Identification and effective regulation of scarb1 gene involved in pigmentation change in autotetraploid Carassius auratus.

The autotetraploid Carassius auratus (4nRR, 4 n=200, RRRR) is derived from whole-genome duplication of Carassius auratus red var. (RCC, 2 n=100, RR). In the current study, we demonstrated that chromatophores and pigment changes directly caused the coloration and variation of 4nRR skin (red in RCC, brownish-yellow in 4nRR). To further explore the molecular mechanisms underlying coloration formation and variation in 4nRR, we performed transcriptome profiling and molecular functional verification in RCC and 4nRR. Results revealed that scarb1, associated with carotenoid metabolism, underwent significant down-regulation in 4nRR. Efficient editing of this candidate pigment gene provided clear evidence of its significant role in RCC coloration. Subsequently, we identified four divergent scarb1 homeologs in 4nRR: two original scarb1 homeologs from RCC and two duplicated ones. Notably, three of these homeologs possessed two highly conserved alleles, exhibiting biased and allele-specific expression in the skin. Remarkably, after precise editing of both the original and duplicated scarb1 homeologs and/or alleles, 4nRR individuals, whether singly or multiply mutated, displayed a transition from brownish-yellow skin to a cyan-gray phenotype. Concurrently, the proportional areas of the cyan-gray regions displayed a gene-dose correlation. These findings illustrate the subfunctionalization of duplicated scarb1, with all scarb1 genes synergistically and equally contributing to the pigmentation of 4nRR. This is the first report concerning the functional differentiation of duplicated homeologs in an autopolyploid fish, substantially enriching our understanding of coloration formation and change within this group of organisms.

[Chromosomal studies of germ cells in diploid and polyploid fish produced by distant crossing].

By observing the chromosomal spreads of germ cells and tissue sections, we studied the chromosomal spreads of germ cells in diploid and polyploid fish produced by distant crossing. The samples covered the second generation of hybrids of red crucian carp (Carassius auratus red var.) (female) x common carp (Cyprinus carpio) (male) (2n = 100) (F2), allotetraploid hybrids of red crucian carp (female) x common carp (male)(4n = 200), triploid hybrids of gold fish (female)x allotetraploid (male) (3n = 150), the second generation of the gynogenetic progeny of allotetraploid hybrids (G2) (2n = 100), and the common carp (2n = 100) used as a control. The results demonstrated that chromosomal number of spermotogonia in common carp was equal to that of their somatic cell (2n = 100), while the chromosomal number of germ cells in diploid hybrid fish and polyploid hybrid fish had doubled obviously, and the frequent of chromosomal doubling in spermotogonia of F2 appeared especially high, accounting for 21.6% of all examined chromosomal spreads, which provided directly cytological evidence for the production of unreduced diploid gametes in F2 and also indicated that distant crossing was an important factor to lead to chromosomal doubling in germ cell. This research had important significance in studies on the formation of polyploidy fish and fish breeding.