So far in this course we have dealt entirely with the evolution of characters that are controlled by simple Mendelian inheritance at a single locus. There are notes on the course website about gametic disequilibrium and how allele frequencies change at two loci simultaneously, but we didn’t discuss them. In every example we’ve considered we’ve imagined that we could understand something about evolution by examining the evolution of a single gene. That’s the domain of classical population genetics. For the next few weeks we’re going to be exploring a field that’s actually older than classical population genetics, although the approach we’ll be taking to it involves the use of population genetic machinery. If you know a little about the history of evolutionary biology, you may know that after the rediscovery of Mendel’s work in 1900 there was a heated debate between the “biometricians” (e.g., Galton and Pearson) and the “Mendelians” (e.g., de Vries, Correns, Bateson, and Morgan). Biometricians asserted that the really important variation in evolution didn’t follow Mendelian rules. Height, weight, skin color, and similar traits seemed to

Human biochemical genetics

Endocrine sex control strategies for the feminization of teleost fish

In fish, pre-embryonic events such as insemination, second polar body extrusion and first mitotic cleavage are manipulable and render 37 different types of ploidy induction possible. A classification of physical, chemical, and biological inductors of ploidy is provided. The amazing ability of fish to tolerate genomes from haploid to heptaploid, genomic contributions from the male or female parent alone, and unequal contributions from parents belonging to the same or different species is highlighted; surprisingly, a single species is amenable for 8–12 different types of ploidy induction. Advantages and limitations of different methods and live or preserved tissues for ploidy confirmation are assessed. Live haploids have been induced in Oreochromis mossambicus. With an ability to synthesize rRNAs and metabolic enzymes such as LDH, the haploid embryos are capable of normal translation and transcription, but suffer mass mortality at hatching, perhaps due to expression of lethal mutant genes. Induction of gynogenesis involves egg activation by irradiated homologous or heterologous sperm, and diploidization by retention of the second polar body (meiotic gynogenesis), or suppression of the first mitotic cleavage (mitotic gynogenesis). UV-irradiation inactivates sperm DNA maximally and avoids chromosome fragmentation. Egg activation by UV-irradiated heterologous sperm under dark conditions, and diploidization by pressure shock result in the highest survival of gynogens; meiotic gynogens survive better than mitotic gynogens. The need for confirmation of genetic purity of mitotic gynogens by one or more methods is emphasized. In different species, survival, growth and fertility improve when gynogens are generated successively for two or more generations. Combinations of induction of ploidy and hormonal sex reversal in gynogens renders the scope for generating all-male or all-female populations. In some gynogenetic species genetic homozygosity leads to growth suppression from 3 to 60% however, meiotic gynogens of Clarias macrocephalus and Paralicthys olivaceus display 18 and 35% faster growth. Hypotheses for the unexpected occurrence of males among natural and artificially induced gynogenetic populations are assessed. In a few species, reproductive performance of gynogens is not equivalent to normal females. Maximal elimination of egg genome by UV-radiation, induction of androgenesis using 2n sperm of a tetraploid, facilitation of dispermy using heterologous eggs, and activation by cryopreserved sperm are land-mark events in the history of androgenesis. In male-heterogametic species, androgenesis may produce supermales to establish broodstock for consistent production of all-male populations. In combination with cryopreservation of sperm, androgenesis may prove to be the best method for conservation of fish genomes. As many as 11 different types of triploids are inducible; and most of them require the retention of the second polar body. The yield and survival of triploids are higher than those of gynogens and these values decrease with the kind of inductor used in the following order: pressure < cold < heat shock. Usually triploidy confers sterility, especially in females, and accelerates growth during post-maturation, when reared solitarily in some (Tinca tinca, Oreochromis Mossambicus) or communally (with diploids) in other species (Oncorhynchus mykiss). The presence of two sets of maternal chromosomes does not disturb the manifestation of morphological sexual characters; however, unexpected sex ratios observed in some triploids indicate a preponderance of females in natural populations and a dominance of males in artificially induced populations. In some species, a certain percentage of female triploids are fertile. Possible pathways, through which the natural fertile triploids may form the base for evolution of tetraploids and origin of diploid new species are suggested. Triploids, especially females, suffer delayed maturity, a low gonado-somatic index, and disrupted gametogenesis due to cytogenetic and endocrine incompatibilities; males do not suffer endocrine incompatibility. Three possible pathways, through which oogenesis may be completed in these fertile triploid species, are indicated. Triploid hybridization between salmonids, which can tolerate salinity changes (e.g. chum and pink salmons) and which cannot tolerate early transfer to sea water but are desired for their meat quality (e.g. Atlantic, chinook and coho salmons) is commercially important. Triploid conspecific salmonids grow faster than diploid conspecifics or triploid hybrids. Protocols for the combination of induced ploidy and hormonal sex reversal to generate all-male, all-female or all-sterile triploid populations are described. In triploids, the increase in nuclear DNA per cell is 1.3 to 1.7 times that of diploids, which results in corresponding volumetric increase of a cell. This, in turn, imposes corresponding decreases in total cell surface area and cell number of a tissue or a triploid individual; however, such a decrease in cell surface area does not impair metabolic processes like oxygen uptake and food utilization. Data for effective temperature required to induce cold or heat shock to retain the second polar body suggest that an elevation of 18, 15 and 14 °C successfully retains the polar body in salmonids, cyprinids and cichlids; the corresponding values for the depression of temperature are 11, 19 and 21 °C, respectively. Seven different kinds of tetraploidy are inducible; however, live, feeding tetraploids have been induced in ten species only. Causes for embryonic or post-embryonic mortality of tetraploids are discussed. Increasing maternal genomic contribution and heterologous insemination appear to enhance tetraploid survival. Survival and growth of Oncorhynchus mykiss progressively improve in F1 and F2 tetraploid progenies. In a rare strain of the loach, Misgurnus anguillicaudatus, pentaploids, hexaploids and heptaploids have been successfully induced. An individual polyploid loach (3n) simultaneously spawns small, intermediate and large eggs carrying n, 2n and 3n genomes. It is not clear how during oogenesis the passage of n, 2n and 3n genomes is regulated into small, intermediate and large eggs. However, evidence from other fish species is accumulating for the simultaneous production of at least two kinds of eggs by a single female. Studies on ploidy induction have shown the magnitude of the complicated genetic mechanisms that control sex determination in fish.

