Pollen - pistil interaction study in interspecific crosses of Medicago sativa x Medicago scutellata for stem weevil resistance

Abstract

Studies were carried out for the pollen tube growth, failure of pollen tubes to effect fertilization and post-fertilization ovule abortion to hybridize Indian tetraploid (2n = 32) Medicago sativa L. cultivar RL 88 with diploid (2n=30) exotic annual shield medics / snail medic genotypes Medicago scutellata L. (Mill.), for the transfer of stem weevil resistance. Pollen fertility of M. scutellata was found to be 89%. In the selfing study of M. scutellata, the pollen grain germinated and reached ovule by 28 hours after pollination (HAP). In cultivar RL 88, the pollen grains reached ovule by 48 HAP after selfing. However, very few pollen grains reached the ovule upon selfing even after 5 days after pollination (DAP). In M. sativa x M. scutellata crosses, the growth of the pollen grains on the stigma and stylar tissues were very slow, during the study period. However, it was observed that after 96 HAP to 120 HAP, hardly few pollen grains reached ovule in the interspecific crosses in M. sativa x M. scutellata, with many callose plugs resulting in putative hybrid embryos. Out of 1287 florets attempted for interspecific crosses between RL 88 x M. scutellata (EC 541685), only five normal appearing pods developed till maturity and containing one seed each. Reciprocal crosses involving M. scutellata as female parent did not yielded any capsule set. Obvious indicators of fertilization as well as the initiation of the embryonic growth in M. sativa are the persistence of the flower on the raceme and increase in the diameter of the ovary. However, the putative hybrids need further confirmation with cytological and molecular studies. Our results demonstrate that interspecific hybrid plants with M. sativa x M. scutellata can be successfully obtained by adapting to suitable tissue culture-based breeding methodologies and embryo rescue.