Syncyte and ‘2n’ Pollen Grain Formation in Heracleum pinnatum: A Possible Mechanism for the Origin of Intraspecific Polyploids

Heracleum pinnatum C. B. Clarke (Apiaceae) collected from the cold deserts of Ladakh Himalaya, Jammu and Kashmir, India, is cytologically analyzed for the first time, revealing a diploid chromosome count of 2n=22. During meiosis, the majority of the pollen mother cells (PMCs) exhibited 11 bivalents, equal segregation of chromosomes during anaphases, regular tetrads, and normal-sized pollen grain formation. Occasionally, two proximate PMCs fused during the early stages of prophase-I and resulted in the formation of syncytes. The frequency of such syncytic meiocytes was low (3.72–3.96%) but these could be easily detected in the preparations due to their large size compared to typical PMCs. Also, the syncytes depicted a meiotic chromosome count of 2n=44 as confirmed from the presence of 22 normal bivalents. Further, meiotic course in such tetraploid PMCs was also observed to be perfectly regular, leading to the formation of ‘2n’ or larger-sized pollen grains that are almost double the size of typical ‘n’ pollen grains. Such larger-sized ‘2n’ pollen grains could also be differentiated on the basis of their shape. Additionally, a significant frequency of PMCs also showed the phenomenon of cytomixis involving transfer of chromatin material resulting in aneuploid meiocytes. Whole chromatin transfer during cytomixis among neighboring PMCs at early stage of meiosis-I lead to the formation of PMCs with double the chromatin material. Syncytes resulted as a consequence of fusion of meiocytes during the early stages of meiosis-I could be attributed to low temperature stress conditions prevailing at the time when the plants enter the flowering stage. It is quite possible that such apparently fertile ‘2n’ pollen grains originating from syncytes might play a role in the origin of polyploids in the species.