Characterization of pollen tube development in distant hybridization of Chinese cork oak (Quercus variabilis L.)

Quercus is the foundation species in the Northern Hemisphere, with extreme prevalence for interspecific hybridization. It is not yet entirely understood whether or how the pollen tube-female tissue interaction contributes to the extensive hybridization in oaks. Pollen storage conditions correlate with distant hybridization. We conducted hybridization experiments with Q. variabilis as female and Q. variabilis and Q. mongolica as male parents. And the differences in pollen tube (PT) development between intra- and distant interspecific hybridization were studied by fluorescence microscopy and scanning electron microscopy (SEM). Our results showed that -20 C-degrees allowed pollen of both species to maintain some viability. Both Q. variabilis and Q. mongolica pollen germinated profusely on the stigmas. SEM results indicated that in the intraspecific hybridization, Q. variabilis pollen started to germinate at 6 h after pollination (hap), PTs elongated significantly at 12 hap, and entered the stigma at 24 hap. By contrast, Q. mongolica pollen germinated at 15 hap, and the PTs entered the stigma at 27 hap. By fluorescence microscopical studies it was observed that some PTs of Q. variabilis gathered at the style-joining at 96 hap, unlike the Q. mongolica which reached the style junction at 144 hap. The above results indicate that the abundant germination of heterologous pollen (HP) on the stigma and the Feeble specificity recognition of the stigma and transmitting tract to HP may create opportunities for the extensive hybridization of oaks. This work provides a sexual developmental reference for clarifying the causes of Quercus extensive hybridization.

Automated summary

This study investigates the differences in pollen tube development between intra- and distant interspecific hybridization in Quercus. The results suggest that the abundant germination of heterologous pollen on the stigma and the feeble specificity recognition of the stigma and transmitting tract to heterologous pollen may contribute to extensive hybridization in oaks.