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Backgrounds Populus and Salix belong to Salicaceae and are used as models to investigate woody plant physiology. The variation of karyotype and nuclear DNA content can partly reflect the evolutionary history of the whole genome, and can provide critical information for understanding, predicting, and potentially ameliorating the woody plant traits. Therefore, it is essential to study the chromosome number (CN) and genome size in detail to provide information for revealing the evolutionary process of Salicaceae. Results In this study, we report the somatic CNs of seventeen species from eight genera in Salicaceae. Of these, CNs for twelve species and for five genera are reported for the first time. Among the three subfamilies of Salicaceae, the available data indicate CN in Samydoideae is n = 21, 22, 42. The only two genera, Dianyuea and Scyphostegia, in Scyphostegioideae respectively have n = 9 and 18. In Salicoideae, Populus, Salix and five genera closely related to them (Bennettiodendron, Idesia, Carrierea, Poliothyrsis, Itoa) are based on relatively high CNs from n = 19, 20, 21, 22 to n = 95 in Salix. However, the other genera of Salicoideae are mainly based on relatively low CNs of n = 9, 10, 11. The genome sizes of 35 taxa belonging to 14 genera of Salicaceae were estimated. Of these, the genome sizes of 12 genera and all taxa except Populus euphratica are first reported. Except for Dianyuea, Idesia and Bennettiodendron, all examined species have relatively small genome sizes of less than 1 pg, although polyploidization exists. Conclusions The variation of CN and genome size across Salicaceae indicates frequent ploidy changes and a widespread sharing of the salicoid whole genome duplication (WGD) by the relatives of Populus and Salix. The shrinkage of genome size after WGD indicates massive loss of genomic components. The phylogenetic asymmetry in clade of Populus, Salix, and their close relatives suggests that there is a lag-time for the subsequent radiations after the salicoid WGD event. Our results provide useful data for studying the evolutionary events of Salicaceae.

Although extensive studies have demonstrated that several genera formerly placed in the family Flacourtiaceae are the closest relatives of Populus-Salix. However, the closest relative or relatives remain uncertain due to the relatively low phylogenetic resolution and insufficient phylogenetic information. In addition, the question as to whether Populus and Salix are truly monophyletic remains open because of the absence of comprehensive studies with large-scale samples of both genera. In the present study, 54 species representing 11 genera of Salicaceae sensu lato (now including the genera of Flacourtiaceae) were studied using a combined rbcL and matK gene sequence dataset with a total length of 2134 bp, to explore the closest genera that are relatives of Populus-Salix and to determine the relationships between Populus and Salix. The resulting phylogenetic trees showed high resolution in each of the main clades. The results confirmed that (1) Idesia and Bennettiodendron are the closest relatives to Populus-Salix, followed by Poliothyrsis, and (2) Populus and Salix are a truly monophyletic lineage in which the most recent ancestor is shared by the two genera. This finding is significant for a wide variety of evolutionary studies and for understanding the classification and biosystematics of Salicaceae.

Bennettiodendron brevipes, which is only distributed in China, is a genus of the family Salicaceae. In this study, we assembled the complete chloroplast genome of B. brevipes using Illumina paired-...

International audience; The circumscription of Salicaceae has recently been enlarged to include a majority of the species formerly placed in the polyphyletic tropical family Flacourtiaceae. Several studies have reported a peculiar and infrequently formed multilayered structure of tension wood in four of the tropical genera. Tension wood is a tissue produced by trees to restore their vertical orientation, and most studies have focused on trees developing tension wood by means of cellulose-rich, gelatinous fibres, as is known in Populus L. and Salix L. (Salicaceae s.s.). This study aims to determine if the multilayered structure of tension wood is an anatomical characteristic common in other Salicaceae, and if so, how its distribution correlates to phylogenetic relationships. Therefore, we studied the tension wood of 14 genera of Salicaceae as well as two genera of Achariaceae, one genus of Goupiaceae, and one genus of Lacistemataceae, families closely related to Salicaceae or formerly placed in Flacourtiaceae. Opposite wood and tension wood were compared with light microscopy and 3D Laser Scanning Confocal Microscopy. The results indicate that a multilayered structure of tension wood is common in the family except in Salix, Populus, and one of their closest relatives, Idesia polycarpa Maxim. We suggest that tension wood may be a useful anatomical character in understanding phylogenetic relationships in Salicaceae. Further investigation is still needed on the tension wood of several other putatively close relatives of Salix and Populus, in particular Bennettiodendron Merr., Macrohasseltia L.O.Williams and Itoa Hemsl.

Poplars (Populus sp.) and willows (Salix sp.) are well known woody plants common throughout the northern hemisphere, both with fully sequenced genomes. They bear compact unisexual inflorescences known as “catkins.” Closely related genera of the “salicoid clade” within the family Salicaceae include the Asian genera Bennettiodendron, Idesia, Itoa, Poliothyrsis, and Carrierea and the Central American genera Olmediella and Macrohasseltia. Like willow and poplar, most of these genera are dioecious, but unlike willow and poplar they generally have loosely branched panicles rather than catkins, and less highly reduced flowers. However, the early developing inflorescences of Carrierea and Idesia show similarities to catkins which suggest possible pathways by which the salicoid catkin may have evolved.