Figure 2. Reverse transcription polymerase chain reaction (RT-PCR) showing temporal expression of Gypsy-1_Hg and Gypsy-2_Hg at different time points of visceral regeneration (Day 3−Day 28) and in the normal (Norm) digestive tube of the sea cucumber H. glaberrima. RT-PCR was performed as described in reference 15. Primer sequences for Gypsy-1_Hg and Gypsy-2_Hg were from reference 10. NADH dehydrogenase subunit 5 was used as an internal control. The gel also includes a no-template control (“-” control) to test for contamination. Whereas Gypsy-1_Hg is not transcriptionally active in the normal gut but overexpressed in regeneration (in particular, at the early time points), Gypsy-2_Hg does not show any significant differences in expression level between the normal and regenerating digestive tube. This contrasts with expression of these genes in the radial organ complex regeneration in the same species (Fig. 3B in Mashanov et al.¹⁰), where both retroelements are barely detectable under the normal conditions, but show dramatic upregulation during regeneration.