Spatiotemporal characteristics of the pharyngeal teeth in interspecific distant hybrids of cyprinid fish: Phylogeny and expression of the initiation marker genes.

As an important feeding organ and taxonomical characteristic, the pharyngeal teeth of cyprinid fish have very high morphological diversity and exhibit species-specific numbers and arrangements. Many genes have been verified to regulate the pharyngeal teeth development and act as the initiation marker for teeth. Six initiation marker genes for pharyngeal teeth were used as RNA probes to investigate the expression pattern, and these genes were further used to construct a phylogenetic tree for cyprinid fish including some distant hybrids. The results from in situ hybridization showed that similarities and differences existed in the expression of dlx2b, dlx4b, dlx5a, pitx2, fth1b, and scpp5 in the pharyngeal region of the hybrids (BT) by the crosses of blunt snout bream (BSB, ♀) × topmouth culter (TC, ♂). Particularly, we found a high specificity marker gene scpp5 for the early development of pharyngeal teeth. The Scpp5 expression pattern established a clear graphic representation on the spatiotemporal characteristics of the early morphogenesis of pharyngeal teeth in BT and BSB. Our results suggested that the scpp5 expression in 4V1, 3V1, and 5V1 in BT occurred earlier than that in BSB, while the replacement rate of pharyngeal teeth (4V2, 3V2, and 5V2) was faster in BSB. Phylogenetic analysis revealed that the six marker genes were highly conserved and could be used as the molecular marker for identifying the parents of the distant hybrids in cyprinid fish. The expression patterns of the scpp5 gene was examined in various tissues, including the brain, gill, heart, liver, muscle, skin, fins, gonad, eye, and kidney, showing that the scpp5 gene was ubiquitously expressed, indicating its important role in cyprinid fish.

An improved hybrid bream derived from a hybrid lineage of Megalobrama amblycephala (♀)×Culter alburnus (♂).

Distant hybridization is an important technique in fish genetic breeding. In this study, based on the establishment of an allodiploid fish lineage (BT, 2n=48, F1-F6) derived from distant hybridization between female Megalobrama amblycephala (BSB, 2n=48) and male Culter alburnus (TC, 2n=48), and the backcross progeny (BTB, 2n=48) derived by backcrossing female F1 of BT to male BSB, an improved hybrid bream (BTBB, 2n=48) was obtained by backcrossing BTB (♀) to BSB (♂). Moreover, the morphological and genetic characteristics of BTBB individuals were investigated; BTBB was similar to BSB in appearance but had a higher body height than BSB. The study results regarding chromosome numbers and DNA content indicated that BTBB is a diploid hybrid fish. The 5S rDNA and Hox gene of BTBB were inherited from the original parents. Gonadal development in BTBB was normal. On the other hand, BTBB had a faster growth rate, higher muscle protein level, and lower muscle carbohydrate level than BSB. Hence, bisexual fertile BTBB is promoted and can be applied as a high-quality fish, and it can also be used as a new fish germplasm resource to develop high-quality fish further. Thus, this study is of great significance for fish genetic breeding.

Characterization of H3 methylation in regulating oocyte development in cyprinid fish.

Histone post-modifications are important epigenetic markers involved in multiple cellular processes via regulation of gene transcription or remodeling of chromatin structure. Oocyte development is a critical process under rigorous control to prevent the generation of aberrant gametes. However, the regulatory mechanism of oocyte early development is not well-understood due to the tiny size and poor distinguishability of the gonad in juvenile stages. Here, two cyprinid hybrid fishes, a sterile allotriploid fish and a gynogenetic hybrid fish with delayed oocyte development, provided research models to investigate the mechanisms involved. We used cytogenetic and molecular methods to confirm the pachytene arrest of oocytes in allotriploid fish and gynogenetic hybrid fish. On the basis of these developmental differences, we screened 21 different histone H3 modifications by ELISA and found that four modifications (H3K4me3, H3K9me3, H3K79me, and H3K79me3) differed significantly in the two cyprinid hybrid fishes. Changes in histone methylation at the three residues (H3K4, K9, K79) were caused by specific methyltransferases and demethylases. Our results provide new insights into the epigenetic regulation of oocyte early development in fish, a process critical for understanding of reproductive biology and with practical applications in the aquacultural breeding industry.