**3. Hybridization in Cyprinidae**

Hybridization is more widespread among members of Cyprinidae than any other groups of freshwater fishes [23]. Several permutation and combinations of interspecific and intergeneric crosses were carried out among Indian major carps (*Catla catla*, *Labeo rohita* and *Cirrhinus mrigala*) and exotic carps (*Hypophthalmichthys molitrix*, *Ctenopharyngodon idella*, *Aristichthys nobilis* and *Cyprinus carpio*) with the major objective to achieve hybrid vigor in economic traits [24–31]. The Indian major carp species are known to be able to hybridize, and hybrids are fertile and can be backcrossed to the parental species [32–34]. Hybridization has been shown to have a significant impact on production-related traits (notably growth), with some studies reporting a growth rate of F1 hybrids intermediate between that of the parent species [35, 36]. Hybrids are reported to have lower rates of growth than either of the parental species of Indian carps, other studies reporting growth rates lower than for either of the parental species [37]. Intergeneric hybrids between catla (*Catla catla*) and fimbriatus (*Labeo fimbriatus*) were produced by employing the technique of hypophysation and dry stripping. Detailed investigations on their embryonic and larval development, taxonomic characters, and aquaculture potential in terms of growth, feed utilization, body carcass composition, meat yield, etc. were carried out [28].

A catla-rohu hybrid produced by hypophysation was found to be intermediate in general appearance to the parent species. Gut content analysis revealed that the hybrid was mainly phytophagous in its diet. Growth rate was observed to be faster than in rohu. It matured within 3 years and was equally responsive to hypophysation [38]. In other cyprinid species, also such efforts of hybridization were carried out [23, 39].

#### **3.1 Purposes and merits of human-mediated hybridization**

It has long been recognized that hybridization can have a variety of evolutionary outcomes, including outcomes that maintain or increase diversity such as stable hybrid zones, the evolutionary rescue of small inbred populations, the origin and transfer of adaptations, the reinforcement of reproductive isolation, and the formation of new hybrid lineages [40, 41].

Hybridization in carps was being carried out to increase growth rate, transfer desirable traits between species, combine desirable traits of two species into a single group of fishes, reduce unwanted reproduction through production of sterile fish or mono-sex offspring, take advantage of sexual dimorphism, increase harvestability, increase environmental tolerances, and increase overall hardiness in culture conditions. Hybrids constitute a significant proportion of some countries' production for certain taxa, for example, hybrid striped bass in the USA, hybrid clarid catfish in Thailand, hybrid characids in Venezuela, and hybrid tilapia in Israel. Hybridization has been used in tandem with polyploidization to improve developmental stability in hybrid progeny [1]. Intergeneric hybrids between catla (*Catla catla*) and fimbriatus (*Labeo fimbriatus*) combined desirable qualities such as the small head of the fimbriatus and the deep body of the catla and exhibited heterosis in terms of meat yield with higher flesh content than either of the parents. Hence the hybrids appear to be of considerable importance to aquaculture [28]. It is also believed that hybrids of parental genotypes might be able to explore ecological niche unavailable

**27**

polyculture systems [46].

*Hybridization in Carps and Early Detection of Carp Hybrids Using PCR-Based Kit*

to the latter. It can lead to adaptation through creation of novel genotypes and morphologies. Further, hybrid vigor or heterosis can also occur [39]. Interspecific hybrids are created purposely to enhance productivity of aquacultural strains [42]. Hybridization is done also to enhance recreational angling opportunities [43]. Evolutionary evidence that hybridization is a constructive process was mentioned, as (a) reproductive barriers, both pre- and post-zygotic, between described species appear incomplete among many fishes, (b) permanent transfer of genetic information apparently is possible even when hybrids and backcrosses are under negative selection, and (c) genetic exchange through introgressive events may have significant effects on the genetic composition of a species, and thereby, actually contribute to diversity within taxa [7]. Studies of interspecific hybridization between the Siberian sturgeon and Russian sturgeon showed that the hybrids had higher survival and growth than the purebreds under provided hatchery conditions. The hybrid crosses displayed positive average heterosis in most of the assessment times for growth and survival traits, and better fitness-related traits than purebreds, thus suggesting that interspecific hybridization provides a survival advan-

*DOI: http://dx.doi.org/10.5772/intechopen.91946*

tage to sturgeons during their evolutionary period [44].

**4. Threats and concerns of human-mediated hybridization**

which can result in outbreeding depression following hybridization [45].

Inadvertent hybridization and backcrossing can lead to unexpected and undesirable results in hybrid progeny, such as failure to produce sterile fish, loss of color pattern, and reduced viability. Uncontrolled and unintentional hybridization could undermine the performance of cultured stocks and restrict future use of the contaminated stocks as broodstock. The level of unintentional or accidental hybridization has important considerations for the conservation of aquatic biodiversity and will influence risk assessment on the use of hybrid fishes in aquaculture and fisheries [1]. Continued hybridization may eventually lead to a breakdown of species barriers, thereby compromising the genetic integrity of the species in the wild and leading to production losses in aquaculture [30]. Hybrid introgression in major carp species is very likely to have negative consequences, as a result of loss of distinct feeding strategies of the pure species, which are the basis of successful

In Indian major carps, inadvertent production of hybrids out of mixed spawning of species has been well documented. Actually the farmers are on the lookout for "mixed seeds," meaning a certain proportion of catla, rohu, and mrigal along with other exotic carps for polyculture [47]. For the sake of time and economy, the hatchery producers keep broodfish of different species, particularly rohu and catla,

The harmful effects of hybridization, with or without introgression, have led to the extinction of many populations and species in many plant and animal taxa. Hybridization is especially problematic for rare species that come into contact with other species that are more abundant [2]. Hybridization can decrease diversity through the breakdown of reproductive barriers, the merger of previously distinctive evolutionary lineages, and the extinction of populations or species. There are two main mechanisms by which hybridization can lead to extinction. If hybrid fitness is strongly reduced relative to that of parental individuals (i.e., outbreeding depression), and hybridization is common, population growth rates of one or both parental lineages may decline below replacement rates due to wasted reproductive effort, leading to extinction [41, 45]. Hybridization involving captive-bred individuals can have harmful consequences beyond the loss of genetic integrity [46]. In many cases, the stocked individuals differ genetically from the target population,

#### *Hybridization in Carps and Early Detection of Carp Hybrids Using PCR-Based Kit DOI: http://dx.doi.org/10.5772/intechopen.91946*

to the latter. It can lead to adaptation through creation of novel genotypes and morphologies. Further, hybrid vigor or heterosis can also occur [39]. Interspecific hybrids are created purposely to enhance productivity of aquacultural strains [42]. Hybridization is done also to enhance recreational angling opportunities [43].

Evolutionary evidence that hybridization is a constructive process was mentioned, as (a) reproductive barriers, both pre- and post-zygotic, between described species appear incomplete among many fishes, (b) permanent transfer of genetic information apparently is possible even when hybrids and backcrosses are under negative selection, and (c) genetic exchange through introgressive events may have significant effects on the genetic composition of a species, and thereby, actually contribute to diversity within taxa [7]. Studies of interspecific hybridization between the Siberian sturgeon and Russian sturgeon showed that the hybrids had higher survival and growth than the purebreds under provided hatchery conditions. The hybrid crosses displayed positive average heterosis in most of the assessment times for growth and survival traits, and better fitness-related traits than purebreds, thus suggesting that interspecific hybridization provides a survival advantage to sturgeons during their evolutionary period [44].
