Tithonia (Asteraceae) in Nigeria
The distribution of Tithonia rotundifolia and T. disversifolia in Nigeria is circumscribed. While the former is an
annual widespread from the forest borders to the southern limits of Sahel Savana, the latter is perennial occurring only as small populations except around Jos metropolis where it tends to be invasive. T. rotundifolia is propagated by seeds while T. diversifolia reproduces exclusively vegetatively. The two taxa are morphologically distinct as they differ with respect to longevity, the size of their capitula and ray florets, the structures of their peduncles, and the shape of their phyllaries; the outermost series of phyllaries are also hairy in T. rotundifolia but glabrous in T. diversifolia. Tithonia rotundifolia is much more variable regarding branching patterns, ligule size and even ray floret colour; a red ray population is encountered around Jos almost as an endemic. This study confirms the chromosome number of n=17 in Tithonia but raises questions as to whether exclusion of polyploidy in the speciation of Tithonia is fully justified. There is no cytological evidence from microsporogenesis to implicate meiotic irregularities in the seed-sterility of T. Diversifolia.
Keywords: Tithonia, Annual, Perennial, Speciation