Spatial distribution of CpDNA and MtDNA haplotypes in a hybrid zone between Pinus pumila and P. parviflora var. pentaphylla (Pinaceae)

Pinus species exhibit paternal chloroplast inheritance and maternal mitochondrial inheritance. This independent inheritance of two cytoplasmic genomes provides an exceptional environment for discriminating female (seeds) and male (pollen) components of gene flow across hybridizing species. We obtained mitochondrial genetic markers diagnostic to P. parviflora var. pentaphylla and P. pumila by PCR amplification of the intron of nad1 on mtDNA, and examined the spatial-distribution pattern of the mtDNA haplotypes in a hybrid zone between P. parviflora var. pentaphylla and P. pumila in the Tanigawa Mountains of Japan. These data, in conjunction with previous information on cpDNA haplotypes and needle morphology, revealed contrastive patterns of introgression of two cytoplasmic genomes. CpDNA introgression has occurred uni-directionally from P. parviflora var. pentaphylla to P. pumila. Conversely, mtDNA introgression has occurred in the opposite direction, from P. pumila to P. parviflora var. pentaphylla. Levels of introgression are roughly equivalent for cpDNA and mtDNA. The contrastive spatial distribution pattern of cpDNA and mtDNA haplotypes could be caused by differential movement of seeds and pollen for interspecific genetic exchange.