4.2.1 Basic Techniques
A. Multiplication
Because cell division is preceded by g., replication of the DNA (chromosomes) ,
the quantity of cellular DNA (number of chromosomes) is doubled if the division
can be artificially blocked. Typically, an egg with two genomes is produced by
checking the maturation division process that occurs in the egg after oviposition and
fertilization (the second maturation division for fish, the first or second for shellfish
and crustaceans). This process of halting division is sometimes called polar body
emission prevention, as one side of the daughter cell during division (the polar
body) is very small and appears to be in a process of emission from the other side.
Doubling can also be achieved by preventing the subsequent first cleavage (division
of the somatic cell), although the success rate is low. Methods used in doubling
include chemical (e.g., through immersion in colchicine or cytochalasin B solution),
physical (e.heating, cooling, and pressure), and biological (e.g., hybridization of
particular combinations) types (Purdom 1983; Thorgaard 1983; Isaacs et al. 2011).
B. Parthenogenesis
Fertilization in which one gamete nucleus is treated with radiation such as
ultraviolet or gamma rays, or chemicals such as toluidine blue, results in development occurring in only the untreated gamete nucleus. Development in the egg
nucleus alone is known as gynogenesis, while development in the sperm nucleus
alone is known as androgenesis; both require artificial fertilization. Naturally, the
genetic traits of the undeveloped nucleus are not transmitted.
Detailed research has considered the question of why gynogenesis should occur
when this treatment is applied to the sperm. Two methods are believed to exist.
C. Aneuploidy
An aneuploid is an organism in which the number of chromosomes is not a
whole number multiple of the genome. Artificial production of aneuploids follows
the same approach as parthenogenesis, but involves fertilization with a gamete
treated under weak conditions so that at least part of the chromosome remains
undestroyed, or artificial insemination with aneuploidy sperm obtained from a triploid male. An aneuploidy embryo may be obtained when an over-ripened egg is
fertilized artificially after losing part of its chromosomes (Hassold and Hunt 2001).
D. Hybridization
If we take the letters “A” and “B” to represent individual genomes, such that the
parents’ somatic cells have karyotypes of 2A and 2B, fertilization of an egg (A) and
sperm (B) obtained from different species results in a hybrid possessing a new
karyotype A + B. While the karyotype does not change when the exact reverse is
4.2 Chromosome Manipulation
83
A. Multiplication
Because cell division is preceded by g., replication of the DNA (chromosomes) ,
the quantity of cellular DNA (number of chromosomes) is doubled if the division
can be artificially blocked. Typically, an egg with two genomes is produced by
checking the maturation division process that occurs in the egg after oviposition and
fertilization (the second maturation division for fish, the first or second for shellfish
and crustaceans). This process of halting division is sometimes called polar body
emission prevention, as one side of the daughter cell during division (the polar
body) is very small and appears to be in a process of emission from the other side.
Doubling can also be achieved by preventing the subsequent first cleavage (division
of the somatic cell), although the success rate is low. Methods used in doubling
include chemical (e.g., through immersion in colchicine or cytochalasin B solution),
physical (e.heating, cooling, and pressure), and biological (e.g., hybridization of
particular combinations) types (Purdom 1983; Thorgaard 1983; Isaacs et al. 2011).
B. Parthenogenesis
Fertilization in which one gamete nucleus is treated with radiation such as
ultraviolet or gamma rays, or chemicals such as toluidine blue, results in development occurring in only the untreated gamete nucleus. Development in the egg
nucleus alone is known as gynogenesis, while development in the sperm nucleus
alone is known as androgenesis; both require artificial fertilization. Naturally, the
genetic traits of the undeveloped nucleus are not transmitted.
Detailed research has considered the question of why gynogenesis should occur
when this treatment is applied to the sperm. Two methods are believed to exist.
C. Aneuploidy
An aneuploid is an organism in which the number of chromosomes is not a
whole number multiple of the genome. Artificial production of aneuploids follows
the same approach as parthenogenesis, but involves fertilization with a gamete
treated under weak conditions so that at least part of the chromosome remains
undestroyed, or artificial insemination with aneuploidy sperm obtained from a triploid male. An aneuploidy embryo may be obtained when an over-ripened egg is
fertilized artificially after losing part of its chromosomes (Hassold and Hunt 2001).
D. Hybridization
If we take the letters “A” and “B” to represent individual genomes, such that the
parents’ somatic cells have karyotypes of 2A and 2B, fertilization of an egg (A) and
sperm (B) obtained from different species results in a hybrid possessing a new
karyotype A + B. While the karyotype does not change when the exact reverse is
4.2 Chromosome Manipulation
83