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The frequency of mature hybrids, including post-F1 individuals, between loblolly (Pinus taeda L.) and shortleaf pine (Pinus echinata Mill.), detectable with a codominant nuclear marker, was studied in a sympatric population from central Arkansas. The direction of introgression was also examined. The marker revealed 10 putative hybrids from the 80 trees sampled. PCR-RFLP analysis of their rbcL gene showed two of the putative hybrids (HL) share loblolly pine chloroplast DNA, and eight (HS) share the shortleaf pine chloroplast DNA. The two putative HL hybrids were morphologically similar to loblolly pine, and the eight putative HS hybrids were morphologically similar to shortleaf pine. Utilizing microsatellite data, Nei's measure of genetic identity showed the putative HL hybrids to be similar to loblolly pine, and the putative HS hybrids as being similar to shortleaf pine. An inferred tree of the individuals, using simple sequence repeat data and the neighbor-joining method, also suggested that some of the putative hybrids were not F1 individuals. Principle component analysis of the morphological characters groups the HL trees with loblolly pine and the HS trees with shortleaf pine. These results suggest bidirectional introgression occurred within the study population, and some of the putative hybrids were likely derived from early-generation backcross(es) with either shortleaf or loblolly pine. [ABSTRACT FROM AUTHOR]

Seed from 126 trees from populations representing 15 geographic locations covering much of the natural range of shortleaf pine (Pinus echinata Mill.) were analyzed using 23 enzyme systems covering 39 loci to determine patterns of genetic variation and structure. Populations were polymorphic (p) at 87.2% of the loci, had 2.18 alleles (A) per locus, and had 2.35 alleles per polymorphic locus (Ap). Mean expected heterozygosity (He) was 0.194 and mean observedheterozygosity (Ho) was 0.174. Western populations had a higher p, higher A, similar Ap, and a higher Hoand He than eastern populations, in part because of six private alleles (alleles seen only in one population) in the west but only one in the east. Genetic structure analysis revealed interpopulation genetic variation at 9%, meaning 91% of the genetic variation inshortleaf pine resides within populations. Interpopulation gene flowwas 2.56, indicating two or three allele migrations per generation, which is relatively high and explains the low interpopulation geneticvariation in the species. There was no apparent relationship betweengeographic distance among populations and their genetic distance. Shortleaf pine populations exist in naturally outcrossing random-matingpopulations and have a relatively large amount of natural variability. Western populations are more diverse than their eastern counterparts. [ABSTRACT FROM AUTHOR]