Diversity analysis reveals genetic homogeneity among Indian populations of legume pod borer, Maruca vitrata (F.)

De Novo Mining and Validating Novel Microsatellite Markers to Assess Genetic Diversity in Maruca vitrata (F.), a Legume Pod Borer

Maruca vitrata (Fabricius) is an invasive insect pest capable of causing enormous economic losses to a broad spectrum of leguminous crops. Microsatellites are valuable molecular markers for population genetic studies; however, an inadequate number of M. vitrata microsatellite loci are available to carry out population association studies. Thus, we utilized this insect's public domain databases for mining expressed sequence tags (EST)-derived microsatellite markers. In total, 234 microsatellite markers were identified from 10053 unigenes. We discovered that trinucleotide repeats were the most predominant microsatellite motifs (61.53%), followed by dinucleotide repeats (23.50%) and tetranucleotide repeats (14.95%). Based on the analysis, twenty-five markers were selected for validation in M. vitrata populations collected from various regions of India. The number of alleles (Na), observed heterozygosity (Ho), and expected heterozygosity (He) ranged from 2 to 5; 0.00 to 0.80; and 0.10 to 0.69, respectively. The polymorphic loci showed polymorphism information content (PIC), ranging from 0.09 to 0.72. Based on the genetic distance matrix, the unrooted neighbor-joining dendrogram differentiated the selected populations into two discrete groups. The SSR markers developed and validated in this study will be helpful in population-level investigations of M. vitrata to understand the gene flow, demography, dispersal patterns, biotype differentiation, and host dynamics.

Amenability of Maruca vitrata (Lepidoptera: Crambidae) to gene silencing through exogenous administration and host-delivered dsRNA in pigeonpea (Cajanus cajan L.)

Insect pests are one of the major biotic stresses limiting yield in commercially important crops. The lepidopteran polyphagous spotted pod borer, Maruca vitrata causes significant economic losses in legumes including pigeonpea. RNA interference (RNAi)-based gene silencing has emerged as one of the potential biotechnological tools for crop improvement. We report in this paper, RNAi in M. vitrata through exogenous administration of dsRNA with sequence specificity to three functionally important genes, Alpha-amylase (α-amylase), Chymotrypsin-like serine protease (CTLP) and Tropomyosin (TPM) into the larval haemolymph and their host-delivered RNAi in pigeonpea. Significant decline in the expression of selected genes supported by over-expression of DICER and generation of siRNA indicated the occurrence of RNAi in the dsRNA-injected larvae. Additionally, the onset of RNAi in the herbivore was demonstrated in pigeonpea, one of the prominent hosts, by host-delivered dsRNA. Transgenics in pigeonpea (cv. Pusa 992), a highly recalcitrant crop, were developed through a shoot apical meristem-targeted in planta transformation strategy and evaluated. Plant level bioassays in transgenic events characterized and selected at molecular level showed mortality of M. vitrata larvae as well as reduced feeding when compared to wild-type. Furthermore, molecular evidence for down regulation of target genes in the insects that fed on transgenic plants authenticated RNAi. Considering the variability of gene silencing in lepidopteran pests, this study provided corroborative proof for the possibility of gene silencing in M. vitrata through both the strategies.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01133-3.

Exogenous administration of dsRNA for the demonstration of RNAi in Maruca vitrata (lepidoptera: crambidae)

The polyphagous spotted pod borer, Maruca vitrata is an important agricultural pest that causes extensive damage on various food crops. Though the pest is managed by synthetic chemicals, exploration of biotechnological approaches for its control is important. RNAi-based gene silencing is one such tool that has been extensively used for functional genomics and is highly variable in insects. In view of this, we have attempted to demonstrate RNAi in M. vitrata through exogenous double-stranded RNA (dsRNA) administration targeting seven genes associated with midgut, chemosensory, cell signalling and development. Two modes of exogenous dsRNA delivery by either haemolymph injection and/or ingestion into third and late third instar larval stages respectively exhibited efficient silencing of specific transcripts. Furthermore, dsRNA injection into the haemolymph showed significant reduction of target gene expression compared to negative controls establishing this mode of delivery to be more efficient. Interestingly, haemolymph injection required lesser dsRNA and led to higher reduction of transcript level vis-à-vis ingestion as demonstrated in dsRNA Serine Protease 33 (ds-SP33)-fed larvae. Over-expression of key RNAi component DICER and detection of siRNA authenticated the presence of RNAi in M. vitrata. Additionally, we have identified inhibitor molecules like morpholine, piperidine, carboxamide and piperidine-carboxamide through in silico analysis for blocking the function of SP33 to demonstrate the utility of functional genomics. Thus, the present study establishes the usefulness of injection and ingestion approaches for exogenous dsRNA delivery into M. vitrata larvae for effective RNAi.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02741-8.