The formation of improved tetraploid population of red crucian carp x common carp hybrids by androgenesis

Bisexual fertile diploid androgenetic individuals (A(0)) (2n=100) were formed by androgenesis. In this way, the diploid spermatozoa from male allotetraploid hybrids (AT) (4n=200) of red crucian carp (Carassius auratus red var.) (female) x common carp (Cyprinus carpio L.) (male) were used to fertilize the UV-treated haploid eggs of goldfish (Carassius auratus), and living androgenetic diploid fish were developed. The A(0) became sexually mature at the age of 2 years, and they fertilized with each other to form their offspring (A(1)). In this study, we observed the chromosomal number, gonadal structure and appearance of A1 fish. The results are as follows: (1) In A(1), there were 85% tetraploids (A(1)-4n), 10% triploids (A(1)-3n) and 5% diploids (A(1)-2n), suggesting that diploid A(0) could produce diploid gametes. It was concluded that the formation of diploid gametes generated from diploid A0 was probably related to the mechanism of pre-meiotic endoreduplication. (2) Among A(1), only A(1)-4n possessed normal ovaries and testes. The mature males of A(1)-4n produced white semen. Under the electron microscope, the head of diploid sperm generated by A(1)-4n was bigger than that of haploid sperm generated by red crucian carp. In the testes of the A(1)-4n, there were many mature normal spermatozoa with a head bearing plasma membrane and a tail having the typical structure of "9+2" microtubules. Between the head and the tail, there were some mitochondria. The ovaries of A(1)-4n developed well and mainly contained II, III and IV-stage oocytes. The IV-stage oocytes were surrounded by inner and outer follicular cells. The micropyle was observed on the oolemma of follicular cells. There were abundant yolks and plenty of endoplasmic reticulum in the cytoplasm of IV-stage oocytes. Because A(1)-2n and A(1)-3n were distant crossing diploid hybrids and triploid hybrids respectively, they possessed abnormal gonads, and no mature semen and eggs were observed. (3) Compared with allotetraploids, the A(1)-4n fish not only had advantages such as fast growth rate and strong resistibility but also showed some new good performances such as high ratio of body width to body length, smaller heads and shorter tails. These results indicated that androgenesis could produce bisexual fertile tetraploids and improve the shape of allotetraploid hybrids as well, which will be of great significance in both the cell genetics research and fish breeding.