Chromosome Pairing in Two Sesquiploid Hybrids and its Bearing on Genome Relationship in the Genus Sesamum

1. The amphilploid derived by doubling the chromosome number of the F₁ hybrid between S. indicum x S. prostratum was backcrossed to the respective parental species. Very few viable seeds were obtained from such crosses though sufficient numbers of pollen tubes reached the ovules.

2. The sesquiploids, di-indicum-mono-prostratum (2n=42) and di-prostratum-mono-indicum (2n=45), thus obtained were studied cytologically. A study of meiosis in PMC showed 13 bivalents or bivalents plus trivalents in the former sesquiploid and 16 bivalents or bivalents plus trivalents in the latter, in addition to a number of univalents. A maximum of three trivalents were observed in both the sesquiploids. This is indicative of intergenomic pairing, though to a restricted extent.

3. The behaviour of the univalents in the two sesquiploids varied. In di-indicum-mono-prostratum sesquiploids, the univalent chromosomes of prostratum tended to get eliminated while in the other sesquiploid, di-prostratum-mono-indicum, the univalent chromosomes derived from S. indicum tended to get included in the gametes.

4. The former sesquiploid (2n=42) showed about 50 per cent fertility on both female and male sides. A large proportion of the progeny obtained by selfing or backcrossing to S. indicum had 2n=26 chromosomes and resembled S. indicum though a few exhibited some prostratum characters. Aneuploid plants with one to three extra prostration chromosomes were also recovered with a low frequency.

5. The other sesquiploid (2n=45) was completely pollen sterile though it set a few seeds when pollinated with pollen from S. indicum and S. prostratum respectively. The chromosome number and morphology of the hybrids so obtained suggested that only megagametes with 16 prostratum chromosomes had functioned in this sesquiploid.