Abstract
Populus, a core genus of Salicaceae, plays a significant ecological role as a source of pioneer species in boreal forests. However, interspecific hybridization and high levels of morphological variation among poplars have resulted in great difficulty in classifying species for systematic and comparative evolutionary studies. Here, we present phylogenetic analyses of 24 newly sequenced Populus plastomes and 36 plastomes from GenBank, which represent seven genera of Salicaceae, in combination with a matrix of eighteen morphological characters of 40 Populus taxa to reconstruct highly supported relationships of genus Populus. Relationships among the 60 taxa of Salicaceae strongly supported two monophyletic genera: Populus and Salix. Chosenia was nested within the genus Salix, and five clades within Populus were divided. Clade I included the three taxa P. euphratica, P. pruinosa, and P. ilicifolia. Clade II contained thirteen taxa [P. adenopoda, P. alba, P. bolleana, P. davidiana, P. hopeiensis, P. nigra, P. qiongdaoensis, P. rotundifolia, P. rotundifolia var. duclouxiana, P. tremula, P. tremula × alba, P. tomentosa, and P. tomentosa (NC)]. Clade III included the ten taxa P. haoana, P. kangdingensis, P. lasiocarpa, P. pseudoglauca, P. qamdoensis, P. schneideri, P. simonii, P. szechuanica, P. szechuanica var. tibetica, and P. yunnanensis. Clade IV included P. cathayana, P. gonggaensis, P. koreana, P. laurifolia, P. trinervis, P. wilsonii, and P. xiangchengensis. The last clade comprised P. angustifolia, P. balsamifera, P. deltoides, P. deltoides × nigra, P. fremontii, P. mexicana, and P. trichocarpa. This phylogeny is also supported by morphological traits, including bark smoothness, bud size, petiole shape, leaf inflorescence, male anther length and male anther tip.
Highlights
The family Salicaceae is primarily distributed in cold, tropical and warm temperate regions worldwide and is a dioecious woody and shrub plant throughout the northern hemisphere (Leskinen and Alström-Rapaport, 1999; Wang et al, 2014)
Because of interspecific hybridization and high levels of morphological variation among poplars, the number of Populus species currently described in the literature ranges from 22 to 85, and hundreds of Populus hybrids and cultivars exist (Eckenwalder, 1977a,b, 1996; Dickmann and Stuart, 1983; Wang and Gilbert, 2007; Wan et al, 2013)
The results showed that Populus is a monophyletic group and can be divided into two main clades: section Leuce and the remaining sections
Summary
The family Salicaceae is primarily distributed in cold, tropical and warm temperate regions worldwide and is a dioecious woody and shrub plant throughout the northern hemisphere (Leskinen and Alström-Rapaport, 1999; Wang et al, 2014). The species of the genus Populus, commonly known as poplars, aspen and cottonwood, are widely distributed throughout the northern hemisphere from subtropical to boreal forests (Brawdshaw et al, 2000), and China is one of the most important poplar distribution areas (Wan et al, 2013). Eckenwalder (1996) classified the genus Populus into 29 species grouped into six sections based on important morphological characteristics: persistence of the floral disk, presence or absence of marked foliar heteroblasty, overall leaf shape, distribution and shape of foliar teeth, number of carpels and flattened vs round petiole. Discrepancies in the total number of Populus species could be attributed to the misinterpretation of some hybrids and difficulties involved in delineating species boundaries (Hamzeh and Dayanandan, 2004)
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