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

This work mainly found that the stigma and style of Q. variabilis did not completely lose the specific recognition towards heterologous pollen, a fact which is different from previous studies. 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 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".