Nonreciprocal recombination between alleles of the chloroplast 23S rRNA gene in interspecific Chlamydomonas crosses.

The inheritance of six polymorphic loci mapping in the rRNA-encoding (rDNA) region of the inverted repeat sequence of chloroplast DNA (cpDNA) was scored in hybrid subclones derived from reciprocal interspecific crosses between the green algae Chlamydomonas eugametos and Chlamydomonas moewusii. In order to enhance the detection of cells that had undergone recombination between parental cpDNAs, hybrids were selected that inherited a chloroplast antibiotic-resistance marker contributed by the mating-type-minus(mt(-)) parent, the parent that normally contributes fewer cpDNA molecules. The major findings of this study can be summarized as follows. (i) The majority of the hybrids (14/17) were recombinant for cpDNA markers in the 10-kilobase-pair rDNA region under study. (ii) Only one allele of each polymorphic cpDNA locus was ever detected in the hybrids, thus suggesting that newly recombined rDNA sequences in one copy of the inverted repeat are rapidly spread to the other by a copy-correction mechanism. (iii) Chloroplast streptomycin-resistance (sr-2) and erythromycin-resistance (er-nM1) loci, although showing little or no genetic linkage, were mapped to the 16S and 23S rRNA gene regions of the cpDNA, respectively, by virtue of their perfect coinheritance with polymorphic markers within these genes. (iv) cpDNA markers associated with a putative intron of the C. eugametos 23S rRNA gene were inherited by all 17 hybrids. Such a result is similar to that observed for certain alleles of the large rRNA gene of yeast mitochondria in crosses between omega(+) and omega(-) strains.