Estrogens interact with classical intracellular nuclear receptors (ESR), and with G-coupled membrane receptors (GPER). In the eel, we identified three nuclear (ESR1, ESR2a, ESR2b) and two membrane (GPERa, GPERb) estrogen receptors. Duplicated ESR2 and GPER were also retrieved in most extant teleosts. Phylogeny and synteny analyses suggest that they result from teleost whole genome duplication (3R). In contrast to conserved 3R-duplicated ESR2 and GPER, one of 3R-duplicated ESR1 has been lost shortly after teleost emergence. Quantitative PCRs revealed that the five receptors are all widely expressed in the eel, but with differential patterns of tissue expression and regulation. ESR1 only is consistently up-regulated in vivo in female eel BPG-liver axis during induced sexual maturation, and also up-regulated in vitro by estradiol in eel hepatocyte primary cultures. This first comparative study of the five teleost estradiol receptors provides bases for future investigations on differential roles that may have contributed to the conservation of multiple estrogen receptors.

Duplicated cyp19a1 genes (cyp19a1a encoding aromatase a and cyp19a1b encoding aromatase b) have been identified in an increasing number of teleost species. Cyp19a1a is mainly expressed in the gonads, while cyp19a1b is mainly expressed in the brain, specifically in radial glial cells, as largely investigated by Kah and collaborators. The third round of whole-genome duplication that specifically occurred in the teleost lineage (TWGD or 3R) is likely at the origin of the duplicated cyp19a1 paralogs. In contrast to the situation in other teleosts, our previous studies identified a single cyp19a1 in eels (Anguilla), which are representative species of a basal group of teleosts, Elopomorpha. In the present study, using genome data mining and phylogenetic and synteny analyses, we confirmed that the whole aromatase genomic region was duplicated in eels, with most aromatase-neighboring genes being conserved in duplicate in eels, as in other teleosts. These findings suggest that specific gene loss of one of the 3R-duplicated cyp19a1 paralogs occurred in Elopomorpha after TWGD. Similarly, a single cyp19a1 gene was found in the arowana, which is a representative species of another basal group of teleosts, Osteoglossomorpha. In eels, the single cyp19a1 is expressed in both the brain and the gonads, as observed for the single CYP19A1 gene present in other vertebrates. The results of phylogenetic, synteny, closest neighboring gene, and promoter structure analyses showed that the single cyp19a1 of the basal teleosts shared conserved properties with both teleost cyp19a1a and cyp19a1b paralogs, which did not allow us to conclude which of the 3R-duplicated paralogs (cyp19a1a or cyp19a1b) was lost in Elopomorpha. Elopomorpha and Osteoglossomorpha cyp19a1 genes exhibited preserved ancestral functions, including expression in both the gonad and brain. We propose that the subfunctionalization of the 3R-duplicated cyp19a1 paralogs expressed specifically in the gonad or brain occurred in Clupeocephala, after the split of Clupeocephala from Elopomorpha and Osteoglossomorpha, which represented a driving force for the conservation of both 3R-duplicated paralogs in all extant Clupeocephala. In contrast, the functional redundancy of the undifferentiated 3R-duplicated cyp19a1 paralogs in elopomorphs and osteoglossomorphs would have favored the loss of one 3R paralog in basal teleosts.