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Abstract
Through distant crossing, diploid, triploid and tetraploid hybrids of red crucian carp (Carassius auratus red var., RCC♀, Cyprininae, 2n = 100) × topmouth culter (Erythroculter ilishaeformis Bleeker, TC♂, Cultrinae, 2n = 48) were successfully produced. Diploid hybrids possessed 74 chromosomes with one set from RCC and one set from TC; triploid hybrids harbored 124 chromosomes with two sets from RCC and one set from TC; tetraploid hybrids had 148 chromosomes with two sets from RCC and two sets from TC. The 5S rDNA of the three different ploidy-level hybrids and their parents were sequenced and analyzed. There were three monomeric 5S rDNA classes (designated class I: 203 bp; class II: 340 bp; and class III: 477 bp) in RCC and two monomeric 5S rDNA classes (designated class IV: 188 bp, and class V: 286 bp) in TC. In the hybrid offspring, diploid hybrids inherited three 5S rDNA classes from their female parent (RCC) and only class IV from their male parent (TC). Triploid hybrids inherited class II and class III from their female parent (RCC) and class IV from their male parent (TC). Tetraploid hybrids gained class II and class III from their female parent (RCC), and generated a new 5S rDNA sequence (designated class I–N). The specific paternal 5S rDNA sequence of class V was not found in the hybrid offspring. Sequence analysis of 5S rDNA revealed the influence of hybridization and polyploidization on the organization and variation of 5S rDNA in fish. This is the first report on the coexistence in vertebrates of viable diploid, triploid and tetraploid hybrids produced by crossing parents with different chromosome numbers, and these new hybrids are novel specimens for studying the genomic variation in the first generation of interspecific hybrids, which has significance for evolution and fish genetics.
Highlights
Polyploidization, the addition of an extra set of chromosomes to the genome, is a predominant mechanism for speciation in plants and animals [1,2]
Formation of Diploid, Triploid and Tetraploid Hybrids The different ploidy levels of hybrid fish were determined by counting the chromosomes and examining DNA content using flow cytometry
The former is a direct and accurate method for determining ploidy of samples, and the latter is a rapid, simple and accurate method. Both chromosome number and karyotype formula of red crucian carp (RCC) were similar to the results of previous studies [11,12]. 2nRT, 3nRT and 4nRT hybrids possessed 74, 124 and 148 chromosomes respectively (Figure 2), in contrast to RCC (100) and topmouth culter (TC) (48)
Summary
Polyploidization, the addition of an extra set (or sets) of chromosomes to the genome, is a predominant mechanism for speciation in plants and animals [1,2]. Polyploid species are frequent in the plant kingdom [2,7], and 40–70% of all plant species are polyploids [3,8]. While it is generally relatively rare in animals [9,10], polyploidy has occurred extensively, independently, and is often repeated in many groups of fish, from the sharks to the higher teleosts [11,12,13]. Artificially induced polyploidy has been used in aquaculture to produce sterility and to improve production [14]
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