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Genetics Terms

Genetics Terms. Selective breeding -the choosing of individuals of a single strain and spp. Hybridization -the crossing of different spp. Crossbreeding -the mating of unrelated strains of the same spp. to avoid inbreeding. Selective Breeding.

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Genetics Terms

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  1. Genetics Terms • Selective breeding-the choosing of individuals of a single strain and spp. • Hybridization-the crossing of different spp. • Crossbreeding-the mating of unrelated strains of the same spp. to avoid inbreeding

  2. Selective Breeding • “Artifical selection” as opposed to natural selection, results in reducing genetic variability in a population. Could be considered inbreeding if not closely monitored. • If selective breeding involves excessive inbreeding physical abnormalities, metabolic deficiencies, and developmental abnormalities may occur. • Inbreeding depression-loss of fitness due to inbreeding • more likely to observe occurence of recessive traits • Decrease in heterozygosity

  3. Selective Breeding • Although potential for deleterious results, selective breeding rationalized by: • size • color • shape • better FCR • reproductive capacity • disease resistance

  4. Hybridization • Already looked at hybrid striped bass. What other examples are out there?? Splake-brook trout/lake trout, Tiger Musky-northern pike/musky Sunfish hybrids (many combinations) • Hopefully achieve “hybrid vigor,” at least in some trait. • Most true hybrids are sterile, but not all.

  5. Gynogenesis • Development of an ovum without fusion of gametes • usually haploid and die • sometimes diploid larva develop • requires retention of 2nd polar body during meiosis • Amazon mollies, cruzian carp do this • Induced by: • radition, chemical, electrical currents, sperm from related species

  6. Mono-sex male tilapia populations can be produced by: • Visual selection 2. Hybridization 3. Sex-reversal 4. Genetic manipulation Male Populations

  7. Using Two Species • Two species of tilapia can be crossed to yield all-male offspring. • Male Hornorum tilapia can be hybridized with the female Nile tilapia and offspring are all-male. • However, the hybrid is fertile and can backcross with female Nile and Hornorum tilapia.

  8. Using Two Species • Only genetically pure Hornorum and Nile tilapia will result in 100% male offspring. • Maintaining pure lines of Nile and Hornorum tilapia over a long period of time is difficult. • Additionally, hybrid fry yield per female is low compared with fry yield from pure species spawns.

  9. Hybridization Male Hornorum tilapia ZZ XZ XX Female Nile tilapia All-male hybrid tilapia

  10. Sex determination by hormones • A popular way to produce all-male populations is with hormone sex reversal of fry. • Tilapia fry (18 day) are fed a diet containing a male steroid (methyl testosterone) for 20 to 28 days. • Females develop as “functional males” following hormone treatment. • All-male populations are hard to produce!! • ~95 to 98% males are commonly produced.

  11. Sex Reversal of Tilapia Fry

  12. Genetic Manipulation 1) XXF + XYM XX + XY normal

  13. More genetic manipulation • What if female Nile tilapia are homozygous (XX) and male Nile tilapia are heterozygous (XY) for sex. • A pairing of genetically normal male and female Nile tilapia will result in 50% male and 50% female offspring. • Now if we feed normal fry estrogen for 28 days, we get phenotypic all-female fry.

  14. Genetic Manipulation 1) XXF + XYM estrogen XXF + XY”F”

  15. Genetic Manipulation 1) XXF + XYM estrogen XXF + XY”F” 2) XY”F” x XYM XXF + 2 XYM + YYM

  16. Genetic Manipulation 1) XXF + XYM estrogen XXF + XY”F” 2) XY”F” x XYM XXF + 2 XYM + YYM 3) XXFx YYM 100% XYM ??

  17. Transgenics: How do you get a transgenic fish? • Transgenic or genetically engineered animals are developed by inserting single or multiple genes from one species into the DNA of another species. • Three basic steps: • Isolate and identify the genes that posses the desired trait. Genes may come from similar fish species, plants, bacteria, animals, and even humans. • Insert isolated material into the intended target fish’s eggs. • Grow your fish!!

  18. Differences in terms? • The terms genetically engineered, genetically modified, and transgenic are used synonymously. • The term transgenic arises from the procedure of transgenesis, one method in which scientists have successfully inserted genes from one species to another.

  19. What is a triploid fish? • Triploid fish contain three sex chromosomes. • Heat and or pressure applied to females’ eggs to generate the extra chromosome. • Proponents of G-EN suggest environmental risks associated with their release into open water would be rendered harmless. • No guarantee of sterility!! • Competition is a reality for sterile fish too!

  20. Polyploidy • Polyploidy has been well-studied in fish and shellfish. • Channel catfish triploids become larger than diploids • Triploids converted feed more efficiently than diploids (Wolters et al., 1982), had six percent greater carcass yield at three years of age.

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