Differential Conservation and Divergence of Fertility Genes boule and dazl in the Rainbow Trout

Abstract

BackgroundThe genes boule and dazl are members of the DAZ (Deleted in Azoospermia) family encoding RNA binding proteins essential for germ cell development. Although dazl exhibits bisexual expression in mitotic and meiotic germ cells in diverse animals, boule shows unisexual meiotic expression in invertebrates and mammals but a bisexual mitotic and meiotic expression in medaka. How boule and dazl have evolved different expression patterns in diverse organisms has remained unknown.Methodology and Principal FindingsHere we chose the fish rainbow trout (Oncorhynchus mykiss) as a second lower vertebrate model to investigate the expression of boule and dazl. By molecular cloning and sequence comparison, we identified cDNAs encoding the trout Boule and Dazl proteins, which have a conserved RNA-recognition motif and a maximal similarity to their homologs. By RT-PCR analysis, adult RNA expression of trout boule and dazl is restricted to the gonads of both sexes. By chromogenic and two-color fluorescence in situ hybridization, we revealed bisexual and germline-specific expression of boule and dazl. We found that dazl displays conserved expression throughout gametogenesis and concentrates in the Balbinani's body of early oocytes and the chromatoid body of sperm. Surprisingly, boule exhibits mitotic and meiotic expression in the male but meiosis-specific expression in the female.ConclusionsOur data underscores differential conservation and divergence of DAZ family genes during vertebrate evolution. We propose a model in which the diversity of boule expression in sex and stage specificity might have resulted from selective loss or gain of its expression in one sex and mitotic germ cells.

Germ cell sex prior to meiosis in the rainbow trout

“…In both rainbow trout and medaka, vasa expression in gonia does not show sex dimorphism. Data for rainbow trout are from the present study and (Li et al, 2011 In press), and data for medaka are according to Together with transplantation data (Okutsu et al, 2006;Yoshizaki et al, 2010), our finding suggests the plasticity of gonial sexuality for reprogramming into both sexes under the influence of the gonadal sex. Our findings suggest that sperm/ egg decision and mitosis/meiosis decision are two independent events, and that sperm/egg decision appears to precede mitosis/meiosis decision.…”

“…Recently, we have established that boule and dazl encoding RNA binding proteins of the DAZ family are germ cell markers in rainbow trout (Li et al, 2011). By using them and vasa as highly conserved germ cell marker also in rainbow trout (Xu et al, 2010), we performed in situ hybridization (ISH) to examine differential expression between male and female germ cells before and during meiosis.…”

“…In many organisms, boule expression has been reported to be meiotic in one sex Li et al, 2011). One exception is medaka, where boule expression occurs in PGCs, mitotic and meiotic germ cells of both sexes .…”

“…The diversity of regulators governing sperm/egg decision and male/female decision provides the complexity to identify premeiotic germ cell sex in different organisms. Among germ cell markers examined so far, boule exhibits exceptional variation in its expression, ranging from male/female meiotic expression to bisexual mitotic and meiotic expression Li et al, 2011). In this regard, it is not surprising that boule becomes a gonial sex marker in one fish but not in another.…”

“…Briefly, pOMbol, pOMdazl and pOMvasa were linearized with Xho I and Sac II and used for the synthesis of sense and anti-sense RNA probes from SP6 or T7 promoter by using the digoxigenin (DIG) Labeling Kit (Roche) for dazl and vasa, or the FITC RNA Labeling Kit (Roche) for boule (Li et al, 2011). The RNA probes were treated with RNase-free TURBO DNase (Ambion) and purified by using the LiCl method according to the supplier's instruction (cat #AM1340, Ambion).…”