by
N.I. Nikoljukin
All-Union Research Institute of Marine
Fisheries and Oceanography
Moscow, U.S.S.R.
Hybridization has become a common practice in fish culture, though it has not yet attained its rightful place in this branch of economy. The hybridization of fish for practical purpose has demonstrated its economic value. In the U.S.S.R. hybridization has solved the problems of carp culture in the northern regions. New productive forms have been successfully created by crossing species and varieties of Carassius, as well as by crossing carp and Carassius. Research on crossing of herbivorous fishes for their acclimatization in new areas has been initiated. Economic characteristics of a number of hybrids of ganoids have been determined. Experiments have shown that hybridization has extensive possibilities in sturgeon culture.
One may ask why hybrids should be created when excellent non-hybrids forms, sturgeon for example, are available. Hybrids are offered along with the pure species and not as substitutes. Preferences for hybrids are based on inherent characteristics of hybrid heterosis (hybrid vigour), which manifest as intensive growth rate, higher viability, adaptive flexibility, and sometimes early sexual maturation.
Charles Darwin attached great significance to hybridization: “If we do not take it for granted, at least we shall consider it highly probable - the existence of a great law in nature, the law of crossing animals and plants which are not closely related to each other, and this is extremely useful and even necessary”.
Study of causes and regularities of hybridization processes occurring in nature reveals possibilities and methods of utilization of hybridization if fish culture. It should be noted that in no other group of the animal world but fish do remote (interspecific and intergeneric) crossings occur so frequently in nature, producing numerous hybrids. This is particularly true of fresh water fishes, as reproductive isolation is violated more frequently among them than among sea fishes because of less constant hydrological conditions in rivers as compared to the seas.
According to the literature available to us, 212 different fish hybrids have been recorded up to 1956, of which only 30 are of sea fishes. The number of hybrids in nature may be much higher, but not all of them are known to investigators or identified correctly.
In nature, hybrids occur mostly as single specimens, but sometimes large scale hybridization may occur as a result of violation of reproductive conditions of the species. Abundance of individuals of one species occurring along with less numbers of other species may result in an increase of interspecific crossing. That is why the probability of crossing is high when large numbers of a species are introduced into a new area, while it remains low in the aboriginal species.
Fish hybrids are of interest from two points of view:
as objects for commercial fish culture when heterosis of the first generation is to be used; sterile hybrids are suitable for this purpose; and
as initial material for producing new hybridogenic forms when the ability for reproduction is a necessary condition. The problem of hybrid reproduction is the most important problem in hybridization.
Great variations in the reproductive ability of hybrids are observed: from complete fertility of intergeneric hybrids of some Cyprinodontidae to complete or incomplete fertility of interspecific and intergeneric hybrids of the sturgeon (Acipenseridae), the carp (Cyprinidae) sunfishes (Centrarchidae) and others.
The degree of fertility of hybrids does not always correspond to the degree of taxonomic relationship of the species crossed. In fact, intergeneric hybrids may be more fertile than intrageneric ones. Sometimes, even reciprocal hybrids produced by crossing the same species, but by substitution of a female of one species by a male of another, and vice versa, may be different in fertility.
Individual hybrids may vary in fertility. In fact, some individuals of so-called sterile hybrids can produce offspring. Therefore, one of the methods of eliminating complete sterility in such hybrids is to carry out hybridization on a larger scale to provide a larger number of hybrid individuals, some of which may be fertile. Fertility may be expected to increase in the second hybrid generation. This is true of such hybrids as Cyprinus x Carassius which, as a rule, are sterile; but among them some exceptional individuals may be fertile. Investigations carried out by A.I.Kuzema has shown possibilities of successful selection work on Ukrainian hybrids of Cyprinus x Carassius.
Fertility of most hybrids is disordered to a certain extent, and they are sometimes sterile. In the process of evolution of various species, they became gradually isolated ecologically, geographically and in other respects. Due to isolation a sort of barrier appeared which prevented them from free crossing. However, such an isolation was not always reliable; the possibility of interspecific crossing still remained. This resulted in the development of a second barrier - disorder in hybrid fertility, preventing them from mixing and destroying specific differences. But where the first barrier acts without failure, fully eliminating the possibility of two species crossing, the second barrier might not appear. If such species are crossed artificially, the first barrier is removed, and a fertile hybrid is produced. Charles Darwin meant these very cases when he wrote “there exist species which are not easy to cross, but their hybrids, once produced, are highly fertile”. Our experiments have shown that man, having succeeded in crossing such species, may find access to reproductive potentials that do not occur in nature.
