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The Research Advances in Animal Distant Hybridization and Polyploid Organisms 1 ����Shaojun Liu�� Shi Wang�� Qingfeng Liu�� Chang Wu�� Yi Zhou�� Min Tao�� Chun Zhang�� Qinbo Qin�� and Kaikun Luo ����Abstract ����Distanthybridization leads to genetic changes which contribute to rapid evolution and heterosis for higher growth rates�� better adaptability�� and stronger disease resistance in offspring. Some studies show that widespread distant hybridizations occur in animals in nature. Hybridization events bring new genetic diversity for animalpopulations which are favorable for genetic breeding. After hybridization�� phenotypic variations are driven by interaction of subgenomes such as genetic recombination. In addition�� the new genotypes provide possibility for speciation. Simultaneously�� polyploidization derived from the distant hybridization is wide-spread in natural organisms including autopolyploid and allopolyploid. Distant hybridization can generate new lineages including the diploid lineages and tetraploid lineages. Through distant hybridization�� a series of diploid and poly-ploid hybrid .sh lineages have been generated�� and they can be used as the new germplasm resources to produce the improved .sh�� which have great signi.cance in biological evolution�� genetics�� and breeding. In this chapter�� we give a brief introduction to animal distant hybridization and polyploid organisms. ����Keyword ����Distant hybridization Polyploid Breeding Speciation Fish Genome ����duplication ����1.1 The Research Advances in Animal Distant Hybridization ����Distant hybridization is de.ned as a crossing between two different species or higher-level taxa�� which can promote the transfer of genomes between species and lead to phenotypic and genotypic changes in progeny. In terms of genotypes�� distant hybridization can lead to changes in genetics�� such as the production of new diploid�� triploid�� and tetraploid. At the level of genomic DNA�� distant hybridization leads to changes in DNA recombination�� deletion�� and insertion in offspring. With regard to phenotypes�� distant hybridization can produce heterosis with higher growth rate�� better adaptability�� as well as stronger disease resistance(Liu2010). ����Anumber of fertile and stable allopolyploid plants were obtained by interspeci.c hybridization�� including allotetraploid Raphanobrassica derived from chromosome doubling and allohexaploid wheat(Triticum aestivum)generated by hybridization (Liu 1991). Researchers thought that gibel carp(C. gibelio)has undergone several rounds of genome polyploidy and experienced an additional�� more recent genome duplication event. In addition�� gibel carp are under an evolutionary trajectory of diploidization (Guiand Zhou 2010). Under natural conditions�� diploid�� triploid�� and tetraploid crucian carp have been con.rmed to exist (Xiao et al. 2011). More and more studies have shown that the evolution of polyploid .sh is related to distant hybridization (Meyer et al. 2006; Saitoh et al. 2010; Nolte et al. 2005). Distant hybridization has been shown to favor speciation and evolutionary radiation. The allopolyploidization caused by distant hybridization is a potential driving force for the occurrence of new species (Grant et al. 2005; Mallet 2007). The progeny of distant hybridization contains genomes from different species�� and the interaction of two genomes providesabasis for gene recombinationin offspring. The formation of fertile distant hybridlineages may create new variations and even new species. ����InSect.1.1ofChap.1��we provideanoverall introductionand reviewofdistant hybridizationin animals�� especiallyin the formationof fertile distanthybrid lineages and their characteristics of genetic variation. The applications of distant hybrid lineages in aquaculture are also introduced. This overview can provide a useful overall reference for the in-depth study of animal hybridization (including .sh hybridization) in the future. ����1.1.1 The Animal Distant Hybridization Profile ����1.1.1.1 Hybridization Between Phyla ����Hybridization between different phyla has been found in severalhybrids�� including sea urchin (A. crassispina�� ��) from Echinodermata x mollusk (Acmaea nanshaensis�� ��)from Mollusca�� sea urchin(��)x blue mussel(Mytilus edulis�� ��) from Mollusca�� and sea urchin(��)x annelid(Nereis succinea�� ��)from Annelida. The larvaeof all three kinds of hybrids weresimilar to those of sea urchins. Although sperm from the paternal parent entered into the egg�� the fusion of distant male and female nuclei was dif.cult. Even if the phenomenon of fusion has occurred�� the chromosomes from paternal parent were eventually degraded and not merged into the nuclei of the offspring. Therefore�� these hybridlarvae resembled the larvae of sea urchin�� the maternal parent (Zhu1961). ����1.1.1.2 Hybridization Between Classes ����Interclass hybridization has been observed in Echinodermata between sea urchin(��) and C. hesperus (��)�� and their larval offspring were simil