Endemic North African Quercus afares Pomel originates from hybridisation between two genetically very distant oak species (Q. suber L. and Q. canariensis Willd.): evidence from nuclear and cytoplasmic markers

Abstract

Hybridisation is a potent force in plant evolution, although there are few reported examples of stabilised species that have been created through homoploid hybridisation. We focus here on Quercus afares, an endemic North African species that combines morphological, physiological and ecological traits of both Q. suber and Q. canariensis, two phylogenetically distant species. These two species are sympatric with Q. afares over most of its distribution. We studied two Q. afares populations (one from Algeria and one from Tunisia), as well as several populations of both Q. suber and Q. canariensis sampled both within and outside areas where these species overlap with Q. afares. A genetic analysis was conducted using both nuclear (allozymes) and chloroplastic markers, which shows that Q. afares originates from a Q. suber  Q. canariensis hybridisation. At most loci, Q. afares predominantly possesses alleles from Q. suber, suggesting that the initial cross between Q. suber and Q. canariensis was followed by backcrossing with Q. suber. Other hypotheses that can account for this result, including genetic drift, gene silencing, gene conversion and selection, are discussed. A single Q. suber chlorotype was detected, and all Q. afares individuals displayed this chlorotype, indicating that Q. suber was the maternal parent. Q. afares is genetically, morphologically and ecologically differentiated from its parental species, and can therefore be considered as a stabilised hybrid species.

Molecular phylogeny and genetic diversity of Tunisian Quercus species using chloroplast DNA CAPS markers

“…This is consistent with the fact that the range of both Q. suber and Q. canariensis includes that of Q. afares. Our results are in agreement with those of Mir et al (2006) using allozymes and chloroplast markers.…”

Evolution of rDNA FISH patterns in the Fagaceae

“…This pattern was also found in other European and North American Quercus species (Zoldos et al 1999), indicating a possible stabilization of their karyotypes despite the high level of radiation present in this genus, and even considering the refuge geographic area of the Iberian Peninsula. This high conservation of karyoptypes could account for the vast number of fertile hybrids occurring naturally between Quercus species (Petit et al 2004), which in turn has been proposed to contribute to the evolutionary processes in oaks, as in Quercus afares (Mir et al 2006). Besides interspecific hybridization, polyploidization has also been detected previously in some Quercus species.…”

Diverse chromosome complements in the functional gametes of interspecific hybrids of MT- and A-karyotype Lycoris spp.

“…5b, d, 7d in this study), which suggests a common occurrence of chromosome morphologic changes in interspecific hybrids. Rapid karyotype changes invoked by interspecific hybridization may facilitate the development of reproductive isolation between a hybrid lineage and parents, and then promote the formation of a new species (hybrid speciation) (Rieseberg 1997;Gross and Rieseberg 2005;Howarth and Baum 2005;Mir et al 2006).…”