title: ITS2 secondary structure for species circumscription: case study in southern African Strychnos L. (Loganiaceae).
abstract: Recently developed computational tools in ITS2 sequence-structure phylogenetics are improving tree robustness by exploitation of the added information content of the secondary structure. Despite this strength, however, their adoption for species-level clarifications in angiosperms has been slow. We investigate the utility of combining ITS2 sequence and secondary structure to separate species of southern African Strychnos, and assess correlation between compensatory base changes (CBCs) and currently recognised species boundaries. Combined phylogenetic analysis of sequence and secondary structure datasets performed better, in terms of robustness and species resolution, than analysis involving primary sequences only, achieving 100 and 88.2 % taxa discriminations respectively. Further, the Strychnos madagascariensis complex is well-resolved by sequence-structure phylogenetic analysis. The 17 Strychnos species corresponded to 14 ITS2 CBC clades. Four of the five taxa in section Densiflorae belong to a single CBC clade, whose members tend to form natural hybrids. Our finding supports the application of ITS2 as a complementary barcoding marker for species identification. It also highlights the potential of comparative studies of ITS2 CBC features among prospective parental pairs in breeding experiments as a rapid proxy for cross compatibility assessment. This could save valuable time in crop improvement. Patterns of CBC evolution and species boundaries in Strychnos suggests a positive correlation. We conclude that the CBC pattern coupled with observed ITS2 sequence paraphyly in section Densiflorae points to a speciation work-in-progress.
authors.affiliation: School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits, Johannesburg, 2050, South Africa. Strychnos009@gmail.com.
authors.forename: Adekunle
authors.identifier: http://orcid.org/0000-0003-4007-6766
authors.initials: A
authors.lastname: Adebowale
authors.affiliation: School of Life Sciences, University of KwaZulu-Natal, New Biology Building, South Ring Road, Westville, 3630, South Africa.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Lamb
authors.affiliation: School of Life Sciences, University of KwaZulu-Natal, New Biology Building, South Ring Road, Westville, 3630, South Africa.
authors.forename: Ashley
authors.initials: A
authors.lastname: Nicholas
authors.affiliation: School of Life Sciences, University of KwaZulu-Natal, New Biology Building, South Ring Road, Westville, 3630, South Africa.
authors.forename: Yougasphree
authors.initials: Y
authors.lastname: Naidoo
mesh_terms: D001483:Base Sequence; D021901:DNA, Intergenic / Q000235:genetics*; D019143:Evolution, Molecular; D010802:Phylogeny; D031451:Strychnos / Q000145:classification* / Q000235:genetics*
chemical_list: D021901:DNA, Intergenic
keywords: Biological species concept; Compensatory base changes; Cross compatibility; DNA barcoding; ITS2 secondary structure