Cytoplasmic and nuclear genome interaction in interspecific crosses of sesame (Sesamum indicum L.)

A study was conducted to identify the source of male sterility cytoplasm among the wild species and transfer to the cultivated species, Sesamum indicum (2n=26). Five wild species (S. malabaricum, S. mulayanum, S. prostratum, S. radiatum and S. occidentale) were crossed as female parent to two cultivars of S. indicum. Two hybrids, S. malabaricum x S. indicum (E8–4) and S. mulayanum X S. indicum (E8–8) showed high percentage of pollen sterility as indicated by acetocarmine and seed setting studies. Cytological studies showed normal chromosome segregation 13:13 during Anaphase I. The sterile hybrids were observed to show regular formation of 13^II at Metaphase - I and large number of cells had 13/13 Anaphase-I segregation. Normal chromosomal segregation was observed in 49.10% and 77.50% of meiocytes in S. malabaricum X E8–4 and S. mulayanum X E8–8 interspecific hybrids, respectively. In S. malabaricum X E8–4 interspecific hybrid, 32 and 19 per cent of meiocytes observed showed 12:14 and 11:15 chromosome separation, respectively, whereas 10 and 7 per cent of meiocytes showed 12:14 and 11:15 chromosome segregation abnormalities respectively, in S. mulayanum X E8–8 hybrid. However, at the end of meiosis all the cells were seen with normal tetrad formation. The significantly higher percentage of pollen sterility in two interspecific crosses [S. malabaricum X E8–4 (24.54%) and S. mulayanum X E8–8 (20.00%)] compared to other crosses shows the genetic interaction between wild cytoplasm and indicum genome resulted in pollen sterility. The pollen sterility percentage increased in BC₁ generation when interspecific sterile hybrid was backcrossed to cultivated parent whereas BC₁ generation developed by backcrossing interspecific sterile hybrid with wild parent showed drastic reduction in pollen sterility further proving the wild cytoplasm and cultivated genome interaction as the cause of pollen sterility.