nasiona marihuany

Heterozja Selekcja cykliczna : etc

Wyszukiwarka Forumowa:
S

sub

Guest

10.3 Crossbreeding

Crossbreeding is a well-known method of genetic improvement, which also has applications in aquaculture. Crossbreeding is mating between, species, breeds, populations, strains or inbred lines. The main objective in crossbreeding programs is to exploit non-additive genetic variance (heterosis or hybrid vigour). When lines are inbred without selection the mean of all their crosses is expected to be equal to the mean of the outbred population from which they were derived. Therefore inbreeding followed by crossing cannot produce any improvement, there must be selection at some stage if any improvement is to be made. Crossbreeding should therefore be looked upon as a supplement to a program for additive genetic improvement.

10.3.1 HETEROSIS

Heterosis also called hybrid vigour can be defined as the phenomenon when the offspring surpasses the average of its parents for one or more traits, it is the reverse of inbreeding depression, which is obtained by mating related animals. The two phenomena are almost universally distributed in plants and animals and are particularly associated with reproductive fitness. There are two methods generally used to estimate heterosis. The first one is to compare crossbred progenies with the average of the parental line/strain, the second is to compare the crossbred progenies with the average of the best parental line/strain. If parents arose from different gene pools, crossbreds have increased heterozygosity and therefore a heterosis increment is expected. The extent of the heterosis increment of a given trait depends on the genetic distance between the parent populations.

The relative gains to be made from crossbreeding and selection depend on the magnitude of additive and non-additive variation for the trait or traits in question. If the non-addtive variance is large, substantial gains can be made by crossbreeding (see section 10.3.4).

10.3.2 COMBINING ABILITY

General combining ability (GCA) results from the average effects of a strain (population or line) if it is combined with other strains (populations or lines) in a cross. Any particular cross has an expected value, which is the sum of the general combining abilities of its two parental lines. The cross may deviate from the expectation and this deviation is called the specific combining ability (SCA) of the two lines in combination

GCA = genetic value

SCA = epistatic and dominance deviation, also called "nicking" (this term is often used when the merit of progeny from a mating deviates significantly from the average of the two parents).

The differences of general combining ability arc due to the additive variance (A) and A x A interactions in the base population. Differences of specific combining ability arc attributable to the non-additive genetic variance and epistasis. Table 10.3 shows how GCA and SCA can be estimated when crossing four populations.

clear.gif


10.3.2.1 Example

General combining ability (GCA) for population A and B can be estimated as follows:

clear.gif


It is generally more difficult to measure differences in specific combining ability, and also to utilise these effects in a breeding program. Making and maintaining inbred lines are almost the only means for utilising SCA commercially, although some use can be made of SCA using crossbreeding. The specific combining ability of a cross cannot be measured without making and testing the particular cross.

10.3.3 RECIPROCAL RECURRENT SELECTION

Reciprocal recurrent selection (RRS) is a crossbreeding scheme designed to utilise both general and specific combining ability. The theoretical basis of RRS has been given by Comstock et al. (1949) and Dickerson (1952). RRS begins with two populations, lines A and B. Crosses are made reciprocally, a number of A being mated to B and a number of B to A (J. The crossbred progeny are then measured for the character to be improved and the parents are judge from the performance of their progeny. The best parents are selected and the rest discarded, together with all crossbred progeny, which are used only to test the combining ability of the parents. The selected individuals must then be mated again to members of their own line to produce the next generation of parents to be tested. These arc crossed again as before and the cycle is repeated. According to Falconer and Mackay (1996) RRS programs are used by commercial breeders of poultry and have given promising results in corn, however, direct comparison with other selection methods has not been encouraging.

The disadvantages of RRS are that the method can be used for multiple spawners only, it requires a complex and extensive testing system and the generation interval will usually be substantially lengthened.

10.3.4 OVERDOMINANCE

When the heterozygote is superior to both homozygotes, the phenomenon is known as ovcrdominancc, figure 10.2 (Falconer and Mackay, 1996). Crossing two lines in which different alleles are fixed gives an F| in which all individuals are heterozygous, and this is the only way of producing a group of individuals that are all heterozygous. In a non- inbred population no more than 50 % of the individuals can be heterozygous for a particular pair of alleles. Consequently, if hetcrozygotes of a particular pair of alleles are superior in merit to homozygots, inbreeding and crossing will be a better means of improvement than selection without inbreeding.

clear.gif


Furthermore it is only when there is overdominance with respect to the desired character or combinations of characters that inbreeding and crossing can achieve what selection without inbreeding cannot. The existence and importance of ovcrdominancc has been much discussed, but the experimental evidence generally suggests the phenomenon of overdominance is not important for most of the characters studied (Falconer and Mackay, 1996).

10.3.5 DIALLEL CROSS

Diallel crossing is a commonly used experimental design for crossing inbred lines or different strains or populations, in which each line/strain/population is crossed with every other line. With p lines this procedure gives rise to a maximum of p~ combinations. A diallel cross is often used to establish the base population before starting a breeding program. Crossbreeding is frequently used to introduce new genes from strange populations into a local strain. This is usually a simple and very cheap way to improve native strains. However, prior to introduction of a new breed, strain or population should be tested under the existing local conditions.

The base population for the GIFT-project (Genetic Improvement of Farmed Tilapia) was a diallel cross. Four Asian farmed strains and four African wild strains were crossed together in a complete diallel cross (8 x 8 = 64 crosses) to study the magnitude of heterosis in growth performance and survival. The results are shown in Table 10.4 (Bentsen etal., 1998 ).

clear.gif


10.3.6 THREE-WAY AND FOUR-WAY CROSSES, BACKCROSSES

In a three-way cross the F1 of two lines, in which for example high productivity is required is crossed with a third line. In a four-way cross two F1's of different pairs of lines are crosscd. Using backcross two lines only are involved, the F1 being mated to one of the lines used in the first cross. Crossing is widely used in animal production most of the animals produced for meat being the progeny of cither a three-way cross or a backcross. In aquaculture these methods arc rarely used.

10.3.7 SYNTHETIC POPULATIONS

A diallel cross to test the potential populations is usually the starting point for making a synthetic population like done for Atlantic salmon, rainbow trout and tilapia. (Gunnes and Gjcdrcm, 1978; Gjcdrem et al., 1987; Refstie, 1990; Bcntscn et , 1998 )

Synthetic populations arc built from different number of parental populations, breeds, stocks or lines. When making a synthetic population the breeders try to create something new which combines the advantages of the parental populations. Crossing a number of selected inbred lines or different populations and allowing the Fl and later generations to mate at random or more usually with planned mating creates this new population.

Synthetic populations arc expected to harbour more heterozygosity than the parental strains and they should show some heterosis gain. The heterosis may, and often will, be rcduccd by inbreeding subsequent to a reduction in the synthetic population size. Also if losses due to recombination arc important, these will be evident in later generations of the synthetic population.
 
Ostatnia edycja:



Z kodem HASZYSZ dostajesz 20% zniżki w sklepie Growbox.pl na wszystko!

nasiona marihuany
Góra Dół