The genome of medicinal leech (Whitmania pigra) and comparative genomic study for exploration of bioactive ingredients.

Background: Leeches are classic annelids that have a huge diversity and are closely related to people, especially medicinal leeches. Medicinal leeches have been widely utilized in medicine based on the pharmacological activities of their bioactive ingredients. Comparative genomic study of these leeches enables us to understand the difference among medicinal leeches and other leeches and facilitates the discovery of bioactive ingredients.

Results: In this study, we reported the genome of Whitmania pigra and compared it with Hirudo medicinalis and Helobdella robusta. The assembled genome size of W. pigra is 177 Mbp, close to the estimated genome size. Approximately about 23% of the genome was repetitive. A total of 26,743 protein-coding genes were subsequently predicted. W. pigra have 12346 (46%) and 10295 (38%) orthologous genes with H. medicinalis and H. robusta, respectively. About 20 and 24% genes in W. pigra showed syntenic arrangement with H. medicinalis and H. robusta, respectively, revealed by gene synteny analysis. Furthermore, W. pigra, H. medicinalis and H. robusta expanded different gene families enriched in different biological processes. By inspecting genome distribution and gene structure of hirudin, we identified a new hirudin gene g17108 (hirudin_2) with different cysteine patterns. Finally, we systematically explored and compared the active substances in the genomes of three leech species. The results showed that W. pigra and H. medicinalis exceed H. robusta in both kinds and gene number of active molecules.

Conclusions: This study reported the genome of W. pigra and compared it with other two leeches, which provides an important genome resource and new insight into the exploration and development of bioactive molecules of medicinal leeches.