In a polyploid derivative of an interspecific hybrid each parental chromosome is represented twice (amphidiploid) or one genome occurs twice and the other only once (sesquidiploid). So there is a choice of pairing between homologous chromosomes (autosyndesis) or homologous chromosomes (allosyndesis). For estimating the amount of allosyndesis in a polyploid derivative of an interspecific hybrid, allosyndetic bivalents should also be taken into consideration in addition to multivalents. Cytologically it is possible to detect whether the pairing is preferential or random by a study of relative frequencies of homo and heteromorphic bivalents and quadrivalents at the pachytene stage of meiosis.
In the present investigation, one sesquidiploid and one amphidiploid involving two sps. of Lycopersicon have been studied.
1. Sesquidiploid of induced tetraploid L. esculentum cv. Punjab chhoara (4n=48) X L. cheesmanii (LA1136) (2n=24).
2. Amphidiploid of L. esculentum cv. Punjab chhoara (2n=24) x L. cheesmanii (LA1136) (2n=24).
Cytological investigations were made on chromosome pairing at pachytene at which stage it is possible to identify homomorphic and heteromorphic bivalents, and multivalents. The present study is limited to chromosomes-2 and 6 in the sesquidiploid and chromosomes-1, 2 and 6 in the amphidiploid as they can be identified easily by their structural dissimilarities and defective pairing in the hybrid complement. It is observed that there are large deviations from the expected 1:2 ratio of homo and homeologous bivalents. So it is concluded that there is a strong tendency towards preferential pairing between chromosomes coming from the same species. Further, a comparison of the multivalent frequency at diakinesis in amphidiploid (1.63/cell) which is also lower than the same in induced tetraploids of L. esculentum cv. Punjab chhoara (2.98/cell) and L. cheesmanii (2.59/cell) indicates the occurrence of non-random pairing of chromosomes.