Dr. Md. Ariful Alam Associate Professor Department of Fisheries Biology and Genetics

Chap – 6 : Hybridization

Hybridization: The act of mixing different species or varieties of animals or plants and thus to produce hybrids is called hybridization. Hybridization is considered as inter-specific between two breeds, strains, species or even genus. Hybridization uses the dominant genetic variance (VD). The phenotype obtained through hybridization is not heritable, i.e. the result of hybridization is unpredictable. It is produced anew in each generation. Two superior parents may not necessarily produce superior offspring.

Uses of Hybridization: 1. It can be used as a quick and dirty method before selection will be employed. 2. It can be used to improve productivity whether h2 is large or small. When h2 is small, hybridization is the only practical way to improve productivity. 3. Hybridization can be incorporated into a selection program as a final step to produce animals for grow-out. 4. Production of new breeds or strains. 5. Production of uniform products. 6. Production of monosex populations. 7. Production of sterile individuals. 8. It can be used to improve a wild fishery.

Types of cross-breeding program: 1. Two-breed crossing: A X B

AB F1 hybrids (for growth)

2. Top-crossing: An inbred line is mated to a non-inbred line or strain.

3. Back-crossing: F1 hybrid is mated back to one of its parents or parental lines. A X B

AB F1 hybrids (for growth) X A

AB-A back cross hybrid (75% A + 25% B)

Following points are considered for hybridization:  Hatching rate  Survival rate at 1 year  Female fertility  Male fertility

Dr. Md. Ariful Alam Associate Professor Department of Fisheries Biology and Genetics

Recurrent selection: A selection program that can be used to improve the results of hybridization. If selection is done with one of the parental groups, it is called recurrent selection. If it is done both parental groups, it is called reciprocal recurrent selection.

Gene introgression: Gene introgression is the incorporation of genes of one species into the gene pool of another species. This process of introgressive hybridization can cause the genetic loss of an entire species, sub-species or unique population.

Heterosis: The superiority or inferiority of hybrids over the parents is called heterosis. Heterosis (H) can be determined by using the following formula:

H = {(Mean reciprocal F1 hybrids – Mean parents) / Mean parents} x 100

Example:

Group Mean Weight (g) Channel catfish ------460 Blue catfish ------440 Channel catfish female x Blue catfish male ------600 Blue catfish female x Channel catfish male ------462

Mean weight of parental group = 450 g Mean weight of hybrids = 531 g

H = {(531 – 450) / 450} x 100 = 18 % ( if H>0, a hybrid vigor has been produced)

Hybridization does not produce good brood stock:  Generally hybridization does not produce good brood stock.

 F1 hybrids do not produce above average progeny (unless they exhibit maternal heterosis) because-

- Their superiority is due to dominant genetic variance VD and - That is disrupted due to independent assortment during gametogenesis  Because hybrid superiority is due to interactions  When hybrids reproduce their progeny exhibit a wide range of interaction effects  Although hybridization can be used to create new breed and thus provide new pools of VA that can be exploited by selection  Hybridization is mainly used to produce superior animals and plants for grow-out  Selection is used to produce superior brood stock

Example: Some of the tilapia sp. have the XY-sex determining system while other have the WZ-sex determining system. The proper combination of sex chromosome in the parents will result in all male progeny. That combination is produced by the hybridization of XX females with ZZ males.

Dr. Md. Ariful Alam Associate Professor Department of Fisheries Biology and Genetics

For example, the hybridization of a Tilapia nilotica female (XX) x Tilapia hornorum male (ZZ) will produce a monosex population.

Tilapia nilotica female (XX) x Tilapia hornorum male (ZZ)

X Z

Offspring (all male) (XZ) But this technique is not 100% successful in producing all male populations, many mating produce some females. There are two reasons why the technique is not 100% successful.

 Firstly, many tilapia culturists cannot or will not be maintain the pure spp.  Secondly, because of autosomal sex influencing or sex modifying genes, such gene turns some males into females. In this why, hybridization cannot produce good brood stock.

