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To the Editor:
In a recent paper, Wang and colleagues described the genomes of two turtles, the Chinese soft-shell turtle (Pelodiscus sinensis) and the green sea turtle (Chelonia mydas)1. A salient finding was an apparent absence of GHRL, the gene encoding the only known circulating orexigen, the peptide hormone ghrelin. The highly conserved GHRL encodes at least two bioactive peptide hormones, ghrelin2 and obestatin3, which are recognized to have a diverse range of functions in a number of cell types and physiological systems4,5. Wang and colleagues hypothesized that the absence of ghrelin was associated with the low metabolic rate observed in these turtle species1.
We interrogated genome and transcriptome data from the Chinese soft-shell turtle, the green sea turtle, the Western painted turtle (Chrysemys picta bellii)6, the spiny soft-shell turtle (Apalone spinifera)6 and the red-eared slider turtle ('Trachemys scripta elegans)7 and from crocodilians and birds (Supplementary Table 1 and Supplementary Note). In the supplementary information provided by Wang and colleagues, it was noted that the loss of GHRL was confirmed by manual investigation of genome sequences, as well as through the interrogation of transcriptome data from the Chinese soft-shell turtle embryo. Although the major site of ghrelin expression is adult...