Cross-breeding is another well-known means of genetic improvement which has application in aquaculture. Heterosis or hybrid vigour enables an offspring to surpass its parents for one or more traits. On the other hand, inbreeding depression caused by mating of closely related parents has a deleterious effect on the progeny.
The inbreeding measure is the coefficient of inbreeding, incorporating the degree of the animal’s homozygosity. It shows what part of the genes in a group of individuals are in the homozygous state. Close inbreeding, especially sib mating (brothers and sisters and parents with offspring), causes homozygosity to increase rapidly, to as much as 0.9–0.95 (expressed as fractions of one) or even more. In most animals, inbreeding results in inbreeding depression, characterized by a drop in viability, growth rate and often fertility. Such depression has been observed by many workers in common carp, brook trout and other fish species. Outbreeding, on the other hand, is accompanied by heterosis in growth rate and viability, especially when fish from different high inbred groups are crossed. The main types of crossings undertaken are:
a) Commercial crossing, directed towards breeding of the first-generation hybrids for commercial purposes. Only the first generation, that has the heterosis of productive qualities or incorporates the advantageous characteristics of both the parental forms, is used. They are maintained for further reproduction.
b) Synthetic or distant crossing, in which distant parents, including those of inter-generic origin, are crossed to develop a new breed, in the course of long selection. It may attempt to combine the qualities of parents of several breeds, species or even genera. Such crossings to produce new breeds should ensure the preservation and perfection of the productive qualities of the breed, the preservation of genetic variability and the prevention of inbred depression.
Distant outbreeding is indispensable in the selection of aquaculture species. The aims of such crossings are as follows (Kirpichnikov,
(1) an overall increase in genetic variability, resulting in an increase in selection response;
(2) achievement of a combination of charac teristics of two or three breeds or two (rarely three) species;
(3) improvement of the productive quality of the local breed by making use of the few valuable traits of another breed;
(4) increase in the viability of the breed by intro ducing genes responsible for resistance to environmental factors and diseases.
Kirpichnikov (1971) describes different cross- breeding patterns to achieve these aims. Reproductive crossing is suggested when valuable properties from both parents are to be combined in the hybrids. It can be done with
complete fertility of the hybrid and requires only meticulous selection in the subsequent generations.
Introductory crossing will be advantageous when one or only a few characteristics from a breed have to be incorporated in the hybrid. Each generation of the hybrid has to be crossed with the local breed and so there is the risk of losing the useful characteristics of the improved breed in back crossing, particularly in the case
of polygenic inheritance of properties selected.
This type of crossing is of considerable use in selection for increased resistance to certain dis- eases, which is often dependent on the presence of one or a few genes. These genes can be pre served by means of proper selection in each generation.
Absorptive crossing differs from introductory crossing only in that its purpose is a nearly com plete substitution of the local breed of geno- type by the genotype of the improved breed. Only some features of the local breed, such as
viability, are preserved.
Alternate crossing is the most complicated system which is most useful when a combina tion of many characteristics from two breeds with polygenic inheritance is required. It allows the preservation of high genetic variability through a number of generations. Selection effi ciency is kept at a high level owing to this variability and does not reach a plateau. The mainproblem of obtaining new hybrid breeds by means of crossing (interspecific or intergeneric) is their complete or partial sterility, which takes a lot of time-consuming work to overcome.
A number of breeding systems have been proposed to utilize completely the advantages associated with heterogeneous crossings. Paral lel breeding of two or more groups within a breed is possible when working with slowly maturing species, without intermingling, allow ing a moderate inbreeding among each and carrying out selection in each generation. In breeding in groups for family selection, a large number of crossings from different groups are carried out for each generation. The parents producing the best offspring are used for subsequent commercial crossings. This systemsuffers from the drawback that the genetic variability gradually decreases during family selec tion. Moav and Wohlfarth (1967) recommend that a reserve group of a sufficient number of individuals should be kept for each group when selecting two groups marked by certain genes.
In case of a drop in genetic variability, addi tional gene pools can be introduced into the exhausted group.
Another possible system is alternate in- breeding and outbreeding. After two or three generations of close inbreeding, the evaluation of hybrids from different inbred lines is per formed. The best combinations are used for
commercial rearing and among the offspring new inbred lines are established. Linear selec tion involving inbreeding for superior ancestors and top cross, where crossing is done between the best inbred individuals (say males) and individuals from the outbred population (say females) to preserve the genetic variability, are
other methods that have applications in aquaculture.
One of the most complicated techniques of breeding is reciprocal recurrent selection (RRS), where the combining capacity of the parents from each of the two groups is evaluated by means of a cross with parents from the
other group. The individuals thus selected are reproduced without recrossing and their off- spring again tested for combining potential. The basic feature of all the systems described is the utilization of heterosis in crossing
individuals from different groups, lines and breeds. Along with this, moderate to very close
inbreeding is employed. The most appropriate system would naturally depend on the species and the traits that are of importance in commercial culture. Gjedrem (1985), however, proposed a cross-breeding scheme for fish farming as summarized below:
a) Test all possible crosses between different strains or species for the economic traits in question and select crosses that are likely to give useful results. It may be better to use strains with very different origins and which, in combination, have favourable traits.
b) Develop inbred lines and test the crosses under natural conditions to find the most valuable cross for farming.
c) Start an RRS programme to ensure continuous genetic improvement, utilizing both general and specific combining abilities simultaneously.