Table 1.

Numbers of species with different sex chromosome systems known in vertebrates. The 4 columns following the taxa are systems with a fusion between a sex chromosome and an autosome: Y-autosome fusions (X₁X₂Y systems), X-autosome fusions (XY₁Y₂ systems), W-autosome fusions (Z₁Z₂W systems), and Z-autosome fusions (ZW₁W₂ systems). From Pennell et al. (2015)

Taxa	Y-A	X-A	W-A	Z-A	XY	ZW
Fish	42	3	0	2	109	38
Amphibians	1	0	0	0	29	16
Reptiles	40	0	2	4	120	240
Birds	—	—	0	3	0	192
Mammals	18	24	—	—	467	0