Homology of the X chromosomes of P. cognata (order: Mecoptera) and 3 other insects: 2 Diptera, C. hominivorax (suborder: Brachycera) and N. appendiculata (suborder: Nematocera), and L. migratoria (order: Orthoptera). (a) Phylogenetic tree of the 4 species. (b) Percentage of genes on each of the C. hominivorax's Muller elements that are X-linked in P. cognata. The dashed line represents the overall proportion of orthologs that are X-linked in P. cognata (i.e. the “X-linkage threshold”). (c) Percentage of X-linked and autosomal P. cognata genes that are X-linked in N. appendiculata. The X-linked genes of P. cognata were divided into 2 sets, based on whether they were F-linked in C. hominivorax (X in F), or not (X not F). (d) Same as (c) but showing the percentage of P. cognata genes that are X-linked in L. migratoria. Statistically significant differences between observed and expected percentages were assessed using a chi-squared test (*P < 0.05, **P < 0.01, ***P < 0.001, NS not significant). (e, f, g) Synteny plots between P. cognata's 25 largest genome scaffolds and the genomes of the 3 other insect species. Note that no synteny was found between P. cognata scaffold 19 and the genomes of the other species.