Abstract
The purpose of this review is to introduce recent advances in research, development and
application of chromosome manipulation techniques in Japan. Triploids have been induced and
utilized to improve growth. In most cases, triploids are produced as all-female populations by
using spermatozoa of artificially sex-reversed males, so as to assure their complete sterility. In
contrast, triploid males show better gonadal development and sometimes generate functional
spermatozoa, which give rise to inviable aneuploids in most species. Although induced tetraploids
can be useful for mass production of triploids by mating with normal diploids, tetraploid lines
have only been produced in rainbow trout by a few institutes belonging to prefectural governments.
In most cases, the techniques to inhibit the first cleavage often result in very low survival
and mosaicism. In the loach Cobitidae , polyploid lines such as hexaploids have been developed Ž .
by using natural tetraploid individuals as an intermediate step. Cloned fish can be produced by the
second cycle of gynogenesis in the eggs of completely homozygous diploids, which were
produced by inhibiting first cleavage after induction of gynogenetic and androgenetic development.
The second cycle of androgenesis, using spermatozoa of completely homozygous males, can
also generate clonal lines. The most serious technical problem of cloning is the extremely low
survival of homozygous gynogenetic and androgenetic diploids, probably due to the expression of
deleterious recessive genes and side effect of treatments. However, cloned populations have been
realized in commercially important ayu Plecoglossus altiÕelis, amago salmon Oncorhynchus
masou ishikawae, coho salmon O. kisutch, hirame Japanese flounder Ž . Paralichthys oliÕaceus,
fancy carp Cyprinus carpio, and red sea bream Pagrus major. In hirame, a practical method for
mass production of clones was proposed and better performance has been reported in a heterozy-