Planning of cross-breeding program:  Like playing cards  Attention to be paid to the phylogenetic tree (stay within a family or within a genus)  Successful inter-specific hybridization depends on the chromosome number and sizes, and morphology of the chromosome  Dissimilarities in the karyotypes between species may enable fish farmers to create sterile hybrids  Biology and reproductive behavior of the two species  Hybridization below species level gives the greatest likelihood of success  The brood stock must be of top quality  Reciprocal crosses are also possible: Female A x Male B and Female B x MaleA

Dominant Genetic Variance and Hybridization

When little or no VA exists and it is either difficult or impossible to improve a phenotype by selection, the breeding technique that can be used to improves productivity by exploiting VD. Hybridization use to exploit

VD even when a large amount of VA also exists.

Dominant genetic variance is created anew and in different combinations each generation and its effects are basicaly those based on luck. Fish that are superior because of certain interactions are superior because of fortuitous combination of alleles that produces the desireable interaction in the progeny and thus improve productivity.

Additive genetic variance and VD are diametrically opposed; consequently, selection and crossbreeding are also diametrically opposed. In selection one can choice fish based on individual or family merit in the hope that the next generation will closely approximate those one have chosen. In crossbreeding, the next generation does not have to approximate the brooders, and unless the cross was made previously, one cannot predict the outcome of a mating to produce F1 hybrids. Dr. Md. Ariful Alam Associate Professor Department of Fisheries Biology and Genetics

Outbreeding Depression

Outbreeding depression refers to cases when offspring from crosses between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population. The resulting offspring may have reduced sruvival and lower reproductive success, i.e., reduced fitness.

This phenomenon can occur in two ways.

1. First, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. Gene flow between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population. 2. A second way outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different populations. Within local, isolated populations, alleles are selected for their positive, overall effects on the local genetic background. Due to nonadditive gene action, the same genes may have rather different average effects in different genetic backgrounds--hence, the potential evolution of locally coadapted gene complexes.

In other words, individuals from Population A will tend to have genes selected for the quality of combining well with gene combinations common in Population A. However, genes found in Population A will not have been selected for the quality of crossing well with genes common in Population B. Therefore outbreeding can undermine vitality by reducing positive epistasis and/or increasing negative epistasis.

As a general rule of hybrid vigor, is strongest in first generation hybrids and gets weaker over time. In contrast, outbreeding depression can be relatively weak in the first generation. But outside the context of ruthless selective pressure, outbreeding depression will increase in power through the further generations as co-adapted gene complexes are broken apart without the forging of new co-adapted gene complexes to take their place.

Outbreeding Depression

Outbreeding depression is a decrease in fitness of progeny upon breakup of coadapted gene complexes resulting from mating of distantly related individuals (Dobzhansky 1937). Although untested in freshwater mussels, outbreeding depression has posed a threat to population viability in some species of marine bivalve mollusks (Lannan 1980a, Lannan 1980b, Gaffney et al. 1993, Boudry et al. 2002). We hypothesize that mussel species and populations that have limited dispersal capabilities and that are subject to local environmental selection pressures may have developed coadapted gene complexes for adaptation to such environments, to include local host fish communities. For example, recent research on fish host specificity has demonstrated that glochidia obtained from allopatric mussel populations can exhibit significant among- population variation in transformation success when exposed to local fish host communities (Rogers et al. 2001, Eckert 2003, Jones et al. 2006). Other factors, such as differences in various life history parameters (e.g., spawning seasonality), population demographic parameters, physiological response to water quality (e.g., differences in local geochemistry) and other potentially adaptive traits should be assessed by biologists. Thus, we suspect that some populations of freshwater mussels may be vulnerable to outbreeding depression, and mixing distinct populations may disrupt genetic adaptation to local environmental conditions.

Dr. Md. Ariful Alam Associate Professor Department of Fisheries Biology and Genetics

Hybrid Hybrid Vigor Hybridization A progeny resulting from a cross The mating of genetically Crossing or fertilization of one between different parental stocks. dissimilar individuals as compared species by the sperm of another to that of the average of the species with the intension of parental types. progeny with the best characteristics of both parents.

Differentiate between cross-breeding and hybridization Sl. Cross-breeding Hybridization 1. Cross-breeding is the mating of two different Hybridization is the mating of two different species. strains or breeding lines of a species. 2. It occurs within a species. It occurs between two species. 3. It occurs naturally, whenever get chance to Normally it does not occur naturally, but accidentally mate can be occur in nature. 4. Normally, they are not sterile Hybrids are normally sterile but not always. 5. Eg. mating between two different strains or Eg. mating between Tilapia nilotica and Tilapia lines of Tilapia nilotica hornorum