In mouse, the three Cyp26 paralogs reside on two chromosomes: Amikacin hydrate Cyp26b1 is on mouse chromosome 6, and Cyp26a1 and Cyp26c1 are adjacent and transcribed in the same orientation on Mmu19, consistent with the origin of Cyp26a1 and Cyp26c1 by tandem duplication rather than by genome 
duplication. In teleosts, however, the three Cyp26 paralogs are on three different chromosomes linkage group 12 and on medaka chromosome 19 ; cyp26b1 on Dre7 and Ola18; and cyp26c1 on Dre17 and Ola15). Genomic analysis at the chromosomal level using circleplots revealed orthology relationships between each Cyp26 chromosomal neighborhood in mouse with its corresponding orthologous chromosomal region in zebrafish and medaka : The mouse Cyp26b1 genomic neighborhood connected to teleost cyp26b1 neighborhoods; and the mouse Cyp26a1/Cyp26c1 neighborhood connected to teleost neighborhoods that harbor cyp26a or cyp26c1, respectively. Local analysis of each Cyp26 genomic neighborhood using the Synteny Database identified conserved gene neighbors near each Cyp26 ortholog: Cyp26b1 with Dysf, cyp26a1 with Pde6c, and cyp26c1 with Tbc1d12. These results provide robust evidence that rules out the hypothesis that reciprocal Cyp26 paralog loss occurred between zebrafish and mouse. Comparative analysis between zebrafish genomic neighborhoods Euphorbia factor L3 surrounding cyp26a1 in Dre12 and cyp26c1 in Dre17 revealed that these two regions are indeed paralogons. This finding suggests the hypothesis that prior to the tetrapod-teleost divergence, cyp26a1 and cyp26c1 were already adjacent in an ancestral chromosome due to a tandem duplication, and that after the teleost genome duplication event, cyp26a1 and cyp26c1 duplicates survived reciprocally in each paralogon, leading to the present situation in which cyp26a1 and cyp26c1 are on different teleost chromosomes. Overall, our phylogenetic and comparative genomic analyses provided robust evidence that rules out the hypothesis of reciprocal Cyp26 gene losses in the zebrafish and mouse lineages and supports the recently postulated Cyp26 ontogeny. We conclude, therefore, that the most parsimonious explanation is that the Cyp26 pro-ortholog predating the expansion of the gene family was already involved in the regulation of RA levels during gonad development, and that both Cyp26a1 and Cyp26b1 maintained this function in the last common ancestor of zebrafish and mouse. Subsequently, independent subfunction partitioning likely led to the reciprocal retention of the gonad function of Cyp26a1 in the lineage leading to zebrafish and Cyp26b1 in the tetrapod lineage. We wondered whether independent partitioning of subfunctions of the ancestral Cyp26 gene that were unrelated to the gonad occurred in the same manner as subfunction partitioning in the gonad. Comparative analysis of the three cyp26 paralogs during late eye development in zebrafish and mouse revealed that the zebrafish ortholog of each of these three genes has the same expression pattern in the retina as its mouse ortholog. This result suggests that at least some ancestral subfunctions partitioned the same way in the three Cyp26 genes, perhaps before the divergence of zebrafish and mouse lineages, but the gonadal subfunction partitioned reciprocally in the two lineages after they diverged. This work provides, to our knowledge, the first comprehensive genomic and molecular analysis of the genetic machinery that regulates the synthesis and degradation of RA at the time that zebrafish gonads tip their sexual fate towards the male or female pathway. Our findings reveal several significant differences between RA-regulated gonadogenesis in zebrafish and tetrapods, including which cells express RAsynthesizing enzymes.