Hybridisation and chloroplast DNA variation in a Pinus species complex from Asia

Xiao-Ru Wang and Alfred E. Szmidt

Evolution (1994) 48: 1020-1031.

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Heterologous hybridization of chloroplast (=cp) DNA involving 30 endonuclease-probe combinations was used to analyze cpDNA variation in multiple individuals and populations of Pinus tabulaeformis (Carr.), P. yunnanensis (Franchèt) and P. massoniana (Lamb.). Restriction fragment patterns detected by several combinations distinguished among the three species. The obtained cpDNA markers were subsequently used to examine cpDNA variation of P. densata (Masters) Fig. 1), a putative Tertiary hybrid between P. tabulaeformis and P. yunnanensis. The analysis demonstrated that P. densata populations harbor three different haplotypes. Two of these haplotypes are characteristic of P. tabulaeformis and P. yunnanensis. However, the third haplotype found in P. densata appears to be absent in other extant Asian Pinus species. It is suggested that the observed cpDNA composition of P. densata populations is a result of past hybridization involving P. tabulaeformis, P. yunnanensis and a third unknown or extinct taxon. CpDNA polymorphism in P. densata was much greater than that for nuclear allozyme markers in this and the other Pinus species. Population differentiation was also substantial in P. densata and exceeded that for allozyme markers. In contrast, no cpDNA polymorphism was detected in populations of P. tabulaeformis, P. yunnanensis and P. massoniana. The study suggests that interspecific gene exchange may lead to the creation of stable cpDNA polymorphism in conifer hybrids.

Key words: Pinus, species hybridization, cpDNA variation, evolution

by Alfred E. Szmidt