It has been possible to produce fertile hybrids of beluga (Huso huso) and sterlet (Acipenser ruthenus), two species which do not cross in nature due to marked ecological differences.
The sturgeons are remarkable for their ability to cross and for their hybrids that are fertile to various degrees. For many years we crossed the sturgeons in most specific combinations in order to produce interspecific and intergeneric hybrids, not only in the first generation, but also in the second and even the third generation (also back-cross and triple-cross). In our work we did not come across any instance in which it was impossible to produce viable offspring.
Without detailed cytogenetic analyses of the causes of fertility and sterility of fish hybrids, I would like to point out that there are more chances of producing completely fertile hybrids if the species crossed have equal chromosome number, which must be homologous and able to conjugate in the course of gametogenesis. When there is unequal chromosome number and violation of the conjugation process, sterility occurs in various degrees.
Until recently the sturgeons and their hybrids were not studied cytogenetically. We have initiated research work on chromosome complexes of parent species and their hybrids, which has already given interesting results. The various species of the sturgeon family such as Huso huso, sterlet (Acipenser ruthenus), starred sturgeon (A. stellatus), ship sturgeon (A. nudiventris) and, probably, some others are characterized by a small number of chromosomes (about 60), whereas the chromosome number of A. guldenstadti is more than twice as much. Therefore, crossing of the first four species in any combination is likely to produce fertile hybrids; crossing each of the four with A. guldenstadti produces hybrids which are either of limited fertility or are completely sterile, the results of abnormal gametogenesis. Hence, before starting work on hybridization, it is necessary to study chromosome complexes of the species to be crossed; this will permit prediction, with a greater degree of probability as to whether or not the hybrids will be fertile or sterile, and will avoid waste of labour and money.
Many scientists, I.V. Mitchurin in particular, have shown that development of some characteristics of hybrids, to a great extent, depends on the environment in which the hybrids were bred. This environmental selection of characters is possible due to the great heterozygosity of the hybrid.
Study of morphological characteristics of certain hybrids of the sturgeons, the carps and others (as compared to their parent species) shows that hybrids have an intermediate heredity not only in the first generation, but also in the second, as well as in the generations produced by back-crossing and triple-crossing. In comparison to the first generation, the following generations, often but not always, are characterized by greater variability; however, no typical morphologic segregation followed by return to the initial species has been observed. Hence it was believed that interspecific hybrids possess the so-called permanent intermediate heredity, which does not obey Mendel's law of segregation. But recent experimental research has proved that the type of heredity observed in crossing different species does obey the law of segregation. But the law, revealed by the behaviour of genes, is impossible to observe visually in morphological characteristics. The species crossed are different in many characteristics, and consequently in a great number of genes. In interspecific crossing high polymeric inheritance occur when numerous genes affect the development of some definite characteristics in a similar way. This is why in the second generation a much greater number of combinations is inevitable; and among the mass of intermediate specimens, only a few of them are homozygous, which, in all characteristics show a return to the parental form. However, often such individuals are not found in the experiment.
In intraspecific crossing, (inter-racial, for example) it does not matter which race is represented by the male or female, since the characteristics of the offspring are not affected. That is to say, reciprocal hybrids do not differ from each other. This is expressed by the formula: A × B = B × A. In interspecific, intergeneric and more remote crossings, reciprocal hybrids may be different, as for example the mule and the hinny which are the reciprocal hybrids between horse and the donkey.
Differences in reciprocal forms of some fish hybrids are revealed in the following: (i) viability, (ii) growth rate (Fig.1), (iii) rates of development of other characteristics of heterosis and (iv) morphological characteristics.
Marked difference in viability is observed in reciprocal intergenetic hybrids of crucian carp (Carassius carassius); the offspring of Carassius females crossed with males of other genera of Cyprinidae (for example Carassius ♀ x Tinca ♂) are viable, whereas the reciprocal offspring (of Carassius males) are not viable. Considerable differences in growth rates of fry were found in reciprocal hybrids of the sturgeons. The hybrid possessed some features characteristic of the maternal species, revealing matroclinic inheritance. Morphologic differences between reciprocal hybrids are even more pronouced: in countable characteristics (number of vertebrae, fin rays) these differences are again matrilinear (Fig.2).
