Hybridization and then what?

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By Greg Lutz*

There are many examples of the use of hybridization as a means to develop superior production animals. Sometimes, it’s just the beginning of a more complex journey.

Many years ago, researchers in the U.S. demonstrated the Channel catfish by Blue catfish hybrid consumed more feed, grew more rapidly, exhibited better food conversion, and had higher overall production, survival and carcass yields than pure Channel catfish. At the time, however, there were too many technical difficulties in producing hybrid fingerlings on a large scale, since these two closely related species usually refuse to spawn together of their own volition.

Most researchers involved in these early efforts to develop “a better catfish” moved on to other topics, but over the years improvements in the administration of maturation hormones, the equipment used for holding broodstock prior to spawning and the understanding of crucial steps in artificial spawning have allowed the industry to shift to hybrid production. Today in many catfish operations in the American south, hybrid catfish and “straight channels” are grown side by side (albeit in separate ponds).

Another common North American example of superior production through hybridization involves the sunfishes from the genus Lepomis. Many common Lepomis hybrids have been shown to exhibit superior growth in recreational ponds, and several even been identified as serious candidates for food fish culture operations. The great thing about these hybrids is that they are usually fairly easy to produce – all that’s required is to stock males of one species in spawning ponds or tanks with females of another species. The downside is that although some crosses produce monosex populations, most are not entirely sterile. This can result in unwanted reproduction and eventual overpopulation by inferior, slow-growing and stunted ‘mongrel’ sunfish.

Hybrid striped bass, including the reciprocal crosses between Morone saxatilis and M. chrysops, have been the basis of a modest aquaculture industry in the U.S. for over 25 years. These days, domestic production has been more or less steady at about 10 million pounds per year, but this fish has also been cultured in a number of other countries, and a significant portion of the “hybrid striper” fry and fingerlings produced each year find their way to far-off lands.
Other examples of hybridization in aquaculture are easy to find. Hybridization has shown promising results with diverse organisms, including sturgeon, oysters, groupers, snappers, carps, and many others. One of the more mundane constraints in the use of hybridization, regardless of whether spawning occurs naturally or artificially, is the need to maintain (or collect) breeding stock from two distinct species.

Typically, in animal or plant production, or in aquaculture, the value of first generation (F1) hybrids is limited simply to grow-out for harvest. But the question continuously arises… what if we could somehow ‘fix’ the desirable traits in the hybrids we are working with and avoid having to continuously use two distinct groups of broodstock year after year? First generation hybrids are comparatively uniform, being isogenic (genetically identical) for all the heterozygous loci that result from combining distinct alleles that were fixed in the parental species. The variability in subsequent generations as this condition is lost through the formation of homozygotes at these various loci often allows the breeder to select for specific combinations of traits that suit his or her needs and markets.

This method has been successfully utilized in production of synthetic lines of tilapia throughout the world, and in Europe and elsewhere in the development of sturgeon “breeds” derived from hybrid broodstocks. The fact that roughly half the gains from heterosis persist indefinitely to complement any available additive genetic variation also opens the door for substantial genetic improvement in populations derived from hybrid parents.

Well… the first hurdle in such a strategy involves getting the F1 hybrids to produce viable F2 offspring. In some aquatic species, that’s no big deal. Fairly good results are often attained with tilapia: most red varieties across the planet are descended from F1 crosses of Oreochromis niloticus and O. mossambicus (the original Taiwanese red tilapia) or O. mossambicus and O. hornorum (the Florida red tilapia), through the F2 generation and onward for many, many more beyond it. Researchers at the Shanghai Fisheries University used this approach to develop a salt tolerant tilapia with good growth under saline conditions by utilizing F2 fish from original crosses of O. niloticus and Sarotherodon melanotheron. Another great example in tilapia is the “Pargo UNAM,” developed at the Centro de Ensenanza en Ganaderia Tropical in Veracruz, Mexico. This strain of fish is the result of using hybrids to produce a population from which desirable characteristics were fixed through selection.

Unfortunately, in most hybrids between aquatic species, getting from the F1 generation to the F3 and beyond is often not easy, or downright impossible. Low hatching rates, low survival and high incidences of deformities are often encountered when crossing first generation hybrids. Similarly dismal results have been reported for hybrid oysters (Crassostrea hongkongensis x C. gigas), hybrid African catfishes (Heterobranchus longifilis x Clarias anguillaris) and many others. This can be explained, at least partly, by a phenomenon referred to as “outbreeding depression.” If fitness depends on specific interactions within gene complexes found in each parental species, the breakdown and separation of these interacting genes in the F2 generation can cause irreparable harm.

Nonetheless, in many cases, if enough viable F2 animals can be produced they can serve as an “artificial center of origin” from which to begin development of a synthetic strain. Let’s examine the case of Morone. In the early 1980’s, researchers showed that striped bass hybrids were fertile and could produce viable offspring. Interest in spawning hybrids at the time stemmed from the fact that traditional fingerling production involved the need to collect, transport, inject, hold and strip spawn wild-caught broodstock of both parental species.

Further pursuit of this line of inquiry by Smith, Jenkins and Snevel (1986), demonstrated that the F2 progeny produced from these hybrids were not, themselves, suitable for commercial production, being much more variable phenotypically and on average only intermediate in performance when compared to their F1 parents and the striped bass parental species. This is exactly what quantitative genetic theory would predict, an inflation of variation and simultaneous conservation of only half the heterosis derived from the original cross (Falconer 1989). Similar efforts in Asia and Europe were not nearly as successful in producing F2 Morone, most likely due to outbreeding but also perhaps to a lack of experience with Morone larviculture.

But, some gems are usually scattered in the rubble of many F2 populations. One interesting result from the Smith et al. (1986) study, in keeping with phenomena plant and animal breeders have utilized for centuries, was that several F2 progeny were the fastest-growing fish in the entire study when generations were compared side by side. In all likelihood, due to the tremendous phenotypic variation in this generation, the smallest fish in the study early on were also probably F2 progeny that were cannibalized by their siblings before contributing to data analysis.

Producing a synthetic strain of Morone would involve raising the very best performing fish from the F2 generation and spawning them to produce F3 fish, and onward to ultimately attain a uniform, high-performing production stock. Large numbers of these F2 fish might be required to offset inbreeding in such an initiative, and the extensive time and facility requirements might never be justified. But if you’re a hybrid striped bass producer, it might just be worth it to be able to collect your broodstock in your own back yard every spring.

 

Dr. C. Greg Lutz is the author of the book Practical Genetics for Aquaculture and the Editor in Chief at Aquaculture Magazine.
editorinchief@dpinternationalinc.com

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