Genome-Wide Identification, Sequence Alignment, and Transcription of Five Sex-Related Genes in Largemouth Bass (Micropterus Salmoides).

BACKGROUND: Largemouth bass (Micropterus Salmoides) is an economically important fish species in China. Most research has focused on its growth, disease resistance, and nutrition improvement. However, the sex-determining genes in largemouth bass are still unclear. The transforming growth factor-beta (TGF-ß) gene family, including amh, amhr2 and gsdf, plays an important role in the sex determination and differentiation of various fishes. These genes are potentially involved in sex determination in largemouth bass. METHODS: We performed a systematic analysis of 5 sex-related genes (amh, amhr2, gsdf, cyp19a1, foxl2) in largemouth bass using sequence alignment, collinearity analysis, transcriptome, and quantitative real-time polymerase chain reaction (qRT-PCR). This included a detailed assessment of their sequences, gene structures, evolutionary traits, and gene transcription patterns in various tissues including gonads, and at different developmental stages. RESULTS: Comparative genomics revealed that the 5 sex-related genes were highly conserved in various fish genomes. These genes did not replicate, mutate or lose in largemouth bass. However, some were duplicated (amh, amhr2 and gsdf), mutated (gsdf) or lost (amhr2) in other fishes. Some genes (e.g., gsdf) showed significant differences in genomic sequence between males and females, which may contribute to sex determination and sex differentiation in these fishes. qRT-PCR was applied to quantify transcription profiling of the 5 genes during gonadal development and in the adult largemouth bass. Interestingly, amh, amhr2 and gsdf were predominantly expressed in the testis, while cyp19a1 and foxl2 were mainly transcribed in the ovary. All 5 sex-related genes were differentially expressed in the testes and ovaries from the 56th day post-fertilization (dpf). We therefore speculate that male/female differentiation in the largemouth bass may begin at this critical time-point. Examination of the transcriptome data also allowed us to screen out several more sex-related candidate genes. CONCLUSIONS: Our results provide a valuable genetic resource for investigating the physiological functions of these 5 sex-related genes in sex determination and gonadal differentiation, as well as in the control of gonad stability in adult largemouth bass.

Whole genome sequencing of the fast-swimming Southern bluefin tuna (Thunnus maccoyii).

The economically important Southern bluefin tuna (Thunnus maccoyii) is a world-famous fast-swimming fish, but its genomic information is limited. Here, we performed whole genome sequencing and assembled a draft genome for Southern bluefin tuna, aiming to generate useful genetic data for comparative functional prediction. The final genome assembly is 806.54 Mb, with scaffold and contig N50 values of 3.31 Mb and 67.38 kb, respectively. Genome completeness was evaluated to be 95.8%. The assembled genome contained 23,403 protein-coding genes and 236.1 Mb of repeat sequences (accounting for 29.27% of the entire assembly). Comparative genomics analyses of this fast-swimming tuna revealed that it had more than twice as many hemoglobin genes (18) as other relatively slow-moving fishes (such as seahorse, sunfish, and tongue sole). These hemoglobin genes are mainly localized in two big clusters (termed as "MNË® and "LAË® respectively), which is consistent with other reported fishes. However, Thr39 of beta-hemoglobin in the MN cluster, conserved in other fishes, was mutated as cysteine in tunas including the Southern bluefin tuna. Since hemoglobins are reported to transport oxygen efficiently for aerobic respiration, our genomic data suggest that both high copy numbers of hemoglobin genes and an adjusted function of the beta-hemoglobin may support the fast-swimming activity of tunas. In summary, we produced a primary genome assembly and predicted hemoglobin-related roles for the fast-swimming Southern bluefin tuna.