SEXUALITY AND UNIPARENTAL INHERITANCE OF CHLOROPLAST DNA IN THE ISOGAMOUS GREEN ALGA ULVA COMPRESSA (ULVOPHYCEAE)1

Ulva compressa L. is a heterothallic macroalga considered to be in the early evolutionary stage between isogamy and anisogamy. Two genetic lines of this species, each consisting of gametophytes with opposite mating types, were collected on the coasts of Ehime and Iwate prefectures: MGEC‐1 (mt⁺) and MGEC‐2 (mt^?) from Ehime, and MGEC‐5 (mt⁺) and MGEC‐6 (mt^?) from Iwate. Their relative gamete sizes (i.e., cell volumes) do not correspond to their mating types: MGEC‐6 (19.8 μm³) > MGEC‐1 (18.6 μm³) > MGEC‐5 (17.0 μm³) > MGEC‐2 (10.1 μm³). The pattern of organelle inheritance is an important sexual characteristic in many eukaryotes. We therefore investigated the relationship between gamete size and the inheritance of chloroplast DNA (cpDNA). Polymorphisms between the cpDNA of the two lines were used as markers. We found a 24 bp insertion between psbF and psbL, and the substitution of a StyI site (from C CAAGG to T CAAGG) in the intergenic region between petD and accD. Two interline crosses (MGEC‐1 × MGEC‐6 and MGEC‐2 × MGEC‐5) produced 42 and 38 zygotes, respectively. PCR and PCR–RFLP analyses showed that the cpDNA of the mt⁺ gametes was consistently inherited in both crosses. The cpDNA is inherited from one parent only, and it depends not on gamete size but on being mt⁺. The cpDNA was observed during crossing and in the zygotes 6 h after mating. In 6% of the zygotes, the cpDNA derived from the mt^? gametes disappeared 3–4 h after mating. Preferential digestion of the cpDNA in the zygote’s mt^? gamete may form the basis for uniparental inheritance of cpDNA.