Studies of the Pinus rigida-Serotina Complex II. Natural Hybridization Among the Pinus rigida-Serotina Complex, P. Taeda and P. echinata

Patterns of hybridization were studied among Pinus taeda, P. echinata and the P. rigidaserotina complex. Efforts were made to determine the relationships of these taxa to help clarify the taxonomnic status of members of the P. rigida-serotina complex. Hybridization between Pinus taeda and the P. rigida-serotina complex was common, and genetic exchange occurred in both directions. Hybridization between P. echinata and the P. rigida-serotina complex was more restricted, as was hybridization between P. taeda and P. echinata. All three of these entities (P. taeda, P. echinata, and the P. rigida-serotina complex) were found to maintain their separate integrities in sympatric populations, in spite of the hybridization. The aforementioned species distinctions were contrasted with the cohesiveness of the types within the Pinus rigida-serotina complex, to explain further the decision by Smouse and Saylor (1973) to denote the components of the complex as subspecies: P. rigida subsp. rigida and P. rigida subsp. serotina. The subsection Australes Loud. of the genus Pinus L. consists of a geographically cohesive assemblage of species which inhabit the eastern United States and the Caribbean. Individual species extend into Canada (P. rigida Mill.) and into Central America (P. caribaea Morelet). The geographic ranges of various species within this group overlap considerably, and it is common to find two or more species in intimate contact. Natural hybridization is not uncommon in sympatric populations, and genetic exchange is sometimes extensive enough to create taxonomic problems. Natural hybridization between the Pinus rigida-serotina complex and P. taeda is one example. Hybridization between P. taeda L. and P. rigida Mill. subsp. rigida has been reported by Little et al. (1967), while similar hybridization between P. taeda and P. rigida subsp. serotina (Michx.) Clausen has been reported by Kang (1966)4. Preliminary field observations for the present study indicated the existence of considerable hybridization between P. taeda and the transitional type of the P. rigida-serotina complex. In addition, hybridization between P. echinata Mill. and P. rigida subsp. rigida has been reported (Austin, 1928). Because of these complex hybridization patterns and because of previous 1 Paper number 3679 of the Journal Series of the North Carolina State University Agricultural Experiment Station, Raleigh, North Carolina, U.S.A. This study was supported, in part, by McIntire-Stennis funds (Project 4016) and by the National Aeronautics and Space Administration and the National Science Foundation, which provided fellowship support for the senior author. 2 Department of Human Genetics, The University of Michigan Medical School, Ann Arbor, Michigan 48104. 3 School of Forest Resources, North Carolina State University, Raleigh, North Carolina 27607. 4 The classification used in this paper for members of the P. rigida-serotina complex follows that proposed by Smouse and Saylor (1973). ANN. MissouRi BOT. CAIRD. 60: 192-203. 1973. This content downloaded from 157.55.39.59 on Sat, 15 Oct 2016 04:34:14 UTC All use subject to http://about.jstor.org/terms 1973] SMOUSE & SAYLOR-HYBRIDIZATION IN THE PINUS RIGIDA-SEROTINA COMPLEX 193 22( Transition Zone i NEW 423 R a rnig/e of P rig/dc ERSEY ssp. ri~gida MARYLAND JRE 27 21 I~~DELAWARE Range of P rigida n s s p. seroline *20 2 -or ~ APLN AN.ESRMNTPOEUE JA nX 9