only species ( and subspecies ) for which complete mitochondrial and plastid genome sequences are available ( as of 1 march 2011 ) ( based on genbank 's entrez organelle genome database : http://www.ncbi.nlm.nih.gov/sites/entrez?db=genome ) were included in the mtpt / ptmt analyses . this is because organelle genomes often contain sequences ( particularly intergenic and repetitive dna ) that are species specific . moreover , organelle gene contents can vary significantly among certain lineages , especially green algae . mitochondrial and plastid genomes were scanned for mtpts and ptmts with blastn ( v2.2.23 ) ( altschul et al . 1990 ) using an expectation value ( e value ) of 0.0001 , a word size of 11 , match and mismatch scores of 2 and 3 , and gap - cost values of 5 ( existence ) and 2 ( extension ) . hits under 30 nt and showing less than 70% sequence identity to the query were ignored . spurious hits , such as those where plastid genes matched to homologous genes in the mitochondrial genome ( e.g. , plastid - encoded ribosomal dna [ rdna ] matching to mitochondrial - encoded rdna ) were ignored . regions of mtdna that contained tight clusters of mtpts ( i.e. , ptdna - like sequences interrupted by mitochondrial sequence that did not show sequence identity to ptdna ) were counted as separate hits . most of the ptdna - derived sequences that were identified within the mitochondrial genomes represented noncoding dna or pseudogenes ; however , instances of functional relocation of ptdna - encoded genes ( e.g. , transfer rnas ) to the mitochondrial genome were considered as mtpts . complete mtpt statistics for the various plastid - bearing species that were investigated are shown in table 1 and figure 1 . none of the monoplastidic or effectively monoplastidic taxa had plastid - like dna in their mitochondrial genomes , whereas all but one of the polyplastidic species contained mtpts . the green alga chara vulgaris was the only polyplastidic species that did not have mtpts , a feature which may reflect the small number of plastids within its reproductive tissues ( 16 ) ( pickett - heaps 1975 ; renzaglia and garbary 2001 ) and its increased ability to expunge excess mtdna relative to other polyplastidic species ( discussed further below ) . on average , the largest numbers of mtpts were found in the mitochondrial genomes of the common grape vitis vinifera and the grass sorghum bicolor , both of which had 15 kb of plastid - derived dna , distributed among 30 different regions ( table 1 and fig . 1 ) . unlike plastid number , being mono- or polymitochondrial appeared to have no effect on the number of observed mtpts within the mitochondrial genomes ( table 1 ) . the results presented here on mtpts are consistent with previous investigations on ptdna - like sequences in nuclear genomes ( nupts ) ( smith et al . 2011 ) , which showed that polyplastidic taxa had on average 80 times more nupts than species that were monoplastidic or effectively so . together , the mtpt and nupt data support the hypothesis that monoplastidy greatly reduces the potential for intercompartmental ptdna migration . bar graph showing the number and accumulative length of ptdna - derived sequences ( mtpts ) within the mitochondrial genomes of various plastid - harboring eukaryotes . for references and notes on the number of organelles per cell , mtpt abundance scaled exponentially with both mitochondrial genome size and mtdna noncoding content ( fig . the most bloated mtdnas that were explored  those belonging to the angiosperms v. vinifera , s. bicolor , carica papaya , and zea mays  had the largest mtpt contents . this relationship is not surprising given that other types of excess dna , including introns , repetitive elements , and intergenic dna , are known to mutually expand as the number of noncoding nucleotides in a mitochondrial genome increases ( lynch et al . it is thought that the burden of harboring surplus mtdna is positively proportional to the mitochondrial mutation rate ( lynch et al . 2006 ) ; thus , the very low mtdna mutation rates that are typically found in angiosperms ( lynch et al . 2006 and references therein ) may promote the accumulation of mtpts . in this context , it is noteworthy that angiosperm mtdnas often contain nucdna - derived sequences as well as genes horizontally acquired from other mitochondrial genomes ( richardson and palmer 2007 ; kleine et al . 2009 ) , reinforcing the notion that they are sponges for all types of foreign dna . among the polyplastidic species that were investigated , the green alga c. vulgaris had the smallest and most compact mtdna ( supplementary table s1 , supplementary material online ) , implying that it is more proficient than its land plant counterparts at eliminating ( or preventing the accumulation of ) excess nucleotides from its mitochondrial genome . if true , this may help explain why it was the only polyplastidic taxa without mtpts . the relationship between mtpts and genome size is in accordance with previous investigations on organelle - to - nucleus dna transfers , which found that bloated nuclear genomes harbor more plastid- and mitochondrial - derived sequences than those that are compact ( hazkani - covo et al . 2011 ) . ultimately , these data suggest that the probability of transferred organelle dna persisting in a genome is largely governed by the forces controlling the expansion and contraction of bulk dna . plot of mtpt content versus mitochondrial genome size ( log scale ) ( a ) and mitochondrial genome noncoding dna content ( b ) . data on mitochondrial genome size and noncoding dna content are shown in supplementary table s1 ( supplementary material online ) . not 1 of the 42 species employed here contained mtdna - like sequences in its plastid genome , irrespective of the number of mitochondria per cell and plastid genome size ( table 1 and supplementary table s1 , supplementary material online ) . this strengthens the commonly held view that the exchange of dna between plastids and mitochondria is a one - way street ( richardson and palmer 2007 ; kleine et al . it has been suggested that because plastid genomes are often more compact than their mitochondrial counterparts ( particularly for land plants and certain lineages of green algae ) they are more efficient than mtdnas at purging excess nucleotides . however , this hypothesis seems unlikely when considering that no ptmts were found within the ptdna of the polymitochondrial green alga volvox carteri , which boasts a plastid genome that is larger ( > 525 kb ) and more expanded ( 80% noncoding ) than many angiosperm mtdnas ( smith and lee 2010 ) . as noted by previous investigators ( richardson and palmer 2007 ) , an intriguing observation concerning the presence of mtpts in mtdnas and the absence of ptmts from ptdnas is that mitochondria ( at least those of land plants and mammals ) have an active dna import system ( koulintchenko et al . as predicted by the limited transfer window hypothesis , mtpts were abundant in polyplastidic species , particularly those with expanded mitochondrial genomes , and were absent from monoplastidic species as well as polyplastidic taxa with monoplastidic meristematic systems . neither the polymitochondrial nor the monomitochondrial taxa contained mtdna - derived sequences in their plastid genomes , which may reflect the absence of an efficient dna import system within plastids . these results , like those on plastid - like sequences in nuclear genomes , support the idea that monoplastidy limits the potential for ptdna to be donated to other organelles . the probability that ptdna sequences will be taken up by those organelles and persist as mtpts or nupts is defined by an organelle 's ability ( or lack thereof ) to both import exogenous dna and retain it within its genome . supplementary tables s1 and s2 are available at genome biology and evolution online ( http://www.gbe.oxfordjournals.org/ ) .