Reproductive biology and variation of nuclear ribosomal ITS and ETS sequences in the Calligonum mongolicum complex (Polygonaceae)

Abstract

To explore the biosystematics of the Calligonum mongolicum complex (Polygonaceae), the flowering phenological period, breeding and pollination characters and seed set of the complex (Calligonum Mongolicum Turze, Calligonum chinense A. Los., Calligonum gobicum A. Los., Calligonum pumilum A. Los. and Calligonum zaidamense A. Los.) were documented in the Turpan Eremophyte Botanical Garden, China. The sequences of the nuclear ribosomal ITS and ETS region were employed to differentiate the Calligonum mongolicum complex and other species in sect. Medusae. The results showed species of the Calligonum mongolicum complex occupied overlapping flowering periods and had consistent pollination agents. Their breeding systems are all self-compatible, tend to be out-crossing and they interbreed amongst each other (out-crossing index, OCI = 4).The crosses within and amongst species had high seed sets (44 - 65%). Phylogenetic analyses of Calligonum sect. Medusae and the network analysis of nrDNA (ITS and ETS) in the complex suggest interbreeding amongst “species” within the complex and provide evidence for taxonomically merging the five species in the complex. The detected hybridisation, occurring within the complex, suggests the need to improve traditional methods of ex situ plant conservation in botanical gardens for maintaining genetic diversity of Calligonum within and amongst species from different geographic areas.

Complete plastome sequencing resolves taxonomic relationships among species of Calligonum L. (Polygonaceae) in China

“…The Calligonum plastomes were highly conserved in organization and structure. They showed a typical quadripartite genome organization, including a LSC (Large Single Copy) region (86,766-88,160 bp) and a SSC (Small Single Copy) region (13,416 bp), which were separated by two IR (Inverted Repeat) regions (30,468-30,552 bp) ( Table 1, Fig. 1).…”

“…Although the Calligonum plastome showed relatively high species resolution in this study, approximately 20% of the species could not be successfully identified. Calligonum species are known to interbreed in sympatry [13,34,36], and it seems likely that in particular, interspecific hybridization may have caused the lack of resolution for C. ebinuricum and C. rubicundum. For example, three C. ebinuricum accessions formed one clade with C. leucocladum (BS = 100%, PP = 1.00), however, C. ebinuricum alone formed a monophyletic clade, with strong support, in the nrITS phylogeny.…”

“…These results verified those of previous studies that also showed relatively low resolution rates [12,36], even though we were able to sequence and screen longer segments (i.e., the complete gene region) of the standard DNA barcodes. There are three possible reasons for the high rates of species identification failure for these DNA barcodes in Calligonum: 1) the current taxonomy for the genus is inaccurate; 2) past hybridization events have blurred species boundaries; and 3) recent speciation events have resulted in coalescent failure of the plastid genome [13,34,38,40]. Although the number of recognized species for Calligonum varies among monographs [3,5], the genus is thought to have undergone recent and rapid diversification in the arid deserts of Western Central Asia [41,42], which may contribute the failure of DNA barcoding to discriminate among Calligonum species.…”

“…As a result, species resolution of nrITS (11%) was even less than the three plastid standard barcodes combined. This result may be due to the young age of this genus [41,42], frequent hybridization [13] and/or introgression, where most hybridization events in Calligonum have been documented between relatively young species that have diverged since the Quaternary [5]. For example, experimental interspecific hybridization among predominantly self-incompatible taxa from sect.…”

“…Although gene regions of the plastid genome (matK, rbcL, trnH-psbA), as well as the nuclear ribosomal internal transcribed spacer (nrITS) region, have been widely used as standard DNA barcodes for species identification in general [7][8][9], DNA barcoding analyses based on these standard regions, as well as other plastid DNA sequences (atpB-rbcL, trnL-trnF, psbK-psbI) fail to discriminate Calligonum species [10][11][12]. Furthermore, recent molecular sequence analysis [13] has treated five species (C. mongolicum, C. pumilum, C. chinense, C. alashanicum, and C. zaidamense) as a complex group, C. mongolicum. Given such discrepancies, more discerning genetic markers for the genus Calligonum are required to solve taxonomic confusion within the group.…”