Characteristics of testis development in autotetraploid fish

Abstract

Fertile autotetraploid fish are an excellent germplasm resource for producing improved triploids. However, males produce more diploid gametes than females during production, which means that males are more valuable in application. To better understand the testis development of autotetraploid fish (4nRR) derived from the distant hybridization of Carassius auratus red var. (RCC, ♀) × Megalobrama amblycephala (BSB, ♂), all-male autotetraploid fish (AM4nRR) were generated. In this study, the tissue structure and transcriptome changes in different periods of gonadal development of AM4nRR were analyzed. Histological sectioning revealed a small number of gonium at 45 days post-hatching (dph), the number of germ cells greatly increased during 65–95 dph, and primary spermatocytes and secondary spermatocytes were observed at 105 dph. Meanwhile, white semen could be extruded from most individuals at 120 dph, and the DNA content of sperm from AM4nRR was approximately twice that from RCC as determined by flow cytometry. These results indicated that AM4nRR reached sexual maturity at 120 dph and produced diploid sperm. In addition, a total of 149.55 Gb of clean data and 56,188 unigenes were obtained from the testicular tissue of AM4nRR at different developmental periods (25 dph, 35 dph, 40 dph, 45 dph, 65 dph, 95 dph and 120 dph). Differential expression analysis showed that the number of differentially expressed genes (DEGs) among the three periods of 40 dph, 45 dph, and 65 dph was the lowest (72–275). However, the expression levels of partial DEGs at 45 dph changed abruptly compared with those at 40 dph and 65 dph according to hierarchical clustering analysis, which was consistent with the expression characteristics of gonadal development-related genes at the early period of sex differentiation. These results suggested that 40–65 dph might be a critical period for the initiation of testis development in autotetraploid fish. These findings not only provide a new gamete resource for the large-scale preparation of improved triploids but also provide a basis for further study of sex differentiation in autotetraploid fish.