Geographic morphological and genetic variation in Xiphophora gladiata (Fucales, Phaeophyta)

Previous studies of two species of Northern Hemisphere algae (Ricus spp., Fucales, Phaeophyta), have shown that the geographical expression of their morphological variation takes the form of a fine-scale mosaic super-imposed on weak clines. Over 80% of the morphological variation (excluding between individuals differences) was expressed over a scale of less than 10 km. This work is here extended to the Tasmanian endemic alga Xiphophora gladiata (Fucales, Phaeophyta) and to still finer geographic scales (100's of kilometres to centimetres). Pattern analysis of the distribution of X. gladiata showed that, within its major beds, it grows in pronounced clumps (typically 10 cm in diameter and 15-25 cm apart), which formed the primary units for morphological analyses. 30 characters were measured on each of 2488 plants from-14 sites around Tasmania. Multivariate and univariate analyses of these data showed that the clumps differ markedly in morphology but that there are only limited differences over larger scales (metres to 100's of kilometres). Since the variation occurs over such fine scales, it is only weakly related to such environmental factors as exposure, in marked contrast to the pattern in Fltcus. In a search for the factors controlling morphological variation in X. gladiata, an improved technique for electrophoresis of brown algal enzymes was developed. An electrophoretic survey of 26 gene loci was performed on all of the individuals from four neighbouring clumps from one site. The clumps were found to differ markedly in allele frequency (Nei's Distance 0.12 to 0.36) and to have a significant excess of heterozygotes. These results are compared with patterns of differentiation observed in other algae and in higher plants. Mechanisms which could promote such a high degree of differentiation over such small geographic scales are discussed. Other possible influences on morphology that were examined included the response of X. gtadiata to wounding, seasonal variation in morphological characters and environmental correlates. Finally, the taxonomy of the genus Xiphophora was reviewed. Some implications of the morphological and genotypic patterns observed in X. gladiata for the speciation process in this genus are discussed.