Differences of reciprocal forms of hybrids are determined by cytoplasmic differences of the species crossed, resulting in different interaction of the nucleus and the plasma in reciprocal crossing. Genetic predetermination of cytoplasm may lead to incompatibility of the cytoplasm in certain species with chromosomes of some other species. As a consequence, some reciprocal hybrids may be viable, whereas some others may not be. The same is true of the factor determining the manifestation of various degrees of heterosis in reciprocal hybrids. Cytolplasmic heredity is a basis of matroclinic inheritance as well.
Fig.1 Growth rate of reciprocal hybrids and parental sturgeon species in aquaria.
| 1 | - | Acipenser güldenstädti, |
| 2 | - | Acipenser ruthenus, |
| 3 | - | A.ruthenus ♀ × A. güldenstädti ♂ |
| 4 | - | A.güldenstadt. ♀ × A.ruthenus ♂ |
Fig.2 Matroclinal differences in number of vertebra between reciprocal hybrids.
| Cr | - | Carassius carassius, |
| CrCp | - | C.carassius ♀ × Cyprinus carpio ♂, |
| CpCr | - | C.carpio ♀ × C.Carassius ♂ |
| Cp | - | C.carpio |
The nature of many phenomena in remote hybridization cannot be explained without consideration of gynogenesis as one fundamental problem of the theory. Gynogenesis is a form of sexual reproduction, when in the process of fertilization, the sperm penetrates the ovum and activates it for development without contribution of paternal chromosomes, so that heredity in the offspring is determined by the female pronucleus alone. As a result, pure matroclinal pseudohybrids are produced, represented as a rule by females only. Some scientists consider gynogenesis to be a special case of parthenogenasis. This is not correct because in parthenogenesis, activation of the ovum is not initiated by sperm but by some other agents.
Gynogenesis occurs in nature and can be demonstrated experimentally. Most interesting instances of natural gynogenesis are Carassius auratus gibelio and the Mexican viviparous fish Mollienesia formosa. They reproduce gynogenetically, spawning with males of some other species.
Some experiments on remote hybridization have resulted in so-called hybrid gynogenesis. Our numerous experiments in crossing Carassius carassius females with Leuciscus cephalus males (different subfamilies) have shown that only a small part of the offspring was viable, and these were matrilinear, i.e. gynogenetic due to the exclusion of the male chromosome complex during development. Most of the embryos died at the very beginning of development owing to incompatibility of maternal and paternal hereditary substance.
Of great theoretical and practical interest, is radiation gynogenesis, produced when eggs are fertilized by sperm exposed to X-rays in dosages sufficient to stop its hereditary function, but not its ability to penetrate the egg and activate its development. In fact, in the fertilization of the sterlet (Acipenser ruthenus) eggs by the sperm of the sturgeon exposed to X-rays, or the beluga (Huso huso) eggs by the irradiated sperm of the sterlet, completely matrilinear offspring were produced (no paternal characteristics were observed). Since gynogenesis produces only females it may be possible to utilize it for control of sexual ratio in fishes, which is of great practical importance. Increase in the number of females of the sturgeon, for example, would be desirable both for commercial fishery and for artificial culture. Experiments in this field should be continued on a larger scale.
Heterosis in fish hybrids is a natural phenomenon observed in various families such as Acipenseridae, Salmonidae, Cyprinidae, Percidae, Centrarchidae and Poecilidae. As a rule heterosis is more prominent in intergeneric hybrids which are not closely related, than in interspecific or intraspecific hybrids.
Heterosis is revealed in various degrees. For example, growth rate of the hybrid may exceed that of the parent species (typical heterosis), or sometimes growth rate of the hybrid may exceed that of one of the parent species and be equal to or less than the other parent (incomplete heterosis).
In the second hybrid generation, heterosis usually declines, but in back-crosses of the first generation hybrid with the parent species (in which the characteristics that are of interest to us is more pronounced) the hybrid may exceed both the parent species; that is, the hybrid will show a prominent heterosis. For example, the hybrid of Huso huso x (Huso huso x Acipenser ruthenus) exceeds Huso huso in growth rate.
Both in evolution and selection, hybrid forms are of more value if their fertility is not violated.
Heterosis should be used on a greater scale in selectional culture of new, valuable fish and in acclimatization as well as in crossing of hybrids for commercial culture in ponds and other inland reservoirs.
