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The evolution of sex chromosomes and mating loci in organisms with UV systems of sex/mating type determination in haploid phases via genes on UV chromosomes is not well understood. We report the structure of the mating type (MT) locus and its evolutionary history in the green seaweed Ulva partita, which is a multicellular organism with an isomorphic haploid-diploid life cycle and mating type determination in the haploid phase. Comprehensive comparison of a total of 12.0 and 16.6 Gb of genomic next-generation sequencing data for mt− and mt+ strains identified highly rearranged MT loci of 1.0 and 1.5 Mb in size and containing 46 and 67 genes, respectively, including 23 gametologs. Molecular evolutionary analyses suggested that the MT loci diverged over a prolonged period in the individual mating types after their establishment in an ancestor. A gene encoding an RWP-RK domain-containing protein was found in the mt− MT locus but was not an ortholog of the chlorophycean mating type determination gene MID. Taken together, our results suggest that the genomic structure and its evolutionary history in the U. partita MT locus are similar to those on other UV chromosomes and that the MT locus genes are quite different from those of Chlorophyceae.

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Using field emission scanning electron microscopy (FE-SEM) and fluorescence microscopy, the respective relationships between the arrangement of the gamete cellfusion site and the inheritance pattern of chloroplast DNA (cp-DNA) were studied for Caulerpa brachypus Harvey, C. okamurae Weber-van Bosse, C. racemosa (Forsskål) J. Agardh var. laete-virens (Montagne) Webervan Bosse, and C. serrulata (Forsskål) J. Agardh var. serrulata f. lata (Weber-van Bosse) Tseng. The eyespot of the biflagellate gamete was visualized using FE-SEM. The female gamete, but not the male, has one eyespot on the cell body posterior. In most mating pairs, the female gamete is fused at the anterior left side of the eyespot and the male gamete at a cell surface that is perpendicular to the plane of the flagellar beat when both gametes are mixed. Then, the inheritance pattern of cp-DNA was observed using fluorescence microscopy after staining with 4′6-diamidino-2-phenylindole. Male and female gametes have one cell nucleus and one chloroplast each. Chloroplasts of the female gamete usually contain 1-11 spherical or rod-shaped nucleoids. In contrast, nucleoids are not usually detected in the male gamete's chloroplast. After mixing male and female gametes, the male gamete without nucleoids and female gametes with nucleoids are always associated at the lateral side and become planozygotes. Such a correlation between the arrangement of the cell fusion site and the inheritance pattern of cp-DNA was found in another member of Caulerpales, Bryopsis maxima Okamura. These results suggest the possibility that the arrangement of the cell fusion site in the gamete is not determined randomly regardless of sex, but is rather correlated with specific mating types. The relation of these results to those for Chlamydomonas is discussed.