Genetic structure of hybrid zones between Pinus pumila and P. parviflora var. pentaphylla (Pinaceae) revealed by molecular hybrid index analysis.

Pinus species have three differently inherited genomes: paternal chloroplast, maternal mitochondrial, and biparental nuclear. Our previous study on the hybrid zones between alpine Pinus pumila and montane to subalpine P. parviflora var. pentaphylla demonstrated contrasting patterns of introgression of two cytoplasmic genomes, i.e., the paternal cpDNA flowed from P. parviflora var. pentaphylla to P. pumila, and the maternal mtDNA flowed in the reverse direction. In the present study, we developed codominant nuclear DNA markers diagnostic or mostly diagnostic for each parental species by single-strand conformation polymorphism (SSCP) of polymerase chain reaction (PCR) products, using expressed sequence tag (EST) primers of Pinus taeda. To describe the introgressive patterns of the nuclear genes, the molecular hybrid index (MHI) showing the overall proportion of alleles inferred to be derived from P. pumila was determined for each plant collected in hybrid zones on Mt. Asahidake and Mt. Higashiazuma, Japan. At Mt. Asahidake, the MHI values changed clinally according to the altitudes at which the plants were collected. However, at Mt. Higashiazuma, there was a gap in the MHI values between the plants above and below the Abies and Tsuga forest zone (alt. 1800-1900 m). This suggested that the zone plays a role in creating an effective barrier to gene flow in the hybrid zone.