), the land remains unutilized as the plant canopy gradually increases over the years. In the present study, Nagpur mandarin (Citrus reticulata Blanco) budded on Rangpur lime rootstock was evaluated under six different planting spacings. It was observed that the organic carbon (1.10-1.82%) and major nutrients viz. N (309-430 kg ha-1), P (20-54 kg ha-1) and K (291-810 kg ha-1) increased vis-à-vis plant density and was highest under 2 × 2 m spacing. Plants were tallest at 2 × 2 m spacing with the higher PAR interception (88.2) and the lowest leaf area index (1.09). Fruit yield on area basis, under 2 × 2 m spacing was 26, 7.1, 4.6 times more as compared to conventional plantation during the first, second and third year, respectively. At fifth year of crop harvest, the highest B:C ratio (6.36) was recorded in 6 × 3 m followed by 4 × 2 m and 2 × 2 m.

Monitoring of Resistance in Field Populations of Scirtothrips dorsalis (Thysanoptera: Thripidae) and Diaphorina citri (Hemiptera: Liviidae) to Commonly Used Insecticides in Citrus in Central India.

Field populations of thrips (Scirtothrips dorsalis Hood) from citrus nurseries and psyllids (Diaphorina citri Kuwayama) from commercial citrus orchards of three districts in Maharashtra State in India were evaluated for levels of resistance to organophosphates (acephate, chlorpyrifos, quinalphos, and dimethoate), neonicotinoids (thiamethoxam, imidacloprid), abamectin and spinosad between 2012 and 2017. Leaf dip bioassays for thrips and shoot tip bioassay for nymph and adult psyllids were used to assess levels of insecticide resistance as indicated by resistance ratios (RRs). In general, levels of insecticide resistance were zero (RR = 1) or very low (RR = 2-10); however, we recorded 21.0- and 28.0-fold increase in resistance (RR = 21-50) to spinosad 45SC in S. dorsalis between 2016 and 2017 that may be attributed to the continuous application of the insecticide for the management of various sucking insect pests of citrus. We found that levels of resistance to insecticides in adult and nymph D. citri collected from the three districts were zero to low (RR = 2-10). Our data show there may be a shift in resistance to neonicotinoid insecticides that are widely used among citrus growers in the study region. Results from this study indicate that the evolution of insecticide resistance and cross-resistance may be decelerated by the rotational use of insecticides with different modes of action.

Relative Susceptibility of Phyllocnistis citrella (Lepidoptera: Gracillariidae) to Commonly Used Insecticides in Maharashtra, India.

Foliar application of insecticides has been the most commonly followed practice to manage Phyllocnistis citrella Stainton in nurseries and flush leaves in citrus groves. Leaf dip bioassays were conducted against insecticides, viz., acephate 75SP, dimethoate 30EC, abamectin 1.9EC, fenvalerate 20EC, imidacloprid 17.8SL, and thiamethoxam 25WG, and were tested against P. citrella larvae for their susceptibility. Among six insecticides tested on second-instar P. citrella larvae collected from Nagpur mandarin/acid lime cultivars during 2013-2016, abamectin was the most toxic insecticide for the initial year (LC50 values ranged from 20.99 to 49.00 ppm), while dimethoate (LC50 of 36.57-160.95 ppm) and thiamethoxam (39.90-71.96 ppm) were consistently effective against P. citrella larvae for the rest of the period. Resistance ratio (RR) values calculated based on the baseline susceptible culture, viz., abamectin (1.24-2.33), acephate (1.03-2.31), fenvalerate (1.54-3.45), dimethoate(1.28-5.63), imidacloprid (1.29-8.64), and thiamethoxam (1.05-1.80), indicated that the current RR values were in low levels (RR < 10).