Microsporogenesis and Pollen Morphology of Triploid and Tetraploid Rubber Trees (Hevea brasiliensis).

Polyploidy plays a key role in plant evolution and breeding, but the reproductive biology of polyploid rubber trees (Hevea brasiliensis) remains poorly understood. Here, we investigated microsporogenesis and pollen morphology in triploid and tetraploid rubber trees using acetic acid-magenta staining. Both polyploids displayed largely regular meiotic progression but exhibited abundant tetrad-stage abnormalities, including monads, dyads, triads, and unequal polyads. Tetraploids additionally exhibited chromosome misalignment, lagging chromosomes, asynchronous segregation, and multipolar spindles, with abnormal tetrads accounting for 24.52%. A total of more than 26,000 pollen grains (15,009 triploid and 11,774 tetraploid) were analyzed. In the triploid, 95.21% were normal-sized (30.7-43.3 µm), 2.21% were large (putative 2n; mean 47.2 µm), and 2.59% were super-large (putative 4n; mean 64.2 µm). In the tetraploid, 95.23% were normal-sized (31.7-46.7 µm), 4.75% were small (mean 19.6 µm), and only 0.025% were large (mean 55.3 µm). Normal-sized pollen from both polyploids was comparable to haploid pollen from diploids. Most large pollen grains exhibited wrinkled or irregular morphology, suggesting non-viability. Thus, functional male gametes in rubber tree polyploids are mostly reduced, and unreduced pollen is rare or nonviable. This study provides the first detailed description of the morphological sequence of microsporogenesis in triploid rubber tree and a cytological basis for understanding the low male fertility potential in breeding programs.