(a,b) The larval cellular immune response includes pigment cell migration to the gut epithelium. An uninfected control larva (a), and a larva exposed to V.d. for 24 hr (b) are shown. The red color of pigment cells is a consequence of echinochrome A, a naphthoquinone that is encapsulated in large granules. Under typical laboratory conditions, pigment cells localize to the outer ectoderm. In response to certain bacterial isolates, these cells migrate through the blastocoel to interact with the gut epithelium (Ch Ho et al., 2016). (c) Genes are activated with a variety of kinetics that varies among functional classes. RNA-Seq was performed on larvae collected at 0, 6, 12, and 24 hr of exposure to V.d. Differentially expressed transcripts (RPKM ≥3, fold-change in expression ≥2; 3238 total transcripts) were hierarchically clustered with average linkages to identify similarly temporally regulated genes using Gene Cluster 3.0. High expression is indicated in dark blue; low expression is shown in white. (d) A subset of IL17 genes is upregulated early in infection. Transcript levels are shown for 16,920 genes that are expressed in larvae during the infection time course. Expression levels (RPKM) are shown for uninfected larvae (x-axis) and larvae exposed to V.d. for 6 hr (y-axis). Most genes were not strongly differentially expressed at these two time points (gray). The 127 genes that exhibit ≥3-fold changes in expression levels between 0 and 6 hr of infection are shown in black. The genes within the SpIL17 subfamilies are indicated (SpIL17-1, red; SpIL17-4, green). The dashed box is enlarged in the inset. (e) Mapped reads were used to identify a novel SpIL17 transcript. RNA-Seq reads were mapped to the S. purpuratus genome. Genomic regions that contained mapped reads but no known genes were investigated for domains that are associated with immunity (Scaffold1325:145,000–160,000 from S. purpuratus genome, v3.1 is shown). The number of reads normalized to library size is plotted for each time point as a stacked bar graph. Reads that map to the positive strand are shown with positive values; negative values indicate reads that map on the negative strand. The exons of the experimentally confirmed SpIL17-4a transcript sequences (SpIL17-4a and SpIL17-4a´) are indicated in black. The location of the IL17 domain encoded by the transcripts is shown in gray.