With the exception of only very few genes, the above-described set of plastome-encoded genes (Table 1) is highly conserved among angiosperm plant species. There is, however, one group of angiosperms whose plastid genomes differ radically in gene content: parasitic plants. A limited number of plant species grows heterotrophically by exploiting green plants as carbon source. Many of these holo-parasites have lost the capacity to carry out photosynthesis and also lack photosyn-thetic pigments. The ability to obtain sugars from a host plant releases the selective pressure on the maintenance of photosynthesis-related genes. Consequently, the plastomes of such parasites suffer dramatic size reductions, mainly caused by the loss of photosynthesis genes or their degeneration to pseudogenes (dePam-philis and Palmer 1990; Wimpee et al. 1991; Bungard 2004). For example, the plastome of the root holoparasite Epifagus virginiana (an Orobanchaceae species) is less than half the size of that in photoautotrophic angiosperms (dePamphilis and Palmer 1990; Wolfe et al. 1992). It contains only 21 intact protein-coding genes, 18 of which belong to the genetic system genes and the remaining three falling into the category 'other genes' (accD, ycfl, and ycf2; see 3.2.3 and Table 1). Remarkably, also some genetic system genes have been lost or degraded to pseu-dogenes (Morden et al. 1991; Wolfe et al. 1992). It is currently unclear, whether or not these missing genes have been substituted by functional nuclear gene copies the protein products of which are imported into plastids. Nonetheless, plastid genes in Epifagus are actively transcribed and their mRNAs are faithfully processed by intron splicing and RNA editing suggesting that the vestigial plastid genome is indeed expressed (Ems et al. 1995). However, whether or not also the translational apparatus in these plastids has remained functional, is not yet clear.
Parasitism in seed plants has evolved several times independently (presumably at least ten times; Bungard 2004). Interestingly, not all parasitic plants grow exclusively heterotrophically. A number of parasitic species have retained at least some photosynthetic activity and thus, strictly speaking, grow mixotrophically. They fix a limited amount of carbon by themselves through photosynthesis, while obtaining the bulk of carbon as sugars from their host plant. Such species are believed to represent evolutionary intermediates that are in the process of losing their ability to photosynthesize. The genus Cuscuta (dodders) provides a particularly striking example for this evolutionary transition. Its more than 150 species vary greatly in their residual photosynthetic activities and also show great interspecific variation with respect to the extent of plastid genome degeneration by gene deletion or gene decay to inactive pseudogenes (Berg et al. 2004; Revill et al. 2005).
Thus, the analysis of ptDNA evolution in Cuscuta provides a unique opportunity to follow the molecular changes associated with the gradual transition to hetero-trophy and to study the mechanisms of plastid genome streamlining as triggered by the loss of photosynthesis.
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