Origin, Evolution, and Biogeography of Juglans: A Phylogenetic Perspective

Abstract

Phylogenetic analyses of extant species of Juglans (Juglandaceae) with the New and Old World distributions were performed using five cpDNA intergenic spacer (IGS) sequences (trnT-trnF, psbA-trnH, atpB-rbcL, trnV-16S rRNA, and trnS-trnfM) to elucidate the origin, diversification, historical biogeography, and evolutionary relationships within the genus. Overall, 3848 characters were sampled, yielding 105 (2.7%) variable sites, of which, 45 (1.16%) were potentially parsimony informative. In all, seventeen in-group (Juglans) and two out-group (Carya and Pterocarya) taxa were analyzed using the parsimony, likelihood, and minimum evolution methods, which produced concordant trees with three well supported clades corresponding to the sections Juglans, Cardiocrayon (butternuts), and Rhysocaryon (black walnuts). Juglans cinerea from the section Trachycaryon was placed within the black walnut clade, probably suggesting historical introgression of black walnut chloroplast during its evolution in North America. Black walnuts were not well resolved perhaps suggestive of their relatively recent origin and diversification or reticulate evolution within the section. Within butternuts, J. hopeiensis is moderately supported as a sister to J. ailantifolia, J. cathayensis, and J. mandshurica, which were well supported as a clade in all three analyses. The Persian walnut, J. regia and its sister taxon, J. sigillata formed a distinct clade sister to both butternuts and black walnuts. The cladogenesis within the genus Juglans appears to deviate from the evolutionary hypothesis based on the fossil history and biogeographic evidence. The results suggest that: (1) the IGS sequence divergence levels observed within and between different sections of Juglans are low; (2) basal placement of the section Juglans in all the three analyses suggests its ancient origin in contrast to fossil evidence, which suggests the earliest divergence of sections Rhysocaryon and Cardiocaryon; (3) the Persian walnuts and butternuts have evolved at different rates as compared to black walnuts; and finally (4) the extant taxa may not represent the entire evolutionary history of the genus.

Genetic variation in walnuts (Juglans regia and J. sigillata; Juglandaceae): Species distinctions, human impacts, and the conservation of agrobiodiversity in Yunnan, China

“…In the field, J. sigillata individuals are distinguished from J. regia by their relatively large compound leaves, rugose trunks, thick endocarp, and deeply pitted nut surfaces (Dode, 1909; Stone, 1993). Previous studies based on DNA sequence data from five chloroplast intergenic spacers demonstrated marginal differences between J. regia and J. sigillata (Aradhya et al, 2006). However, a recently published study of J. regia and J. sigillata from central and southwestern China (Wang et al, 2008), based on different microsatellite loci than those employed in this study, failed to distinguish J. regia from J. sigillata , leading Wang et al to suggest that J. regia and J. sigillata belong to the same species.…”

“…Juglans regia and J. sigillata were designated as the sole members of section Juglans (section Dioscaryon Dode); the other Asian Juglans species, J. ailantifolia and J. mandshurica , are classified within section Cardiocaryon . Subsequent molecular work based on chloroplast sequence data supported the sister‐group relationship of J. regia and J. sigillata (Aradhya et al, 2006). However, Wang et al (2008), using different microsatellite markers, were unable to distinguish between the two species in southwestern China, and they concluded that these two should be considered a single species.…”

Completion of the Chloroplast Genomes of Five Chinese Juglans and Their Contribution to Chloroplast Phylogeny

“…Aradhya et al (2007) used ITS, RFLP, and cpDNA sequence data to suggest J. regia and J. sigillata are distinct species. J. cathayensis and J. mandshurica were combined into one species in Flora of China (English version) (Lu et al, 1999), which does not consider J. hopeinesis (Kuang and Lu, 1979; Aradhya et al, 2004, 2007) a valid taxon. In addition, some previous phylogenetic studies of Juglans omitted J. hopeiensis and J. sigillata (Fjellstrom and Parfitt, 1995; Stanford et al, 2000; Aradhya et al, 2007).…”

Comparison of Different Processing Methods of Iron Walnut Oils (Juglans sigillata): Lipid Yield, Lipid Compositions, Minor Components, and Antioxidant Capacity

“…J. regia and J. sigillata are readily distinguishable from each other based on nut morphology, that J. sigillata has seal‐like depressions on the surface of the nuts, whereas J. regia has a wrinkled nut surface. However, at the species level, these two Juglans exhibit a closest relationship, they have similar morphologic and chloroplast sequences, and even indistinguishable microsatellite markers . Because of the shell of iron walnut hard to be separated, it is not suitable as a dietary nut.…”