Unreduced diploid sperm from diploid hybrids and formation of a new type of tetraploid hybrid.

The production of unreduced gametes is an important resource for the formation of polyploids. Here, we obtained a second generation of diploid hybrids (2nKCF 2) derived from koi carp (KOC; 2n = 100; ♀) × colored crucian carp (CCC; 2n = 100; ♂) with the unusual reproductive trait of producing unreduced diploid sperm. Compared with KOC and CCC, 2nKCF 2 had unique morphological characteristics, including a gray and golden head, golden-brown scaly outer edge, and smooth golden-yellow back. FISH and 5S rDNA analysis revealed that 2nKCF 2 not only inherited genomic DNA from their parents but also lost some parental DNA fragments. Chromosomal spreads of germ cells provided direct cytological evidence for the production of unreduced diploid sperm by male 2nKCF 2. Electron microscopy results showed that the heads of the diploid sperm generated by 2nKCF 2 were twice as large as those of the haploid sperm generated by CCC (P < 0.05), confirming that the sperm of 2nKCF 2 were diploid. A new type of tetraploid hybrid (4n = 200) was produced by crossing 2nKCF 2 males with improved female allotetraploid hybrids of red crucian carp (♀) × common carp (♂). This study provides insights into reproductive biology and aquaculture. • Male individuals capable of producing diploid sperm and infertile female ones were obtained in the F 2 through distant hybridization of koi carp (KOC, 2n=100, ♀) and color crucian carp (CCC, 2n=100, ♂) in our laboratory. • At present, a new type of tetraploid hybrid (4nAK, 4n=200) was produced by crossing F 2 males hybrids with improved female allotetraploid hybrids. The 4nAK had very important significance in the establishment of the new type of tetraploid fish lineage with special genotypes with four sets of chromosome and the hybrid genomes, and unique phenotypes including gold head, golden-brown scaly outer edge, and smooth golden-yellow back. • Both 2nKCF 2 and 4nAK enriched the resource of diploid gametes, which was of great significance in fish genetic breeding and biological evolution. [ABSTRACT FROM AUTHOR]