Fig. 5: Transitions of sexual systems in platyhelminths and nematodes.

a Gene synteny relationships between Schistosoma mansoni chrZ (Sman-chrZ) and Clonorchis sinensis linkage group CM030370 (Csin_CM030373, red) and CM030373 (Csin_CM030373, blue). The evolutionary strata of S. mansoni are shown in different colours, and the positions of the candidate sex-determining genes U2AF2 and mag-1 on the chrZ are indicated by arrows. b The distribution of the reported genes in the C. elegans sex determination pathway on different Nigon elements. c Lineage-specific duplication of C. elegans sex-determining genes across the phylogeny of nematodes. d Density plots comparing the testis or ovary (e) transcription levels, or the testis (f) or ovary (g) transcription specificity (measured by the transcript level ratios of gonad vs. soma) of gonad-biased genes of C. sinensis (light blue or red) and their S. mansoni orthologs (dark blue or red). Masculinisation, i.e., increase of testis transcription levels, is indicated with a larger male sign, and vice versa for feminisation and demasculinsation etc. Comparison of whole-body male (h) and female (or hermaphrodite, i) transcriptomes between C. elegans (dotted line) vs. H. contortus (solid line). j Masculinised S. mansoni ortholog of C. sinensis gonad-enriched genes (defined as the former having at least 2-fold higher transcription level in testis than the latter), and demasculinised, feminised, or defeminised genes. Their testis or ovary transcription levels of each gene are shown in the heatmap, as well as its functional category, chromosome location in S. mansoni, and information about the knockdown phenotype of its C. elegans ortholog (Supplementary Data 10). k A proposed model for the transition of sexual systems in platyhelminths and nematodes. We hypothesised a 2-step model for the transition from a hermaphroditic ancestor to the gonochoristic schistosomes (see text). The recent independent transitions from gonochorism to androdioecy in different Caenorhabditis species have been studied previously^88,120, as have the genes reported here. Changes in gonad transcription levels in relation to the sexual systems are indicated using larger or smaller male or female/hermaphrodite symbols.