Sex is generally determined by sex chromosomes in vertebrates, but sex evolution and sex chromosome origin remains elusive. Recently, the research team led by Professor GUI Jianfang from Institute of Hydrobiology, Chinese Academy of Sciences (IHB) has revealed that the extra microchromosomes may play significant role in male determination during the evolutionary trajectory of reproduction mode transition from unisexual to sexual reproduction in the polyploid gibel carp (Carassius gibelio). 

They firstly isolated a male-specific sequence via analysis of amplified fragment length polymorphism (AFLP) in gibel carp. Subsequently, they found out that males contained several extra microchromosomes with repetitive sequences and transposable elements than females through fluorescence in situ hybridization (FISH) analysis. Moreover, sex-reversed physiological females with male-specific marker were produced by estradiol hormone treatment, and two gynogenetic families were established from them. And male incidence rates of two gynogenetic families were revealed to be closely associated with extra microchromosome number of the sex-reversed physiological females. Thus, it is suggested that the extra microchromosomes in males might resemble common feature of sex chromosomes and might play significant role in male determination of gibel carp.  

In summary, these findings provide clear evidence for understanding the association among sex chromosome origin and sexual reproduction evolution in vertebrates. This study was performed by Dr. LI Xiyin et al., and the corresponding author was Professor GUI Jianfang. This work was supported by National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences and the earmarked fund for Modern Agro-industry Technology Research System. The paper entitled “Extra microchromosomes play male determination role in polyploid gibel carp” has been published in the journal Genetics. 

Extra microchromosomes in males of gibel carp (A) FISH analysis in metaphase of females. (B), (C) FISH analysis in metaphases of males. (D) Statistical data of microchromosome number in females. (E) Statistical data of microchromosome number in males. (F) Morphological comparisons of microchromosomes in (A), (B) and (C). Extra microchromosomes in males are indicated in white box.