Figure 6. Model of NR5A/NR6A gene evolution.

The ancestral gene segment contains one NR5 gene. After tandem duplication, this gene became the ancestor of NR5 and NR6 in vertebrates. Two more rounds of gene amplification ensue, resulting in four sets of the gene segment. Some of the genes were subsequently lost in the lobe-fin lineage, leading to the current human genome with one NR6A1 and three NR5A genes. In the ray-fin lineage, the genome was duplicated further, followed by loss of chromosomal segments, forming zebrafish chromosomes with two nr6a1 and four ff1 genes.