The term “polyploid” is typically applied in cases of whole number multiples of
genomes; depending on the number of genomes, they may be haploid (n), diploid
(2n), triploid (3n), and so forth. The majority of fish and shellfish are thus diploid.
Why are polyploids used in breeding? Since they have more chromosomes and
genes than diploids, they also yield more genetic products, resulting in large
organisms. In the case of odd-number polyploids, clonal propagation appears to
have developed as a means of survival because gamete pairs cannot be formed.
A representative example of this is the triploid seedless watermelon. While ordinary
watermelons are diploid, treatment with colchicine to check the formation of
spindle fibers (microtubules) during meiosis (the characteristic form of cell division
to create reproductive cells, or eggs and sperm) resulted in the development of 2n
sex cells, in which the number of chromosomes has not been halved. Subsequent
pollination of the polyploid sex cells (2n) and diploid sex cells (n) results in the
formation of a triploid (3n). Because the inability of chromosomes to pair during
meiosis prevents the formation of normal cells, odd-number polyploids are sterile.
The seedless watermelon is one such result.
Quadruploids can sometimes bear especially large flowers or fruit, resulting in
large yields. In 1936, Blakeslee and Avery found that treatment of seed sprouts with a
solution of colchicine (a type of alkaloid contained in colchicum, a perennial of the
lily order) resulted in multiples of the chromosome set, most of them quadruploid.
This method holds great significance for breeding science, as it can be used to
produce large pumpkins, radishes with large roots, cotton with long fibers, high-yield
leaf tobacco, vitamin-rich tomatoes, and large-blossomed flower-of-the-hours.
Quadruploids can also be obtained with Quercus variabilis and other forest plants.
Examples of well-known crops for which polyploids are used are bananas (triploid), peanuts and potatoes (quadruploid), and sweet potatoes (sextuploid). These
are larger in form than their diploid counterparts because the chromosomes replicated in the somatic cell division process do not divide.
While active breeding with polyploids has taken place in agriculture, chromosome manipulation for marine organisms was introduced very late. Indeed, serious
application has only taken place since the 1980s, and its history is relatively brief.
The situation today in research and development of chromosome manipulation
for marine organisms is one of a near total absence of such techniques for marine
microalgae. In the case of marine plants, cultured cells from the kelp class have
been used to attempt chromosome set manipulation through apospory and apogamy. In terms of marine animals, chromosome manipulation methods have been
established for several varieties of fish from the salmon and flatfish families and for
several kinds of shellfish, including pearl and other oysters and abalone. Production
of female-only populations is currently under way, along with the commercialization of grains with superior growth properties, such as triploid shellfish.
The following are the four basis techniques used in chromosome manipulation,
which can be combined to obtain the genetic variomes seen in the application
examples.
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4 Fish Breeding and Biotechnology
genomes; depending on the number of genomes, they may be haploid (n), diploid
(2n), triploid (3n), and so forth. The majority of fish and shellfish are thus diploid.
Why are polyploids used in breeding? Since they have more chromosomes and
genes than diploids, they also yield more genetic products, resulting in large
organisms. In the case of odd-number polyploids, clonal propagation appears to
have developed as a means of survival because gamete pairs cannot be formed.
A representative example of this is the triploid seedless watermelon. While ordinary
watermelons are diploid, treatment with colchicine to check the formation of
spindle fibers (microtubules) during meiosis (the characteristic form of cell division
to create reproductive cells, or eggs and sperm) resulted in the development of 2n
sex cells, in which the number of chromosomes has not been halved. Subsequent
pollination of the polyploid sex cells (2n) and diploid sex cells (n) results in the
formation of a triploid (3n). Because the inability of chromosomes to pair during
meiosis prevents the formation of normal cells, odd-number polyploids are sterile.
The seedless watermelon is one such result.
Quadruploids can sometimes bear especially large flowers or fruit, resulting in
large yields. In 1936, Blakeslee and Avery found that treatment of seed sprouts with a
solution of colchicine (a type of alkaloid contained in colchicum, a perennial of the
lily order) resulted in multiples of the chromosome set, most of them quadruploid.
This method holds great significance for breeding science, as it can be used to
produce large pumpkins, radishes with large roots, cotton with long fibers, high-yield
leaf tobacco, vitamin-rich tomatoes, and large-blossomed flower-of-the-hours.
Quadruploids can also be obtained with Quercus variabilis and other forest plants.
Examples of well-known crops for which polyploids are used are bananas (triploid), peanuts and potatoes (quadruploid), and sweet potatoes (sextuploid). These
are larger in form than their diploid counterparts because the chromosomes replicated in the somatic cell division process do not divide.
While active breeding with polyploids has taken place in agriculture, chromosome manipulation for marine organisms was introduced very late. Indeed, serious
application has only taken place since the 1980s, and its history is relatively brief.
The situation today in research and development of chromosome manipulation
for marine organisms is one of a near total absence of such techniques for marine
microalgae. In the case of marine plants, cultured cells from the kelp class have
been used to attempt chromosome set manipulation through apospory and apogamy. In terms of marine animals, chromosome manipulation methods have been
established for several varieties of fish from the salmon and flatfish families and for
several kinds of shellfish, including pearl and other oysters and abalone. Production
of female-only populations is currently under way, along with the commercialization of grains with superior growth properties, such as triploid shellfish.
The following are the four basis techniques used in chromosome manipulation,
which can be combined to obtain the genetic variomes seen in the application
examples.
82
4 Fish Breeding and Biotechnology