Occurrence of Syncytes: A Possible Mechanism Owing to the Origin of Polyploid Cytotypes in Achillea millefolium L. within Indian Himalayas

摘要

Present study records the existence of tetraploid and hexaploid meiocytes and subsequently the formation of large and very large-sized pollen grains for the first time in the diploid individuals of Achillea millefolium. In majority of the diploid accessions, meiocytes exhibited nine bivalents, equal segregation of chromosomes during anaphases, regular tetrads and normal-sized pollen grain formation. In the accessions scored from Palchan (2400m) and Dhundhi (3050 m) regions of Solang Valley, two to three proximate PMCs fused during early stages of Prophase-1 and Metaphase-1 resulting into tetraploid (4x) and hexaploid (6x) meiocytes with 18 and 27 bivalents, respectively. Although the frequency of polyploid PMCs was rather low these are detectable due to their larger size compared to diploid PMCs. In both the accessions, the diploid and polyploid PMCs exhibited the multiple chromosomal associations of four to eight chromosomes indicating the existence of structural heterozygosity. Analysis reveals that there is an increase in the chiasma frequency with structural heterozygosity for reciprocal translocations. Syncyte meiocytes followed a regular meiotic course resulting into the formation of normal tetrads but the products of such sporads yielded atypical (large and very large-sized) pollen grains compared to normal sized typical pollen grains. Although the exact cytological status of aforesaid pollen grains could not be ascertained but such fertile larger-sized pollen grains with possible '2n' or '3n' genetic constitution might be involved in fertilization to generate polyploid offsprings and the origin of intraspecific polyploid cytotypes (4x, 6x) which are known to exist in some populations of Himalayas in India.