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So far one of the translocation lines, 'Weique Zuchter',
has been found deviating from all,other by the complete or
almost complete lack of the telomeric band of the long arm
of chromosome 1B-1R, which is accompanied by an adequate
reduction of length (FRIEBE 1976; MUNZER 1977). At present
neither the appropriate meiotic pairing data (Table
2, data taken from ZELLER and SASTROSUMARJO 1972;
BLUTHNER and METTIN 1973; BLUTHNER and METTIN 1977) nor the
Giemsa analyses provide any evidence on the origin of this
particular chromosome arm. Actually the data presented in
Table 2 only demonstrate the
capability of the deviating interchange chromosome to pair
with 1RS as well as 1BL. Whether the long arm of chromosome
1B-1R of 'Weique Zuchter' is capable to pair with 1RL has
not yet been investigated. And so there is at the moment no
reason to speculate about the possible loss of telomeric
heterochromatin effecting a bilateral pairing ability
similar to the behaviour of the presumed 2D(2R) substitution
in 'Rosner' as suggested by KALTSIKES et al. (1977).
On the other hand it seems very likely that the long arm of
the common type of the 1B-1R interchange has derived from
chromosome 1B completely or almost completely. In addition
to previous results SCHLEGEL (unpubl.) by using the Giemsa
banding technique was able to demonstrate pairing of
chromosome 1B-1R with rye chromosome 1R being restricted to
the short arms, i.e. closed bivalents were never
observed.
Because of the difficulty to locate the possible breakpoint
of the interchange by genetic means, we tried to get some
additional evidence by the presumed mode of origin of the
1B-1R interchange. Tracing back the origin of most of the
translocation lines listed in Table
1 there is no doubt about the parentage of 1R(1B) whole
chromosome substitutions. So it is very likely that the
substitution was prior to the translocation. Because
substitutions were often crossed to karyotypically normal
wheats it seems reasonable to suppose a connection between
the double-monosomic state for 1B and 1R in the hybrids on
the one hand and the origin of the interchange on the other.
Considering 1B, 1R double-monosomic wheat hybrids there is a
certain probability for centromeric misdivision of the
univalents followed by fusion of the telocentrics as shown
by MORRISON (1954), SEARS (1972) and SHEPHERD (1973), though
casual homoeologous pairing and recombination can no longer
ruled out. For that reason some of the prerequisites for
either centromeric fusion or recombination have been
analyzed.
Taking the amount of telosomic offspring in appropriate
wheat lines and hybrids as a measure for centromeric
misdivision our previous data show that 1B, 1R
double-monosomics yield up to 21% telocentrics. A
comparative analysis of the frequency of telosomics in the
progenies of wheat hybrids being monosomic for either 1B or
1R indicated a rather high instability of rye chromosome 1R,
at least in a wheat background. This suggestion was
confirmed in Giemsa-stained metaphase I preparations of 1B,
1R double-monosomics. As shown in Table
3 misdivisions of rye chromosome 1R exceeded by far
those of wheat chromosome 1B. Considering the low number of
PMC's scored it seems worthwhile to emphasize the occurrence
of simultaneous misdivisions of both chromosomes under
question, which is the prerequisite for centromeric
fusion.
The alternative mode of origin of the 1B-1R interchange by
means of homoeologous recombination must be taken into
consideration, since several authors, by using the Giemsa
banding technique, have proven the occurrence of wheat-rye
pairing even in the presence of wheat chromosome 5B (METTIN
et al. 1976; DHALIWAL et al. 1977; POHLER and
KISTNER 1977). Our additional data on a large number of
bivalents and multivalents in euhaploid wheat-rye hybrids
suggest a figure of about 1.5% Wheat-rye pairings of all
associations (Table 4). This is,
however, the average for the total of the rye chromosomes.
Whether or not rye chromosome 1R, having a large amount of
telomeric heterochromatin, participates proportionally in
the wheat-rye associations must be left open at the
moment.
Summarizing all the data available there is a lot of reason
to suppose that the 1B-1R interchange derived from the whole
chromosome 1R (1B) substitution by means of centromeric
fusion. Therefore the breakpoint should be located at or
very close to the centromere. This seems to be true, at
least, for the common type of the interchange.
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