preferentially into a functional megaspore during megasporogenesis; and certain gametes (carrying deficiencies in
euchromatin, presumably in chromosome 3.2) were inviable. All of these circumstances led to the nonrecovery of
the expected genotypic constitutions in the progeny.. But invariably the susceptibility was found whenever the
smaller translocated chromosome (1.4, on which this supposed dominant mutant gene was located: and which can
be identified by its size and multivalent formation) was present in the complement, highlighting that the inheritance of
the susceptibility reaction followed that of the translocated chromosome.
Rao, R. N., and Panuganti N. Rao Pachytene chromosome pairing in autotetraploid tomato.
In autotetraploid tomato cv. Marglobe, pachytene pairing in individual chromosome sets was analyzed to
facilitate an understanding of the relationship between chromosome length, extent of heterochromatin and
centromere, and the frequency and localization of partner exchanges. There were interchromosomal differences in
the frequency of partner exchanges in the three regions of the chromosome (heterochromatin, euchromatin and
centromere). Though the exchanges were distributed all along the, chromosome, the short chromosomes, such as
10, 11 and 12, had predominantly centric exchanges, but such exchanges were not frequent and were even absent
in some chromosomes, 2, 4 and 8, in which they were more frequently in eu- or heterochromatic regions.
The exchanges were more often localized in proximal than in distal regions of the chromosomes. They also
occurred more frequently in euchromatin than in heterochromatin which might be due to the greater proportion of
euchromatin. The number of exchanges per set varied from 1 to 2. The occurrence of two exchanges was as
frequent in medium-long chromosomes, such as 7 and 8, as in long chromosomes, such as 1 and 3, but was not
found in short chromosomes.
The four homologues of chromosome 2 (the nucleolus organizer) were stuck together most of the time in the
heterochromatic short arm and many times also at the centromere and heterochromatin of the long arm. On the
basis of multivalent formation by this chromosome at diakinesis, such associations between centromeres were
considered not to involve a partner exchange (Rao and Rao, 1978). But in the rest of the chromosomes, the centric
associations also were taken to represent exchanges, otherwise the multivalent frequency per cell realized at
diakinesis and metaphase I cannot be accounted for.
The results in the present study are not in complete agreement with those of the earlier investigation
(Gottschalk, 1955). The differences are believed to be due to genotypic differences between the varieties involved in
these studies.
Rick, C. M.
Further delimitation of the centromere of chromosome 11.
Thanks to the large population of genes and the availability of useful aneuploids, good progress has been
made in the localization of the centromere on the linkage map of chromosome 11. The last contribution in this series
was made by Carl Clayberg (TGC 22:5), who delimited ms-12 to the short arm by the modified ratio technique with
the tertiary trisomic 2n + 7S*11L. This discovery confined the centromere to the interval between ms-12 and a, in
terms of the present map, between positions 58 and 68. Since the only her locus in this interval is that of yg-6, we
utilized the same technique with the same trisomic with the following results.
The cross was made between 2n + 7S*11L and yg-6, and several trisomics were identified in the F1. The
latter were selfed and the F2 was produced with the following segregation: 145 2n+, 26 2n yg-6, 13 trisomic +, 5
trisomic yg-6 As expected, the ratio of yg-6 segregation amongst diploids is not modified because, even if the
segregation were trisomic, no viable haploid gametes can transmit the translocated chromosome. Trisomic
segregation should, however, be detected amongst the trisomic progeny if the gene were located on the long arm.
Since the segregation is normal and significantly different from the all vs. none expected of trisomic segregation, the
gene cannot be situated on the long arm, hence has to reside on the short arm. The centromere is thereby delimited
to the yg-6 - a interval, i.e. between positions 61 and 68.
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