Fig. 1

The isomorphic life cycle of Ulva. A: The sexual life cycle of Ulva involves alternation between gametophytic and sporophytic life stages with identical morphologies. Sporophytes produce quadriflagellate meiotic spores (zoospores) that develop into haploid mt+ or mt- gametophytes with similar morphology. Gametophytes produce mt+ and mt- gametes by gametogenesis. Gametes of the opposite sex fuse in the water column to form a zygote (syngamy). Dashed arrows show the parthenogenetic development of gametophytes derived from unfused gametes. Zygotes develop to produce diploid sporophytes, completing the cycle. B: Boxplots of the area of mt+ and mt- gametes of U. mutabilis derived from flow cytometry (n = 307 and 3170, respectively). The mean area of mt- gametes and mt+ gametes are 19.75 and 21.72 μm², respectively (P < 0.001, Student’s t-test).