Rao, R. N., and Panuganti N. Rao A case of spontaneous breakdown of self-incompatibility in Lycopersicon
peruvianum f. glandulosum.
One plant in a population of L. peruvianum f. glandulosum grown from seed obtained from Plant Introduction
Centre, I.A.R.I., New Delhi, exhibited a variation in flower form. Normally, self-incompatible plants have their styles
exerted which facilitates cross pollination. The variant has a slightly smaller flower with its style and stigma
completely enclosed within the anther column, and it has well formed fruits and seeds even though there was no
possibility for cross pollination. It is, therefore, presumed to be a case of breakdown of self-incompatibility. Absence
of exsertion of the style is due to shortening in length rather than twisting as observed in L. peruvianum by
Hogenboom (Euphytica 17:220-223, 1968). To verify the assumption of breakdown of incompatibility, the flowers
were selfed and there was good fruit and seed set. Also this variant was crossed to L. esculentum to test its cross-
compatibility. Tetraploid tomato (cv. Marglobe) was used in crosses since the diploid was not available at that time.
To serve as control, crosses were made between 4n tomato and self-incompatible f. glandulosum having exserted
style, with the former as the seed parent, since in some earlier reports it was mentioned that crosses were
successful when 2n f. glandulosum or L. peruvianum was crossed as male with 4n tomato. But in the present study,
whilst the latter cross did not produce any plump seed despite good fruit set, in the former cross involving the variant
f. glandulosum there was fairly good fruit and seed set. Although the germination of these seeds was not very good,
the few that germinated turned out to be sesquidiploids (triploid hybrids). Whether this success in crosses can be
taken as breakdown of cross-incompatibility is to be ascertained by making crosses at the 2n level.
Rao, R. N., and Panuganti N. Rao Non-random chromosome pairing in sesquidiploids and an amphidiploid of
Lycopersicon.
In a diploid F1 hybrid between two closely related species, since every chromosome of one species has only
a single homeologous counterpart from the other species, no competition for pairing exists and the two
chromosomes form a bivalent. Thus bivalent formation may be considered obligatory here. But in a polyploid
derivative of such an interspecific hybrid each parental genome occurs twice and there is a choice of pairing
between chromosomes of the two genomes. Similarly, in a sesquidiploid one genome occurs twice and the other
only once. Theoretically these chromosomes can form multivalent, bivalents and univalents. When a multivalent is
formed, it is by allosyndesis. But when two bivalents, as in amphidiploid, and bivalent and univalent, as in
sesquidiploid, are formed it can be by autosyndesis or allosyndesis. Random pairing between chromosomes of the
two genomes is expected to yield homologous and homeologous bivalents in the proportion of 1:2 (in both
amphidiploids and sesquidiploids). Any deviation from this ratio of homo- and homeologous bivalents implies
nonrandom or preferential chromosome pairing between the two species. Thus a study of the relative frequencies of
these two types of bivalents will be a more realistic approach in understanding the nature of chromosome pairing
than a mere study of multivalent frequency at diakinesis and metaphase I. However, a prerequisite for this sort of
study is the proper discrimination between the two types of bivalents and their correct identification.
In the present investigation, two sesquidiploids and one amphidiploid of the following parentage have been
studied.
1. sesquidiploid - L. esculentum (4n) x L. peruvianum f. glandulosum (2n)
2. sesquidiploid - L. esculentum (4n) x L. peruvianum var. humifusum (2n)
3. amphidiploid - L. esculentum (2n) x L. hirsutum (2n) F1 hybrid doubled
Direct cytological observations were made on chromosome pairing at pachytene at which stage it is possible
to identify the homologous and homeologous bivalents with exactitude. The study, however, was restricted to
chromosome-2 and chromosome-6 whose morphology differs considerably and could be recognized readily in the
complement in the cultivated and the three
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