Introgression between shortleaf pine (Pinus echinata Mill.) and loblolly pine (Pinus taeda L.)

Scope and method of study. The purpose of this study was to determine the degree and nature of introgression between shortleaf pine and loblolly pine in a natural population utilizing molecular markers from nuclear, chloroplast and mitochondrial genomes and morphological data. Two controlled crosses, shortleaf pine (seed parent) X loblolly pine (pollen parent) and slash pine (seed parent) X shortleaf pine (pollen parent), and 80 individuals sampled from a central Arkansas population of loblolly and shortleaf pine were used in this study.; Findings and conclusions. A codominant DNA marker, from PCR-RFLP analysis of nuclear ribosomal DNA internal transcribed spacer 1 region, revealed ten hybrids from the eighty population samples. Two hybrids (HL) were morphologically similar to loblolly pine, while the other eight hybrids were similar to shortleaf pine. PCR-RFLP analysis of the rbcL gene showed that the two HL hybrids share loblolly pine restriction pattern, while the other hybrids (HS) all have the shortleaf pine pattern. Based on the data analyses of eleven low-copy microsatellite data analyses, the expected heterozygosity of the hybrids was higher than shortleaf or loblolly pine. UPGMA (the Unweighted Pair Group Method with Arithmetic Mean) analysis based on genetic distance from SSR data analysis showed that the hybrids morphologically similar to loblolly pine were clustered with loblolly pine, and the hybrids morphologically similar to shortleaf pine were clustered with shortleaf pine. These results indicated that bi-directional genetic introgression exists between the two pine species within this population, and the hybrids were likely derived from later generations backcross(es) with either shortleaf pine or loblolly pine. The inheritance and variation of mitochondrial DNA (mtDNA) markers were also studied. Maternal inheritance of mtDNA in these Pinus species was confirmed by the artificial cross of slash pine (seed parent) X shortleaf pine (pollen parent). High variation of mtDNA markers exists among shortleaf and loblolly pine populations, but low variation of mtDNA markers occur within the population selected. In addition, the trnL- trnF intergenic region nucleotide sequence polymorphism was found to be useful for paternal analysis among closely-related pine species.