GARCIA, MIGUEL A.*, ERICA H. NICHOLSON, and DANIEL L. NICKRENT. Department of Plant Biology, Southern Illinois University, Carbondale, IL 62901-6509. - Intra-individual variation in plastid rDNA sequences from the holoparasite Cynomorium (Cynomoriaceae).
It is now generally recognized that intraspecific variation in cpDNA
is relatively common; however, less is known about plastid genome
variation within individuals. cpDNA chimeras (plants with different
cpDNA genotypes) are well-known from variegated cultivars and may
result from biparental inheritance or somatic mutations. We document
here an example of intra-individual variation in plastid rDNA in a
wild species, Cynomorium coccineum, a nonphotosynthetic
holoparasite. PCR amplifications of all portions of the ribosomal DNA
cistron (16S, 23S, 4.5S, 5S and intervening spacers) were made. The
PCR products were cloned and the inserts sequenced from 19 colonies.
Sequence variation has been observed for all portions of the cistron,
however, we will here focus on variation in the large-subunit (LSU,
23S) rDNA which is 2.7 kb in length. Of the 19 clones, five distinct
types were identified. Fifteen of the sequences were nearly identical
(11 differences or less) and these were designated Type I. The
remaining types (II-V) were each represented by a single clone and
differed from Type I by 90-235 changes. Of the five types, Type I
sequences were the most derived when compared with a tobacco outgroup
sequence (range across Types 250-370 differences). Higher-order
structures were constructed for representatives of the five Types to
address questions of functionality. Major structural changes (mainly
multiple base deletions relative to tobacco) were noted in the
variable regions V2, V6, V12, V15, and V16. These deletions varied in
size, and those of lesser magnitude retained sequences that were
alignable with tobacco. Relatively few mutations were seen in the
peptidyl transferase loop, thus suggesting retention of functionality.
Given that the PCR products were obtained from total genomic DNA, it
is not presently known whether heteroplasmy in Cynomorium
derives from variation among cpDNAs of different plastids, cells or
tissues within the individual plant.
Key words: 23S ribosomal DNA, heteroplasmy, parasitic plant, plastid