Biosystematics and Diversification in the Genus Striga Lour. (Scrophulariaceae) in Africa
Of the 34 taxa of Striga (29 species) that occur in Africa, 25 are endemic. Species diversity differs from region to region. The subtropical zone has 74% of the taxa including 41% endemics. Within this region, the West African part (from Senegal to eastern Nigeria) has 62% of the total taxa including 24% endemics. South of the equator the number of taxa is 56% of the total with 21% endemics, a proportion slightly less than that for West Africa. I recognize 29 species based on extensive herbarium and field studies. A new species from Angola is described. Striga yemenica previously considered an Arabian endemic, is reported from Ethiopia. Multivariate Statistics was used to compare the effects of the groups (species, geographical separation, host) on 13 vegetative and floral features. The affinities within the different species groups and in polymorphic species were determined based on a large number of randomly (using a random table) chosen plants that represent the range and the various hosts parasitized by a given species or host-specific strain. In general, the autogamous species which constitute the majority of the taxa, are more variable than the allogamous. Striga asiatica is divided into three species based on morphology, phenology, host range, and biogeography. The taxonomy adopted in the present study in general agrees with that of Hepper (1960) who recognized five subspecies of S. bilabiata. I recognize S. ledermannii as a fifth subspecies of S. bilabiata and not as a separate species as advocated by some workers, and propose S. linearifolia as the sixth subspecies of S. bilabiata. Special attention has been given to the problem of the development of host specific strains in S. gesnerioides, and S. hermonthica. While these strains may have distinct morphotypes, I accord them no taxonomic status. The Phylogenetic Analysis Using Parsimony (PAUP) and the Lundberg Rooting were adopted to infer the phylogeny of Striga. Thirty characters were polarized and ordered prior to data entry following the Outgroup Algorithms of Maddison et al. (1984). Results suggested that Striga favored the natural global changes in climatic conditions towards less rains, and the destruction and disturbance of the natural grassland savanna of Africa by man and his livestock. I favor the argument of allopatric over sympatric speciation for Striga. For the allogamous species, available evidences support the idea that the geographical separation is more important than host in contributing to population differentiation. In autogamous species, although the chances of self-pollination (intrinsic factor) are high, this alone cannot guarantee the reproduction isolation necessary for speciation.
Scrophulariaceae, Diversification, Africa, Agriculture, Ecology and Evolutionary Biology, Plant Pathology
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