Maternalalk8Promoter Fragment Directs Expression in Early Oocytes

Abundance of Early Embryonic Primordial Germ Cells Promotes Zebrafish Female Differentiation as Revealed by Lifetime Labeling of Germline

Teleost sex differentiation largely depends on the number of undifferentiated germ cells. Here, we describe the generation and characterization of a novel transgenic zebrafish line, Tg(piwil1:egfp-UTRnanos3)^ihb327Tg, which specifically labels the whole lifetime of germ cells, i.e., from primordial germ cells (PGCs) at shield stage to the oogonia and early stage of oocytes in the ovary and to the early stage of spermatogonia, spermatocyte, and spermatid in the testis. By using this transgenic line, we carefully observed the numbers of PGCs from early embryonic stage to juvenile stage and the differentiation process of ovary and testis. The numbers of PGCs became variable at as early as 1 day post-fertilization (dpf). Interestingly, the embryos with a high amount of PGCs mainly developed into females and the ones with a low amount of PGCs mainly developed into males. By using transient overexpression and transgenic induction of PGC-specific bucky ball (buc), we further proved that induction of abundant PGCs at embryonic stage promoted later ovary differentiation and female development. Taken together, we generate an ideal transgenic line Tg(piwil1:egfp-UTRnanos3)^ihb327Tg which can visualize zebrafish germline for a lifetime, and we have utilized this line to study germ cell development and gonad differentiation of teleost and to demonstrate that the increase of PGC number at embryonic stage promotes female differentiation.

A promoter fragment of the sycp1 gene is sufficient to drive transgene expression in male and female meiotic germ cells in zebrafish

The synaptonemal complex protein 1 (Sycp1) is required for the formation of crossovers that occurs during the meiotic prophase. The tissue and cell-specific expression pattern of the Sycp1 protein have been studied in mammals and fish, but data on the corresponding transcript remain scarce. In this report, we described for the first time in zebrafish the tissue- and cell-specific expression pattern of the sycp1 gene. In ovary, the expression of the sycp1 transcript was restricted to the early primary oocytes. In testis, the sycp1 transcript was observed in primary spermatocytes in agreement with a previous report describing the localization of the Sycp1 protein in those cells. Unexpectedly, sycp1 transcript expression remained high in spermatids. To gain insight on the genomic region responsible for the sycp1 gene expression pattern, we generated four independent Dr_sycp1:eGFP transgenic zebrafish lines carrying the -1482/+338 gene fragment fused to the enhanced green fluorescent protein reporter gene. We demonstrate that this promoter fragment contains the information required for the cell-specific expression of the endogenous sycp1 gene in males and in females. However, the GFP protein and its associated fluorescence persist in spermatozoa and maturing oocytes. The Dr_sycp1:eGFP zebrafish lines have the potential to be valuable models to trace meiosis onset in zebrafish and constitute the first transgenic lines expressing the GFP reporter protein only in the male meiotic and postmeiotic cells in fish.

Spatiotemporal expression of zebrafish keratin 18 during early embryogenesis and the establishment of a keratin 18:RFP transgenic line

Zebrafish cytokeratin 18 (K18) is one of the type I keratin genes expressed the earliest after amputation of the zebrafish fin, but its spatiotemporal expression during early development is unclear. Whole-mount in situ hybridization revealed that k18 was a maternally inherited gene and that its expression is restricted to the single layer of enveloping cells on the surface of embryos during the gastrula stage. At later stages, K18 expression was detected in the epithelial cells, pronephric duct, digestive tract, dorsal aorta, and fins. By using microinjection, we generated the transgenic line Tg(k18(2.9):RFP), which carries an upstream 2.9-kb segment of k18 gene fused with a red fluorescent protein (RFP) reporter. The spatiotemporal distributions of red fluorescent signal of Tg(k18(2.9):RFP) line correlated well with endogenous k18 transcripts detected by whole-mount in situ hybridization, indicating that this line is capable of recapitulating endogenous k18 expression patterns. We noticed that the red fluorescence appeared strongly in the dorsal, pectoral, pelvic, anal, and caudal fins when transgenic fish became adults. Interestingly, we also found that when F1 female from the Tg(k18(2.9):RFP) line were mated with wild-type males, 100% (326/326) of F2 offspring expressed red fluorescence at the one-cell stage. In contrast, when F1 male from the Tg(k18(2.9):RFP) line were mated with wild-type females, only 49.8% (138/277) of F2 embryos exhibited red fluorescence. On the basis of these findings, we suggest that the transcript of zebrafish K18 is inherited as a maternal effect. We believe that Tg(k18(2.9):RFP) fish should be an excellent experimental animal for studying the zygotic regulatory mechanism of k18.