A SYSTEMATIC AND EVOLUTIONARY STUDY OF SELECTED TAXA IN THE GENUS ARCEUTHOBIUM (VISCACEAE) (ISOZYME, ENDOSPERM, DWARF-MISTLETOE)
Abstract (summary)
A systematic and evolutionary study of selected taxa in the genus Arceuthobium (Viscaceae) was conducted owing to the high degree of morphological reduction. The taxonomy of these parasites has been difficult because of the reduction in the number of characters available for analysis. For this reason an electrophoretic study was begun to contribute additional independent evidence on genetic relationships in view of constructing an evolutionarily meaningful phylogeny.
Triploid seed endosperm tissue was used for enzyme extraction and interpretations of the resulting gel banding patterns were made. The types and frequencies of electromorphs were compiled for 19 Arceuthobium taxa from 40 populations and then subjected to cluster analysis. Some similarities and differences emerged compared to the most recent monographic treatment of this group. A major difference involves two taxa, A. douglasii and A. pusillum, which do not appear to reside within one section (Minuta) as previously circumscribed. Another taxon, A. divaricatum, thought to reside within the largest group of the genus (Section Campylopoda), appears to have greater genetic affinity with A. douglasii. The above examples provide preliminary evidence of convergent evolution with respect to morphological features. For this reason caution should be exercised when proposing phylogenies based on morphology in such parasitic groups.
The Campylopodum grouping of taxa (here comprising 11 members), were linked at similarity values of 70-80% or more, values generally greater than the level at which other clearly defined species of the genus were joined. It is proposed that either (1) the Campylopodum complex does not contain distinct, reproductively isolated species, or (2) the complex is composed of closely related (sibling) species, possibly undergoing more rapid (and recent) radiation than other dwarf mistletoe species such as A. douglasii, A. pusillum, or A. americanum.