OPTIONAL ELEMENTS IN THE CHLOROPLAST DNAS OF CHLAMYDOMONAS-EUGAMETOS AND C-MOEWUSII - UNIDIRECTIONAL GENE CONVERSION AND CO-CONVERSION OF ADJACENT MARKERS IN HIGH-VIABILITY CROSSES

Unlike most polymorphic markers in the Chlamydomonas eugametos and Chl amydomonas moewusii chloroplast DNAs (cpDNAs), the C. moewusii 6- and 21-kb extra sequences and the C. eugametos-specific CeLSU . 5 intron a re transmitted to all of the few viable progeny in reciprocal crosses between the two green algae. To determine whether this unidirectional transmission pattern is due to gene conversion or to selection for F1 hybrid survival, we followed the inheritance of the parental alleles a t the loci featuring these three deletions/additions and at several ot her polymorphic cpDNA loci in zygospore clones derived from high-viabi lity crosses. The great majority of the zygospore clones examined inhe rited exclusively the long alleles from the mt(-) parent at the loci c ontaining the three optional cpDNA elements, but as expected, they pre ferentially inherited the markers from the mt(+) parent at most other loci. Our results therefore indicate that all three optional cpDNA seq uences propagate themselves very efficiently by gene conversion in cro sses between strains differing by the presence of these elements. The co-conversion tracts associated with these sequences are longer (>3 kb ) than those previously reported for mobile elements spreading by gene conversion. Our results also revealed that less efficient gene conver sion events occurred at two other cpDNA loci.