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2. Hypoaneuploid
a) Monotelodisomic: Sears (1966) first reported gene-centromere
distance with the aid of telocentrics in wheat. Neatby's virescent
gene (v) on the short arm of chromosome 3B was located at 0.28
crossover unit from the centromere. And the loci on the long arm of
6B, Sr11, Ki, and B2 were 45.1%, 41.3% and 0.44%
apart from the centromere, respectively. A dominant marker on the
monosomic arm of complete chromosome in montelodisomic, Co in
the case of Dr. Sears, marks transmission of the complete chromosome
to the offspring and can more or less save cytological determination
of chromosome complement. In comparison with disomic or conventional
analysis, reduction of amout of crossing over in the centromere
region is a problem that deserves careful attention. Sears (1972)
investigated crossover values in the heterozygote both for the
markers (Sr11 and B2) on the long arm and for
umbellulata segment substituted for most of the short arm of
6B, but the remaining segment being still enough to make the pairing
conditions different from a monotelodisomic for the long arm of 6B.
The recombination value of 3.5% was obtained in the region from B2
to the proximal end of umbellulata segment across the
centromere, which indicates four fold increase in frequency of
crossing-over in comparison with those obtained by using the
respective telosomes. Endrizzi and Kohel (1966) had reported decrease
in recombination value comparable to Sears as well as compensatory
increase in recombination value in monotelodisomic cotton.
Nishikawa et al (1974) reported recombination values of two
complementary genes for progressive necrosis with the centromere,
10.5% for Ne1 and 9.4% for Ne2, obtained by the
telocentric method. The Distance from the centromere of alpha-amylase
isozyme loci on the long arm of three chromosomes of homoeologous
gourp 6 were determined as shown in Fig.
2 (Nishikawa et
al unpublished). There is no information on shift of recombination
frequency in these cases.
b) Doubletelotrisomic: Doubletelotrisomic is as vigorous as disomic
and can be used for genetic analysis in diploid as well as polyploid
plants. An albino plant (ditelo2BL-monotelo2BS) of durum LD
222 occurred from the cross of a green plant of ditelo2BL-monotelo2BS
with heterozygous (A/a) doubleditelosomic for 2B, indicates that the
albino gene is located on short arm of 2B. Recombination value of
6.0% with the centromere was obtained from progenies of selfed
heterozygous (A/a) doubletelotrisomic. Though there seems to be few,
if any, paper reporting it, doubletelotrisomic analysis should be
recommended for genetic mapping, because it is not only applicable to
diploid and polyploid plants, but makes it possible to estimate the
recombination value and position of centromere, if the both arms of a
chromosome involved are properly marked.
Mase et al (1989) investigated F2 segregation of the
hybrid of doubleditelosomic for chromosome 1 with a multiple marker
line (BGN 1008: 1k2, n on the long arm and f8 on the
short arm, Tsuchiya 1985). The recombination values obtained are
shown in Fig.
3 together with
those from conventional method. It is obvious that recombination in
n-f8 region is reduced, and that in 1k2-n region
further apart from centromere is increased in the telocentric method
as compared with the conventional method. This seems to be a
phenomenon that commonly occurs in use of the telocentrics. In
contrast to Tsuchiya (1985) the recombination values indicate that
gene f8
is located on the
long arm.
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