Transcriptome analysis of the desert locust central nervous system: production and annotation of a Schistocerca gregaria EST database

Abstract

Background: The desert locust (Schistocerca gregaria) displays a fascinating type of phenotypic plasticity, designated as 'phase polyphenism'. Depending on environmental conditions, one genome can be translated into two highly divergent phenotypes, termed the solitarious and gregarious (swarming) phase. Although many of the underlying molecular events remain elusive, the central nervous system (CNS) is expected to play a crucial role in the phase transition process. Locusts have also proven to be interesting model organisms in a physiological and neurobiological research context. However, molecular studies in locusts are hampered by the fact that genome/transcriptome sequence information available for this branch of insects is still limited.

Methodology: We have generated 34,672 raw expressed sequence tags (EST) from the CNS of desert locusts in both phases. These ESTs were assembled in 12,709 unique transcript sequences and nearly 4,000 sequences were functionally annotated. Moreover, the obtained S. gregaria EST information is highly complementary to the existing orthopteran transcriptomic data. Since many novel transcripts encode neuronal signaling and signal transduction components, this paper includes an overview of these sequences. Furthermore, several transcripts being differentially represented in solitarious and gregarious locusts were retrieved from this EST database. The findings highlight the involvement of the CNS in the phase transition process and indicate that this novel annotated database may also add to the emerging knowledge of concomitant neuronal signaling and neuroplasticity events.

Conclusions: In summary, we met the need for novel sequence data from desert locust CNS. To our knowledge, we hereby also present the first insect EST database that is derived from the complete CNS. The obtained S. gregaria EST data constitute an important new source of information that will be instrumental in further unraveling the molecular principles of phase polyphenism, in further establishing locusts as valuable research model organisms and in molecular evolutionary and comparative entomology.

Figure 1. Sequence length distribution of the 12,709 unique S. gregaria EST sequences (4,785 contigs/7,924 singletons).

Figure 2. Second level GO distribution of the S. gregaria EST sequences (Biological Process).

Distribution of the S. gregaria EST sequences in the major subclasses of the main ontology Biological Process (A) and a more detailed overview of sequence distribution in the two subclasses Metabolic process and Cellular process (B).

Figure 3. GO distribution of the S. gregaria EST sequences classified under Signal Transduction.

Distribution of the S. gregaria EST sequences classified under the GO term Signal transduction and a more detailed overview of the sequence distribution in the two subcategories Intracellular signaling cascade and Cell surface receptor-linked signal transduction.

Figure 4. Relative transcript levels for PRP, SPARC and SSG in desert locusts in the two phases.

Relative transcript quantity (RQ) for PRP, SPARC and SSG in isolated- and crowded-reared desert locust ventral nerve cords (VNC) and head ganglia (HG). Results were obtained by analyzing four independent pooled samples of ten individuals per condition and are represented as means ± standard error. Statistical analysis consisted of a Student's t-test for comparing two independent groups. Significantly higher transcript levels (p<0.05) are indicated by an asterisk (*).

Figure 5. Relative transcript levels for genes showing higher expression levels in nervous tissue of crowded-reared locusts.

Relative transcript quantity (RQ) for genes showing higher expression levels in crowded-reared desert locusts (VNC: ventral nerve cord, HG: head ganglia). Results were obtained by analyzing four independent pooled samples of ten individuals per condition and are represented as means ± standard error. Statistical analysis consisted of a Student's t-test for comparing two independent groups. Significantly higher transcript levels (p<0.05) are indicated by an asterisk (*).

Figure 6. Relative transcript levels for genes showing higher expression levels in nervous tissue of isolated-reared locusts.

Relative transcript quantity (RQ) for genes showing higher expression levels in isolated-reared desert locusts (VNC: ventral nerve cord, HG: head ganglia). Results were obtained by analyzing four independent pooled samples of ten individuals per condition and are represented as means ± standard error. Statistical analysis consisted of a Student's t-test for comparing two independent groups. Significantly higher transcript levels (p<0.05) are indicated by an asterisk (*).