Figure 3.

Origin of the SPANX-C locus in humans. The figure shows the proposed scenario for the origin of the SPANX-C locus in humans. The scheme is based on the replication-based model of recombination with long-tract gene conversion (Richardson et al. 1998; Richardson and Jasin 2000). For simplicity, both loci are drawn in the same orientation from left to right, but, in fact, this duplication is accompanied by an inversion. The initial stimulus for recombination was a DNA break in an L1PA7 copy (dark blue, contains an AluSx insert) in the pre-C locus (preserved in the chimpanzee, bonobo, and gorilla), followed by an invasion of one end in the 44-bp-long stretch of identity (black) in the L1PA4 copy (light blue) of the SPANX-B locus. Repair synthesis continues further down past the end of L1PA4 until a random interruption in the LTR1B copy (green). The repair is then completed by nonhomologous end joining (NHEJ) of the invading strand with the original break. This model thus combines homologous recombination in the first step with nonhomologous recombination in the terminal stage. The new SPANX-C locus contains (1) a chimeric LINE1 element composed of an original L1PA7 fragment and a tail derived from L1PA4 (light blue), (2) a duplicated SPANX-B locus that is terminated with a LTR1B fragment, and (3) the terminal part of the L1PA7 element. The net result is duplication of the SPANX-C locus, while the second copy (SPANX-B) remains intact.