Sexual reproduction and crossing barriers in white pines: the case between Pinus lambertiana (sugar pine) and P. monticola (western white pine)

The sexual reproductive process in Pinus iambertiana has not been completely described, and previous attempts to generate hybrids with Pinus monticola and other North American pines have not been successful. The nature of incompatibility barriers between P. iambertiana and P. monticoia is unknown. This needs to be understood if strategies are to be developed to overcome the said barriers. In this paper, development on interspecific crosses is compared with that on intraspecific crosses on the same parent trees. Pollen grains of both species germinated on the nucellus of both species within a week after pollination. Seed cone receptivity in P. iambertiana came approximately 2 weeks after receptivity in P. monticoia, and this delay was perpetuated throughout ovule development in the first year of the reproductive process. Development of the second-year seed cones proceeded more gradually in P. iambertiana. However, seed cones reached maturity only for P. monticoia x P. iambertiana. In both crosses, the barriers to hybridization occurred during the second year of the reproductive process. With the P. lambertiana as the seed parent, it was manifested through the failure of the megaspores at the free-nuclear stage to resume development. When P. monticoia was used as the seed parent, the male and female gametes failed to fuse. Our results clearly show that the barriers to hybridization in these species occur before or at fertilization. However, the exact mechanisms behind these are still unknown. Based on the results of this study, we present several strategies to bypass the developmental barriers and possibly produce hybrid progenies.