Genetic engineering by the manipulation of chromosomes

Abstract

The manipulation of chromosomes becomes feasible during the nuclear cycles of cell division and basically comprises the addition or subtraction of a complete haploid or diploid set. In animals, meiosis in the egg is the principal cell division phase where manipulation is possible, and in fish, and other animals with external fertilization, artificial processes can be applied to either gamete before fertilization or to the fertilized egg at any period during the formation of the zygote. Control of the first mitotic division is also feasible in eggs but reports of it so far are unsubstantiated. Two basic fields of practical importance involve the processes of parthenogenesis and induced polyploidy, respectively. Both derive conceptually from the work of embryologists during the early years of this century but are now the subject of widening interest particularly in relation to the domestication of fish. Gynogenesis, the parthenogenic development of eggs after activation by genetically inert spermatozoa, emerges as a relatively rapid method for inbreeding, and a study it affords of the recombination frequencies between individual gene loci and the centromere permits quantitative assessment of its power. Radiation-inactivated spermatozoa are normally used but untreated spermatozoa from a different species may also be effective without transfer of genetic material. The useful gynogenomes are, of course, the diploid ones, and various physical or chemical treatments which disrupt metaphase of cell divisions have been used successfully to increase the normally small frequency of diploid gynogenomes. Diploidy restored at metaphase II of meiosis leads to a level of inbreeding in excess of 50%, depending on the extent of recombination. The possibility of producing isogenic lines by the restoration of diploidy at first mitosis has yet to be fully assessed. Similarly, the production of isogenic lines through diploid androgenesis, the development of an embryo solely from the genetic content of the fertilizing spermatozoan, is yet to be achieved in fish. Polyploidy is produced using physical or chemical treatments which are similar to those used for disrupting metaphase in gynogenesis. Triploidy has been produced in a wide variety of fish and promises soon to be a routine tool in fish breeding. Tetraploidy is not so far confirmed in fish but remains a desirable goal because of its potential for further production of triploids or for line breeding of otherwise difficult hybrids. Among the other practical considerations for the future is the assessment of triploids in a variety of fish farming or sea ranching contexts, the further refinement of diploid gynogenesis, possibly combined with self-fertilization following induction of hermaphroditism and the exploration of the manipulation of single chromosomes, or parts of chromosomes, either by incomplete destruction of the genome in gynogenesis or by segregation within spermatogenesis in triploids. Longer term aspirations should also include the scrutiny of female triploids, of hybrid or pure species derivation, for ameiotic oogenesis. Evidence from natural populations supports the feasibility of this potentially very useful attribute. Chromosome engineering has immediate applicability in fish farming and great promise for the further domestication of fish.