Fig. 2: B. malayi Chr X mapping across O. volvulus Chr 1 (NX) and Chr X (ND).

An analysis of the B. malayi X chromosome indicates that it is the product of a fusion of two Nigon elements: ND and NX, in contrast with the O. volvulus X chromosome, which is composed of a fusion between ND and NE. The middle panel, which includes combined mummerplots to highlight Nigon element matches, shows the synteny of the B. malayi X chromosome to its corresponding two Nigon elements in O. volvulus. Each axis is marked with a corresponding sequencing depth plot of that chromosome. Thus, the X-axis of the main plot is linked to the sequencing depth of the Brugia malayi X chromosome, which is divided into ND (gray) and NX (blue). The left Y-axis shows the corresponding O. volvulus chromosome X, composed of ND (gray) and NE (yellow), while the right Y-axis shows the corresponding O. volvulus chromosome 1, composed of NA (red) and NX (blue). In both B. malayi and O. volvulus, ND is fused to a Nigon element that was likely autosomal in the most recent common ancestor, but those fusions occur at opposite ends of ND in the two nematodes.