Among the most profound biological insights gleaned from the past decade of comparative genomics has been the realization that horizontal gene transfer (HGT) has impacted the genetic make up of virtually everything we have chosen to sequence. Long recognized as a major force in the evolution of prokaryotic genomes [1
] and somewhat more recently for microbial eukaryotes [2
], HGT can also impact the genomes of complex, multicellular organisms: case in point, plant mitochondrial DNA (mtDNA). Parasitic plants and their hosts (Figure ) have proven to be avid donors and recipients of mtDNA [3
], and two recent studies, one in the pages of BMC Biology
], have provided new mechanistic detail on the causes and consequences of plant HGT [4
]. HGTs involving plant nuclear and chloroplast DNA appear (for now at least) to be rare [6
]. In contrast, plant mtDNA appears to be highly mobile, a fact that has significant practical and theoretical implications for plant biology.
The parasitic plant Cuscuta wrapping around one of its many possible hosts, Arabidopsis. Image kindly provided by Dr Collin Purrington, Swarthmore College.
The first suggestions that plant mitochondria might be exceptional in terms of DNA uptake came from studies of their fungal-derived, homing group I introns (for example, [8
]), and a growing body of evidence for plant-to-plant HGT has since accumulated [3
]. The current champion is Amborella trichopoda
, a 'primitive' flowering plant found exclusively on the island of New Caledonia, the mtDNA of which is littered with foreign genes acquired from both angiosperm and non-angiosperm donors [9
How exactly does plant HGT happen? An important clue comes from the fact that while examples of ancient and 'recent' HGT events involving chloroplast DNA have been documented [2
], such events appear to be very infrequent [6
]. This makes sense given that plant mitochondria possess active DNA uptake systems and are capable of fusion; chloroplasts do not and are not (see [3
] and references therein). Thus, given direct physical contact between host and parasite tissue, ample opportunity for mtDNA uptake and exchange would seem to exist. Yet despite extensive phylogenetic evidence supporting the notion that plant mitochondrial HGT is rampant, numerous mechanistic uncertainties remain. These include the question of whether DNA or RNA is the donor molecule and whether a virus or some other vector mediates the transfer.