Phylogeny of Salix subgenus Salix s.l. (Salicaceae): delimitation, biogeography, and reticulate evolution

Abstract

BackgroundThe taxonomy and systematics of Salix subgenus Salix s.l. is difficult. The reliability and evolutionary implications of two important morphological characters (number of stamens, and morphology of bud scales) used in subgeneric classification within Salix remain untested, and a disjunct Old–New World distribution pattern of a main clade of subgenus Salix s.l., revealed by a previous study, lacks a reasonable explanation. To study these questions, we conducted phylogenetic analyses based on 4,688 bp of sequence data from four plastid (rbcL, trnD–T, matK, and atpB–rbcL) and two nuclear markers (ETS and ITS) covering all subgenera of Salix, and all sections of subgenus Salix s.l.ResultsSubgenus Salix came out as para- or polyphyletic in both nrDNA and plastid trees. The plastid phylogeny successfully resolved relationships among the major clades of Salix, but resolution within subgenus Salix s.l. remained low. Nevertheless, three monophyletic groups were identifiable in subgenus Salix s.l.: the ‘main clade’ of subgenus Salix s.l., with New and Old World species being reciprocally monophyletic; the section Triandroides clade; and the subgenus Pleuradenia clade. While nrDNA regions showed higher resolution within subgenus Salix s.l., they failed to resolve subgeneric relationships. Extensive, statistically significant gene-tree incongruence was detected across nrDNA–plastid as well as nrDNA ETS–ITS phylogenies, suggesting reticulate evolution or hybridization within the group. The results were supported by network analyses. Ancestral-state reconstructions indicated that multiple stamens and free bud scales represent the plesiomorphic states within Salix, and that several significant shifts in stamen number and bud scale morphology have occurred.ConclusionsSubgenus Salix s.l. is not monophyletic, and the evolutionary history of the subgenus has involved multiple reticulation events that may mainly be due to hybridization. The delimitation of subgenus Salix s.l. should be redefined by excluding section Triandrae and subgenus Pleuradenia from it. The evolutionary lability of bud-scale morphology and stamen number means that these characters are unreliable bases for classification. The disjunct Old–New World distribution of subgenus Salix s.l. appears to be linked to the profound climatic cooling during the Tertiary, which cut off gene exchange between New and Old World lineages.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0311-7) contains supplementary material, which is available to authorized users.

Bringing Elton and Grinnell together: a quantitative framework to represent the biogeography of ecological interaction networks

“…In the left panel, open circles represent the absence of both species, whereas closed circles represent co-occurrence and plus signs the occurrence of only one of the two species. These observations within Europe are also matched by the ones found at a global scale for Salix (Argus 1997, Cronk et al 2015, Wu et al 2015 and sawflies (Kouki et al 1994, Kouki 1999. Figure 4.…”

Conflicting phylogenomic signals reveal a pattern of reticulate evolution in a recent high‐Andean diversification (Asteraceae: Astereae: Diplostephium)

“…Future research should focus on understanding the role of hybridization in diversification and on developing methods to quantify ancient introgression and accurately infer reticulate networks. The evidence presented in our study builds on numerous reports (Sessa et al ., ; Sochor et al ., ; Sun et al ., ; Wu et al ., ) that emphasize a central role of reticulation in plant evolution.…”

Sex‐biased gene expression in flowers, but not leaves, reveals secondary sexual dimorphism in Populus balsamifera

“…that the pattern of sex‐biased expression in flowers seems to be largely uncorrelated with the expression patterns in leaf tissues, potentially because of differences in the regulation of common gene networks, or sex‐ and tissue‐specific selection on energetic demands. The Salicaceae are a large family with primarily dioecious members (Cronk et al ., ), and the two most speciose genera, Populus and Salix , diverged over 45 Ma in the early Eocene (Wu et al ., ); thus, sexual specialization has had a long time to evolve and mature. As patterns of sexual dimorphism and sex‐biased expression are explored in a more diverse array of plants, we will be able to discern whether the patterns of sexual dimorphism in gene expression found in P. balsamifera are common among groups with ancient dioecy, or are related to other aspects of their biology or ecology, such as the relative investment by males and females in reproduction or their long lifespan.…”

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