- The dioecious Mercurialis annua L. was used as a model plant to study some aspects of the molecular basis of sex determination in plants. We report in this paper the characterization of a previously identified male specific DNA marker, OPB01-1562, from diploid dioecious M. annua. The marker co-segregated with male sex in the progeny of hormonally feminized males. Sequence analysis showed the presence of approximately 0.6 kb retrotransposon-like sequence at its 3′ end. Homologous sequences were isolated from diploid female, hexaploid male and monoecious plants. These sequences contained RNaseH and integrase domains of reverse transcriptase and were most similar to pineapple retrotransposon dea1, hence were named M. annua retrotransposon-like sequences (MARL-1 to MARL-5). A 771 bp fragment isolated from a diploid female, named fem771, was homologous to the 5′ end of OPB01-1562. Results from DNA blot hybridization suggested OPB01-1562 and fem771 to be from the same locus and MARL-1 from a different one. RNA blot hybridization with OPB01-1562 and MARL-1 detected an approximately 2.8 kb transcript which was expressed strongly in stems and flowers of females but not males. This transcript was named M. annua female expressed (Mafex). Sex linkage of OPB01-1562 and expression of Mafex detected by OPB01-1562 strongly suggested Mafex to be a candidate gene involved in sex determination in M. annua.