Ploidy induction and sex control in fish

Publisher Summary This chapter describes the techniques used in chromosome set manipulation and reviews the results and prospects in the application of gynogenesis, androgenesis, and induced polyploidy to fish. Chromosome-set manipulation techniques of sperm chromosome inactivation (with radiation or chemicals) and suppression of cell divisions (with heat shock, cold shock, or pressure) can be readily applied to fish to produce gynogenetic and polyploid individuals. Gynogenetic individuals have all their chromosomes from the female parent and should all be females in species with XX females. Polyploids include triploids that are expected to be sterile, and tetraploids that have the potential of being fertile and producing sterile triploids when crossed to normal diploids. Partially inbred gynogenetic diploids and triploids may be produced by treatments causing retention of the second polar body of the egg. Completely homozygous gynogenetic diploids and tetraploids may be produced by treatments blocking the first mitotic division.

8 Chromosome Set Manipulation and Sex Control in Fish

Androgenesis, gynogenesis and the production of clones in fishes: A review

CURRENT ideas on fish farming have stimulated interest in fish genetics and highlighted the need for more genetic analysis, particularly of fish of commercial importance. A major difficulty in breeding fish of commercial interest is the long generation time of 3 years or more. The work reported here is an investigation of the possibility of producing homozygous clones of fish rapidly by gynogenesis—a special form of artificial parthenogenesis in which activation of the eggs is achieved by fertilisation with genetically inert spermatozoa. Gynogenesis was first observed in frogs by Hertwig (1911), who showed that a low frequency of apparently normal embryos appeared when eggs were fertilised by spermatozoa which had received radium gamma ray doses much higher than levels normally required to produce 100 per cent, abnormality. It was concluded that, at these very high doses, the genetic material of the spermatozoa was so thoroughly destroyed that it played no part in the subsequent parthenogenetic development of the egg. This \"Hertwig effect\" has been confirmed several times in amphibia (reviewed by Beatty, 1964), and, as in other forms of parthenogenesis in vertebrates, the resulting parthenogenomes are usually haploid, and although developing normally at first are grossly abnormal at hatching. However, sporadic occurrences of more normal hatchlings have been observed following parthenogenesis by pricking and these have been shown to be diploid (Parmenter, 1933; Kawamura, 1939). Several mechanisms have been proposed to explain the process of diploidisation (see Tyler, 1941; Beatty, 1964), but the experimental evidence in amphibia seems to favour diploidisation by doubling of the haploid female genome during cleavage. Thus Parmenter (1933) and Kawamura (1939) observed delayed cleavage in parthenogenetic eggs which subsequently produced diploid organisms, and Subtelny (1958) produced diploid individuals following transplantation of haploid nuclei into enucleated eggs, i.e. diploidisation in the absence of polar bodies. Increased frequencies of both haploid and diploid gynogenomes have been reported by Rostand (1934, 1936) following post-fertilisation cold treatments of amphibian eggs; gynogenesis was produced by irradiated spermatozoa and also by fertilisation of eggs with foreign spermatozoa (\" false hybrids \"). Gynogenesis is a natural form of reproduction in the teleost Mollienesia formosa (Hubbs and Hubbs, 1932), and the existence of distinct clones in laboratory fish has been demonstrated by tissue transplantation tests (Kallman, 1962). Perfectly normal and fully viable broods can easily be reared, and this suggests that diploidy is retained through suppression of meiosis Kaliman, bc. cit.) leading to fixed heterozygosity. Recent work in Russia (Romashov, Belyaeva, Golovinskaia and Pro-

/pdf/radiation-induced-gynogenesis-and-androgenesis-in-fish-43s89qxgqm.pdf

Radiation-induced gynogenesis and androgenesis in fish.

Genetic analysis was performed on five enzyme systems (G3PDH; GPI-A; GPI-B; PGM; MDH-A) in plaice (Pleuronectes platessa) collected in spawning condition from the North Sea. Conventional crosses, induced gynogenesis and induced triploidy were performed. The data conclusively demonstrated the inheritance of isozymes by co-dominant alleles at individual loci for each system. No linkage was observed but tests did not include MDH nor the possibility of linkage between G3PDH and GPI-A. Some anomalous segregation ratios were observed, particularly a deficiency of heterozygotes for GPI-A, but the data were largely in conformity with Mendelian expectations. At the PGM locus, five independent anomalous individuals were scored and interpreted as mutations with a mutation rate of 1.1 X 10(-3) per gamete. Recombination with the centromere was assessed and induced triploidy and cross-over values of 41 per cent for PGM, 19 per cent for MDH-A and 9 per cent for GPI-B were derived on the assumption of complete interference. Amongst the parent fish, genotypic and phenotypic frequencies were largely consistent with the expectations of the Hardy-Weinberg Law, and allelic frequencies were not significantly different between year of collection or location of collection ground.

/pdf/genetic-analysis-of-enzyme-polymorphisms-in-plaice-1dz1nn0loc.pdf

C E Purdom

Papers

Radiation-induced gynogenesis and androgenesis in fish.

Genetic analysis of enzyme polymorphisms in plaice (Pleuronectes platessa).