Systematics of Alectra (Orobanchaceae) and phylogenetic relationships among the tropical clade of Orobanchaceae.

摘要

Parasitic plants are either hemiparasitic or holoparasitic, depending upon the degree of dependence on their host plant for nutrition, and their level of photosynthetic ability. Alectra (Orobanchaceae) consists of primarily hemiparasitic herbaceous species distributed mainly in sub-Saharan Africa, with two species native to tropical America, and two widespread species extending out of Africa into India and China. Despite containing an economically important noxious agricultural weed, Alectra has never been the subject of a phylogenetic analysis. The monophyly of Alectra was assessed using DNA sequences from the nuclear (ITS) and chloroplast (rpl16, trnT-L) genomes. One of two holoparasitic species, A. alba, was placed outside of Alectra. Due to lack of adequate outgroup sampling, no robust hypothesis of relationships could be established for this species; Alectra alba was supported as either the first branch within Harveya, or as sister to Aeginetia + Christisonia , depending on the dataset being analyzed. Similarly, Alectra fruticosa was shown to either be the first branch within Alectra, or sister to the African species of Melasma. The monophyly of the remaining species of Alectra was highly supported in all analyses.;A larger scale phylogenetic analysis was undertaken to examine the placement of Alectra alba within the framework of the tropical clade of Orobanchaceae, to which Alectra belongs. Previous studies did not agree on the branching order of major lineages within the tropical clade, nor the nearest relatives, so outgroup sampling extended as far as Lindenbergia, the earliest diverging lineage within Orobanchaceae (15 outgroup taxa were included representing 12 genera), and ingroup sampling focused on including as many taxa as was feasible (68 taxa in 16 genera). The analyses were based on the same three gene regions utilized for the Alectra phylogeny, and a robust hypothesis of relationships for the taxa within the tropical clade was obtained. The woody genera Asepalum and Cyclocheilon, included here for the first time, were shown to comprise the earliest diverging lineage within the tropical clade. Alectra alba was strongly supported within Harveya in all analyses, and Alectra fruticosa was supported as the earliest diverging lineage within Alectra in the combined analysis, albeit with weak support. All of the holoparasitic genera were shown to comprise a single lineage, although two genera were only represented by single species (Aeginetia, Christisonia), thus their relationship to the other holoparasitic genera (Harveya, Hyobanche) may change once more taxa are included. The hemiparasitic Melasma was shown to comprise two lineages, one African and one New World, and the New World Melasma lineage may be more closely related to Escobedia than to the African Melasma lineage, but increased sampling of Escobedia will be necessary before any taxonomic changes are undertaken.;The widespread and poorly understood Alectra sessiliflora complex consists of three varieties, and extends from the entirety of sub-Saharan Africa through the Middle East and India into China and the Philippines. Previous treatments of the complex in regional floras failed to unequivocally define the three varieties, and relied on stamen filament and calyx pubescence and relative leaf size and shape to distinguish the varieties. As continuous variation in these and other characters was observed from specimens across the range, a phenetic study was undertaken to see if specimens grouped into identifiable varieties. Principal Coordinates Analysis and the Unweighted Pair-Group Method with Arithmetic Averages were both used to explore the data, and in both analyses no groupings were recovered, indicating that the currently accepted varieties are taxonomically meaningless. Instead a concept of A. sessiliflora without any infraspecific divisions is supported.;Alectra has not been treated comprehensively in over 60 years, and all species descriptions and taxonomic work since that time have been restricted to regional floras. Even in the last monograph species delimitations were often vague, and with more species names being published with only regionally restricted material available, many species were described multiple times, and a new monographic treatment became necessary. Seventy-seven names have been effectively published under Alectra, and after reevaluating species delimitations using loaned herbarium material and field collections, 11 species are currently recognized. A diagnostic key, species descriptions and distribution maps are presented along with discussion of taxonomic issues.