An mt+ gamete-specific nuclease that targets mt− chloroplasts during sexual reproduction in C. reinhardtii

Although the active digestion of mating-type minus (mt−) chloroplast DNA (cpDNA) in young zygotes is considered to be the basis for the uniparental inheritance of cpDNA inChlamydomonas reinhardtii, little is known about the underlying molecular mechanism. One model of active digestion proposes that nucleases are either synthesized or activated to digestmt− cpDNA. We used a native-PAGE/in gelo assay to investigate nuclease activities in chloroplasts from young zygotes, and identified a novel Ca2+-dependent nuclease activity. The timing of activation (∼60–90 min after mating) and the localization of the nuclease activity (in mt− chloroplasts) coincided with the active digestion of mt− cpDNA. Furthermore, the activity of the nuclease was coregulated with the maturation of mating-type plus (mt+) gametes, which would enable the efficient digestion of mt− cpDNA. Based on these observations, we propose that the nuclease (designated as Mt+-specific DNase, MDN) is a developmentally controlled nuclease that is activated inmt+ gametes and participates in the destruction ofmt− cpDNA in young zygotes, thereby ensuring uniparental inheritance of chloroplast traits.