High-resolution time-course observations of pollination drop withdrawal in Chinese fir (Cunninghamia lanceolata)

Gymnosperms have few mechanisms for shaping the mating pool between the time of pollen reception on a pollination drop (PD) and fertilization. The duration of the PD on the ovule may be one mechanism if it changes depending on relatedness of the pollen source and the recipient. Here, we timed the duration of the PD after pollination with pollen from different clones of the outcrossing conifer Cunninghamia lanceolata.

We collected unpollinated strobili from C. lanceolata in the field and grew them hydroponically in a controlled chamber to induce PD production. Using high-resolution imaging, we monitored PD dynamics in real time after selfed, inbred backcross, and outcross pollination. Interparental genetic distances (GD) were determined for each cross using 21 polymorphic microsatellite loci.

The high-resolution imaging revealed that PDs in nonpollinated ovules were exposed for an average of 10.63 ± 1.31 h, whereas PDs in ovules after outcrossing and selfing were withdrawn within 0.46 ± 0.06 h and 3.43 ± 0.84 h, respectively. The three inbred backcrosses were intermediate between selfing and outcrossing. High-resolution videos further confirmed the withdrawal was rapid and abrupt after outcrossing and slower after most inbred crosses, especially selfing. There was a strong negative correlation between genetic distance and PD withdrawal time (r = -0.92, P < 0.0001), implying a regulatory mechanism sensitive to the degree of relatedness.

These novel findings suggest that the longevity of the PD is a mechanism for post-pollination mate recognition.