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

Abstract

Bisexual fertile diploid androgenetic individuals (A₀) (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.) (♀) × common carp (Cyprinus carpio L.) (♂) were used to fertilize the UV-treated haploid eggs of goldfish (Carassius auratus), and living androgenetic diploid fish were developed. The A₀ became sexually mature at the age of 2 years, and they fertilized with each other to form their offspring (A₁). In this study, we observed the chromosomal number, gonadal structure and appearance of A₁ fish. The results are as follows: (1) In A₁, there were 85% tetraploids (A₁-4n), 10% triploids (A₁-3n) and 5% diploids (A₁-2n), suggesting that diploid A₀ 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 A₁, only A₁-4n possessed normal ovaries and testes. The mature males of A1-4n produced white semen. Under the electron microscope, the head of diploid sperm generated by A₁-4n was bigger than that of haploid sperm generated by red crucian carp. In the testes of the A₁-4n, 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 A₁-4n 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 A₁-2n and A₁-3n 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 A₁-4n 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.