Expression and Sequence Evolution of Aromatase cyp19a1 and Other Sexual Development Genes in East African Cichlid Fishes

Abstract

Sex determination mechanisms are highly variable across teleost fishes and sexual development is often plastic. Nevertheless, downstream factors establishing the two sexes are presumably conserved. Here, we study sequence evolution and gene expression of core genes of sexual development in a prime model system in evolutionary biology, the East African cichlid fishes. Using the available five cichlid genomes, we test for signs of positive selection in 28 genes including duplicates from the teleost whole-genome duplication, and examine the expression of these candidate genes in three cichlid species. We then focus on a particularly striking case, the A- and B-copies of the aromatase cyp19a1, and detect different evolutionary trajectories: cyp19a1A evolved under strong positive selection, whereas cyp19a1B remained conserved at the protein level, yet is subject to regulatory changes at its transcription start sites. Importantly, we find shifts in gene expression in both copies. Cyp19a1 is considered the most conserved ovary-factor in vertebrates, and in all teleosts investigated so far, cyp19a1A and cyp19a1B are expressed in ovaries and the brain, respectively. This is not the case in cichlids, where we find new expression patterns in two derived lineages: the A-copy gained a novel testis-function in the Ectodine lineage, whereas the B-copy is overexpressed in the testis of the speciest-richest cichlid group, the Haplochromini. This suggests that even key factors of sexual development, including the sex steroid pathway, are not conserved in fish, supporting the idea that flexibility in sexual determination and differentiation may be a driving force of speciation.

Genetics of Sexual Development: An Evolutionary Playground for Fish

“…Data: Oreochromis niloticus Ijiri et al 2008;Baroiller et al 2009a;Poonlaphdecha et al 2011Poonlaphdecha et al , 2013Tao et al 2013; http://www.uniprot.org/uniprot/ E9RGV7), Oryzias latipes Klüver et al 2005;Nakamoto et al 2007;Shibata et al 2010;Siegfried 2010;Okubo et al 2011;Nakamura et al 2012), Odontesthes hatcheri (Hattori et al 2013), Oncorhynchus mykiss (Baron et al 2005;Sawatari et al 2007;Gohin et al 2011;Vizziano-Cantonnet et al 2011), D. rerio (Chiang et al 2001;Guo et al 2005;Von Hofsten and Olsson 2005;Kallivretaki et al 2007;Siegfried 2010;Gautier et al 2011b;Nakamura et al 2012), Astatotilapia burtoni (Böhne et al 2013;A. Böhne and W. Salzburger, unpublished results).…”

“…This is evidenced by varying and species-specific expression patterns between sexes and throughout development (see, e.g., Vizziano et al 2007;Ijiri et al 2008;Hale et al 2011;Herpin et al 2013;and Table 1). For example, sox9 and the aromatase cyp19a1 show expression patterns in East African cichlid fishes that are not consistent with conserved testis and ovary functions, respectively (Böhne et al 2013).…”

“…After the teleost-specific genome duplication, teleost fish retained two copies of this gene (cyp19a1A and cyp19a1B), which are active in ovary and brain tissue, respectively. Some derived East African cichlid species express the otherwise ovary-specific gene cyp19a1A in testis (Böhne et al 2013). This finding asks for a critical revision of the hormonal control of sexual differentiation.…”

Inhibition of Aromatase Induces Partial Sex Change in a Cichlid Fish: Distinct Functions for Sex Steroids in Brains and Gonads

“…Estradiol is responsible for inducing and maintaining ovarian development [Baroiller et al, 1999] and is produced via the aromatization of T by the aromatase enzyme Cyp19a1. Fishes have 2 gene copies for this enzyme, cyp19a1a and cyp19a1b [Piferrer and Blázquez, 2005], which emerged from the teleost specific genome duplication [Böhne et al, 2013]. Cyp19a1a is usually highly expressed in the gonad whereas cyp19a1b is higher expressed in the brain [Böhne et al, 2013].…”

“…Fishes have 2 gene copies for this enzyme, cyp19a1a and cyp19a1b [Piferrer and Blázquez, 2005], which emerged from the teleost specific genome duplication [Böhne et al, 2013]. Cyp19a1a is usually highly expressed in the gonad whereas cyp19a1b is higher expressed in the brain [Böhne et al, 2013]. Overall, aromatase activity in the brain of fishes is reported to be 100-1,000 times higher than in other vertebrate taxa [Forlano et al, 2006].…”