Phylogeny based on morphological traits

The first cladistic analysis of the genus Quercus relying on morphological characters is by Nixon (1993a) and is based on the evolutionary variation of the pistillate flower and fruit subtending the cupule (Kaul and Abbe 1984). Phylogenetic relationships based on morphology could be proposed once polarization of flower and fruit traits were inferred from the existing fossil remains (Nixon 1989, 1993a). Morphological synapomorphies of the genus Quercus include a single pistillate flower, valveless cupules, decurrent styles, expanded stigmatic surfaces, unisexual inflorescences, lax staminate inflorescences and scabrate pollen exine structure (Manos et al., 2001). Two subgenera were recognised by Nixon (1993a): Cyclobalanopsis (cycle-cup oaks, only present in Asia) and Quercus (oaks, widespread over the 3 continents).

Subgenus Quercus can be distinguished from sub genus Cyclobalanopsis by the presence of expanded stigmatic surfaces on the pistillate flowers and small inconspicuous bracts which subtend single-staminate flowers.

Within subgenus Quercus, Nixon recognized three clades:

  • Section Lobatae (= Erythrobalanus, sensu Camus, red and black oaks, present only in North and Central America) showing most of the plesiomorphic morphological traits within Subgenus Quercus. This section is defined by a derived pistillate floral morphology (a calix “flange”) that is absent in the rest of Quercus (Nixon, 1993b).
  • Section Protobalanus (= Protobalanus, sensu Camus, intermediate or golden cup oaks, present only in North America), with abortive ovules apical to lateral.
  • Section Quercus (= Lepidobalanus, sensu Camus, white oaks, present in the three continents), with abortive ovules always basal. Because of the synapomorphy of the basal position of the abortive ovules on the surface of the development seed, this section includes also two groups that were identified in previous classifications and comprise many plesiomorphic characters: the Cerris and Ilex group.

More recently pollen features as patterns of tectum ornementation assessed by electronic microscopy allowed to resolve morphological phylogenetic relationships within section Quercus (Denk and Grimm, 2009). Tectum ornamentation can be used to polarize character states within Quercus by comparison with other genera in the Fagaceae. Species of the ilex group exhibit distinct simple type tectum ornamentation also found in Fagus and in extinct lineages of Trigonobalanus, Colombobalanus and Formamendron.

Phylogeny based on molecular data

Phylogenetic reconstructions were further enriched with molecular data coming from various sources:

  • chloroplast DNA restriction analysis: Manos et al. (1999)
  • chloroplast DNA sequence data (rbcl, atpBE intergenic spacer;3’ trnK intron): Manos and Stanford (2001)
  • chloroplast DNA sequence data (intergenic spacer trnD-trnT): Xu (2004)
  • ribosomal sequences (ITS region): Samuel et al. (1998), Manos et al, (1999), Manos et al. (2001)
  • single copy nuclear gene CRABS CLAW (CRC), that regulates carpel development in angiosperms: (Oh and Manos, 2008)

To date, phylogenetic reconstructions based on DNA sequences in oaks are poorly resolutive, and result in polytomic trees at the sectional level. This was the case when cpDNA sequences were used, where a star like phylogenetic tree was obtained comprising 5 sister groups that correspond to 4 sections described by Camus (Manos et al., 1999): Erythrobalanus (red oaks), Lepidobalanus (white oaks), Protobalanus (intermediate groups), Cerris (cerris oaks), Lepidobalanus-Subsection Virentes, Cerris-Subsection Cocciferae (= ilex group).

Analysis of the combined cpDNA and ITS data were more resolutive and ended into four monophyletic groups (sensu Camus): (Cerris-(Erythrobalanus-Protobalanus+Lepidobalanus); The same resolution was obtained within cpDNA sequence diversity within the interspacer trnD-trnT (Xu, 2004), although this analysis additionally separated sub section Brachylepides at a higher hierarchical level: (Sub section Brachylepides–(Cerris (Erythrobalanus + Protobalanus + Lepidobalanus)))

The most complete analysis based the nuclear sequences (CRC and ITS) resolves two clades with strong support:

  • the Eurasian Cerris clade, that is actually closely related to the subgenus Cyclobalanopsis.
  • The mostly new world clade composed of Erythrobalanus-Protobalanus-Lepidobalanus.

Although poorly resolutive, the different phylogenetic reconstructions show congruent results and challenge the traditional taxonomic classification. They suggest that the following amendments should be made in current taxonomical classifications.

Among the 6 botanical sections identified by Camus (Erythrobalanus, Lepidobalanus, Protobalanus, Cerris, Mesobalanus, and Macrobalanus), only the four first are distinguished as monophyletic groups in the phylogenetic reconstruction. Species assigned to Mesobalanus, and Macrobalanus by Camus are members of the Lepidobalanus group (Xu, 2004). Two new sections should however be added based on the phylogenetic analysis: the Ilex group, that is part of the Lepidobalanus section in Camus’ classification, and the Brachylepides subsection (sensu Camus). Species of this subsection are located on high altitudes on the eastern side of the Himalayas.

Concerning Nixon’s classification (1993a), the three groups  (Quercus, Cerris and Ilex) that were assembled within section Quercus appear as three different clades in the phylogenetic reconstructions and should be considered as three separate sections.


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