The chromosomes and relationships of Welwitschia mirabilis

Abstract

W. mirabilis is unique in the plant kingdom by having a very peculiar habit. Its karyotype is very asymmetrical. Two of the chromosomes in the complement are satellited and this coincides with 2 nucleoli in the metabolic nuclei. The longest pair in the complement is nearly 3.25 times longer than the shortest pair. Between the two extremes there is a gradual transition in size in the karyotype. Such a karyotype has no relation (quantitative or qualitative) with those in Ephedra and Gnetum. Arguments have been advanced against interrelating Ephedra and Welwitschia through hermaphrodite flowers. Such flowers appear to be due to the sex polymorphism ensuing as the result of tetraploidy (in originally diploid-dioecious species) and do not represent therefore the ancestral condition. Only after these abnormalities are discovered in diploids they will represent atavistic mutations and then alone phyletic reliance can be placed in them. In view of the strong morphological differences the few resemblances between the three genera are possibly due to parallel evolution, because many of these are also found in other gymnosperms. The best arrangement is to treat the three genera as belonging to three distinct orders under one class.