Fig. 1: Chromosome evolution of Pristionchus nematodes with independent evolution of androdioecy.

a Proposed evolutionary path between sexual systems after Weeks, 2006⁷. b XO system in C. elegans and P. pacificus. Inheritance of X chromosomes (shown as green bars) and sex determination are visualized in selfing (left) and outcrossing (right). c Chromosome evolution between the closely related species Pristionchus occultus, P. exspectatus and P. pacificus. Coloured bars indicate the seven conserved chromosomal elements (i.e. Nigon elements). Three chromosome elements, P. pacificus ChrI left (NigonE, orange), ChrI right (NigonN, purple) and ChrX (NigonX, green), which are involved in fusion events, are arranged in a tri-radial pattern. Schematic diagrams in boxes represents diploid chromosome number of the three elements in the different sexes. Chromosomes stained by Hoechst 33342 in sperm cells of P. neolucani (d, n = 5), P. mayeri (e, n = 6), P. fissidentatus (f, n = 7), and P. maupasi (g, n = 12). In those species, we tested 259, 204, 86 and 83 cells, in which 252, 199, 33 and 82 cells indicated the same chromosome number, respectively. The scale bar indicates 2 µm. h. Karyotype analysis of 47 Pristionchus species with phylogenetic estimation. Proportion of male meiotic cells and sperms with each chromosome number is shown in the colored bar graph on the right side of the species name. The sample number is shown in Supplementary Data 1. Karyotypes (left of the species names) were categorized based on those chromosome numbers in male meiosis and sperm, as well as variations of chromosome number in sperm. Species labelled in bold letters are androdioecious, while others are gonochoristic species. The phylogenetic tree is based on a previous phylotranscriptome analysis⁹. The branch length is fitted by chronos function in R package, ape (lambda = 1)⁷⁶. Evolutionary events were parsimoniously estimated and are shown as rhombuses.