Nuclear behavior and roles indicate that Codiolum phase is a sporophyte in Monostroma angicava (Ulotrichales, Ulvophyceae)

Abstract

The life‐cycle system of Ulotrichales, a major order of Ulvophyceae, remains controversial because it is unclear whether the Codiolum phase, a characteristic unicellular diploid generation in ulotrichalean algae, is a zygote or a sporophyte. This controversy inhibits the understanding of the diversified life cycles in Ulvophyceae. To distinguish between zygotes and sporophytes, we have to examine not only whether diploid generations function as sporophytes, but also whether mitosis occurs before meiosis in diploid generations. However, the nuclear behavior in the Codiolum phases is largely unknown, probably because no suitable methods are available. Using fluorescent microscopy with ethidium bromide and transmission electron microscopy of cell‐wall‐dissected specimens, we report the nuclear behavior in the Codiolum phases of an ulotrichalean alga with a representative life cycle, Monostroma angicava. Each vegetative Codiolum phase had a single polyploid nucleus due to endoreduplication, a type of mitosis without nuclear division. During zoosporogenesis, the nucleus had a structure that would be a meiosis‐specific complex. We quantitatively showed that Codiolum phases grew extremely large and produced numerous zoospores. Our results suggest that an event comparable to mitosis occurs before meiosis in the Codiolum phase of M. angicava. This nuclear behavior and the functions (growth and zoospore production abilities) correspond to those of sporophytes. Therefore, the life‐cycle system of M. angicava is a heteromorphic haplo‐diplontic cycle. This system appears to be widely adopted among other ulotrichalean algae.

Evolution of Anisogamy in Organisms with Parthenogenetic Gametes

“…, respectively, for x and y). In the sexual pathway, after fusion (syngamy) the diploid zygote typically grows considerably to form a (diploid) sporophyte that undergoes a meiotic division, ending in many cell divisions to produce many small haploid zoospores (e.g., Horinouchi et al 2019), each of which grows into an adult gametophyte thallus, so gametogenesis and meiosis are not directly linked in haplodiplontic cycles. In the parthenogenetic (asexual) cycle, an x or y gamete may develop into a (haploid) parthenosporophyte, which releases zoospores as in the sexual pathway, each of which grows to form a new gametophytic adult.…”

A comparative test of the gamete dynamics theory for the evolution of anisogamy in Bryopsidales green algae

“…figure 1 b,c ). However, unlike diplontic taxa, haplodiplontic species may first undergo many mitotic divisions followed by the formation of meiotic progeny (zoospores) [19], each of which is released and germinates to produce a haploid adult (gametophyte) (figure 1 c ). In such species, there is, therefore, no direct translation of zygotic reserves into the haploid adult (gametophyte).…”

“…Some taxa have diplontic life cycles without a haploid adult (gametophyte) stage (electronic supplementary material, table S1), as do higher plants and most animals: after syngamy, the zygote develops directly into the diploid adult (sporophyte) in which meiosis occurs during gametogenesis [14], and the gametes are the only haploid stage (figure 1 b ). However, other taxa have haplodiplontic life cycles (electronic supplementary material, table S1) in which the diploid zygote grows into a diploid adult (sporophyte) before undergoing meiosis, mitosis and release of many unicellular haploid progeny (stephanokont zoospores) [18,19] (figure 1 c ). Each zoospore develops into a haploid adult (gametophyte), which may be multicellular and complex, and which releases gametes to produce the following generation.…”

Identification of genomic differences between the sexes and sex‐specific molecular markers in Monostroma angicava (Ulvophyceae)

“…Here, we focused on Monostroma angicava (Ulotrichales, Ulvophyceae), which has a heteromorphic haplo‐diplontic life cycle with dioicous multicellular haploid gametophytes and unicellular diploid sporophytes (Tatewaki 1969, Horinouchi et al. 2019). M. angicava produces anisogamous male and female gametes (Fig.…”