DO NONASTERID HOLOPARASITIC FLOWERING PLANTS HAVE PLASTID GENOMES

Past work involving the plastid genome (plastome) of holoparasitic pla nts has been confined to Scrophulariaceae (or Orobanchaceae) which hav e truncated plastomes owing to loss of photosynthetic and other genes. Nonasterid holoparasites from Balanophoraceae (Corynaea), Hydnoraceae (Hydnora) and Cytinaceae (Cytinus) were tested for the presence of pl astid genes and a plastome. Using PCR, plastid 16S rDNA was successful ly amplified and sequenced from the above three holoparasites. The seq uence of Cytinus showed 121 single base substitutions relative to Nico tiana (8% of the molecule) whereas higher sequence divergence was obse rved in Hydnora and Corynaea (287 and 513 changes, respectively). Seco ndary structural models for these 16S rRNAs show that most changes are compensatory, thus suggesting they are functional. Probes constructed for 16S rDNA and for four plastid-encoded ribosomal protein genes (rp s2, rps4, rps7 and rpl16) were used in Southern blots of digested geno mic DNA from the three holoparasites. Positive hybridizations were obt ained using each of the five probes only for Cytinus. For SmaI digests , all plastid gene probes hybridized to a common fragment ca. 20 kb in length in this species.' Taken together, these data provide prelimina ry evidence suggestive of the retention of highly diverged and truncat ed plastid genome in Cytinus. The greater sequence divergence for 16S rDNA and the negative hybridization results for Hydnora and Corynaea s uggests two possibilities: the loss of typically conserved elements of their plastomes or the complete absence of a plastome.