DISAPPEARANCE OF MALE MITOCHONDRIAL DNA AFTER THE FOUR-CELL STAGE IN SPOROPHYTES OF THE ISOGAMOUS BROWN ALGA SCYTOSIPHON LOMENTARIA (SCYTOSIPHONACEAE, PHAEOPHYCEAE)

Mitochondrial DNA (mtDNA) of the isogamous brown alga Scytosiphon lomentaria (Lyngb.) Link is inherited maternally. We used molecular biological and morphological analyses to investigate the fate of male mitochondria. Ultrastructural observations showed that the number of 25 mitochondria in a zygote coincided with the number of mitochondria derived from male and female gametes. This number remained almost constant during the first cell division. Strain-specific PCR in single germlings suggested that mtDNA derived from the female gamete remained in the germling during development, while the male mtDNA gradually and selectively disappeared after the four-cell stage. One week after fertilization, male mtDNA had disappeared in sporophytic cells. Using bisulfite DNA modification and methylation mapping assays, we found that the degree of methylation on three analyzed sites of mtDNA was not different between male and female gametes, suggesting that maternal inheritance of mtDNA is not defined by its methylation. This study indicates that the mechanism of selective elimination of male mtDNA is present in each cell of a four-celled sporophyte and that it does not depend on different degrees of DNA methylation between male and female mtDNA.