Chapter 8. Ecology and Evolution

Parasitic Flowering Plants

Chapter 8. Ecology and evolution

Parasitic flowering plants have been able to get a not have been the same as that of, for example, the foothold in practically every type of plant com- mistletoes or Orobanchaceae. In fact it is agreed munity in all geographically areas of the world that angiosperm parasitism has evolved at least 8 where flowering plants occur. There are parasites or 9 times independently. Molecular analysis of from the northernmost arctic areas to the driest gene sequences even suggests that it may have regions of the deserts. The only exceptions seem happened as often as or 2 times. to be communities of floating and submerged Perhaps the simplest explanation to account for plants. This is understandable since competition the origin of parasitism points to the frequent oc- for water has been one of the main driving forces currence of root grafts in forest trees. This is of- in the evolution of land plants. In an aqueous en- ten evident in the survival and continued growth vironment, water availability is no problem for of a stump that draws materials from neighbour- plant growth, and there is no advantage in being ing, living trees via a root graft. However, this ex- a parasite. Competition for light is another im- planation loses its credibility because root grafts portant driving force, but only for photosynthetic tend to be between individuals of the same spe- plants. Competition for nutrients, pollinators etc. cies and are basically pressure-generated when can also drive evolution. Ultimately we end up two older roots press against each other. Root with highly specialized plants which may look grafts always concern older root areas far away very different from the starting point as, e.g., the from the subterminal locations where haustoria holoparasitic member of Balanophoraceae in Fig. develop in parasitic plants. 466. Needless to say a plant which, especially in A second possible explanation for the origin its early part of its life cycle, manages to draw on of root parasites is the participation of fungal another plant’s fully developed root system and mycorrhiza, somewhat similar to what is seen photosynthetic products has a very significant in saprophytes and myco-heterotrophic plants ecological edge. It is this advantage that has been (Box ) or in Parasitaxus usta (Figs. 16-19). exploited by parasitic plants. One may speculate that, in the course of evolu- The exact evolutionary path along which some tion, the fungus partner has gradually “dropped plants became parasitic remains a mystery, and out”, setting the stage for the evolution of an in- there are no completely convincing explanations timate, direct connection like the kind of root today. Since parasitism has evolved indepen- parasitism we see today. There are problems dently in a number of different places in the Plant with this theory also, however. Why should my- Kingdom, there is no reason to believe that there corrhiza almost completely drop out in parasites has been only one way to become a parasite. It when it is so benificial for other plants? It would only takes a look at Cuscuta and Cassytha to re- seem that saprophytic relationship is an evolu- alize that the origin of their parasitic habit could tionary path that is not related to parasitism. It is 385 Parasitic Flowering Plants

unfortunate that the early developmental stages of Parasitaxus are not known, for without such information the comparison with parasitic angiosperms is rather meaningless. What at pres- ent seems the most likely course of evolution is discussed in the following section. Evolution of parasitism in dicots

A It seems a fact that every time parasitic plants have evolved, the starting point has been a photosynthetic terrestrial plant. Apart from Cuscuta and Cassytha, as discussed in a later paragraph, establishment of root parasitism has been the first step. How early in evolutionary time did parasitism appear, and is there any relevant fossil evi- dence? Unfortunately the available information is very modest. As in true generally, many geo- logical and environmental conditions have to be fulfilled for plant material to be preserved as fossils. Preservation happens rarely, and therefore very few fossils of parasites are known. Fossil B pollen of loranths from the Oligocene was found in Puerto Rico. Besides this it is hard to find reli- able examples. Only a few decades ago, the origin of angiosperms was considered to be in the Cretaceous but it is now generally agreed that it was in the early Triassic or even earlier. So far it is safe to assume that parasitic flowering plants did appear fairly early in angiosperm evolution and probably more than 00 million years ago in the Cretaceous period. It is quite possible, however, that some are of more recent origin.

C Evolution of different modes of parasitism. As already noted the most ancestral parasitic Fig. 467. Olax scandens. A, 5-merous flowers. B, plants must have been hemiparasitic root para- Immature fruit with persistent calyx. C, Shrub-like sites, except in the case of Cuscuta and Cassytha. growth that may become scandent. Sandy seashore in A primitive version of such a parasite might have Pahang Province, Malaysia. - J Lai. developed under the following circumstances: 386