RNA-Seq reveals divergent gene expression between larvae with contrasting trophic modes in the poecilogonous polychaete Boccardia wellingtonensis

Abstract

The polychaete Boccardia wellingtonensis is a poecilogonous species that produces different larval types. Females may lay Type I capsules, in which only planktotrophic larvae are present, or Type III capsules that contain planktotrophic and adelphophagic larvae as well as nurse eggs. While planktotrophic larvae do not feed during encapsulation, adelphophagic larvae develop by feeding on nurse eggs and on other larvae inside the capsules and hatch at the juvenile stage. Previous works have not found differences in the morphology between the two larval types; thus, the factors explaining contrasting feeding abilities in larvae of this species are still unknown. In this paper, we use a transcriptomic approach to study the cellular and genetic mechanisms underlying the different larval trophic modes of B. wellingtonensis. By using approximately 624 million high-quality reads, we assemble the de novo transcriptome with 133,314 contigs, coding 32,390 putative proteins. We identify 5221 genes that are up-regulated in larval stages compared to their expression in adult individuals. The genetic expression profile differed between larval trophic modes, with genes involved in lipid metabolism and chaetogenesis over expressed in planktotrophic larvae. In contrast, up-regulated genes in adelphophagic larvae were associated with DNA replication and mRNA synthesis.

Figure 1

Characteristics of genes. (A) Top ten species distribution. (B) Functional annotation based on Gene Ontology (GO) categorization. (C) COG categories distribution of EggNOG classifications.

Figure 2

Adult individuals and larval state of B. wellingtonensis show a unique transcriptional signature. (A) Principal component analysis of the 21 RNAseq sample shows clustering into adult and larval groups. Upper circles show larval samples: EAL (red), AL (black), PLTIII (green) and PLTI (yellow). Lower circles show adult samples: FTIII (pink), FTI (lilac), and M (calypso). (B) Volcano plot comparison of differential gene expression between larvae and adults. Red dots highlight differentially expressed genes with threshold FDR < 0.01 and Log2 Fold Change [≥ 1]. The black zone indicates the number of transcripts that do not show significant differential expression.

Figure 3

Plot of the enriched biological processes (FDR < 0.05) identified in adults, larval stages, planktotrophic (PLTI and PLTIII) and adelphophagic (EAL) larvae. The GO terms were collapsed in REVIGO. The size of the dot indicates the number of genes grouped in a GO term, and the color represents the significant enrichment -Log10(FDR). The vertical line represents the number of DEGs.