Inheritance pattern of chloroplast DNA is correlated with gamete types based on sex‐specific arrangement of the cell fusion site in Caulerpa (Ulvophyceae, Chlorophyta)

Abstract

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.

Sex-Specific Cell Fusion Pattern of Isogametes in Marine Green Alga, <i>Acetabularia caliculus</i> (Ulvophyceae, Chlorophyta)

“…The results also support the proposal that a sex-specific arrangement of the mating structure/cell fusion site of the gamete is a universal feature in advanced chlorophytes (Nakayama and Inouye 2000) and that it is correlated with specific mating types (Miyamura and Nagumo 2007).…”

“…2f, g) was a result of sex-specific fusion. Therefore, gametes of A. caliculus are suggested to be divided into two types (type α and β) based on the cell fusion site position, as reported previously in other chlorophyte algae (Miyamura and Nagumo 2007) (Fig. 1b).…”

“…The mating pairs consisting of three gametes aligned side-by-side were rarely observed in two mating pairs in which three eyespots aligned on the same side of the cell (n= 62). The alignment of two eyespots on the same side of the planozygote was also reported in A. acetabulum (Crawley 1966) and other chlorophyte algae (Miyamura et al 2015), and is explained as a consequence of sex-specific cell fusion between the two gametes belonging to the opposite mating types: one gamete fuses on the opposite side of the flagellar beat plane to the eyespot (type α gamete), whereas the other gamete fuses on the same side as the eyespot (type β gamete) (Holmes andDutcher 1989, Miyamura andNagumo 2007) (Fig. 1).…”

“…To date, sex-specific arrangement of the mating structure/ cell fusion site and the consequent sex-specific cell fusion pattern has been reported mainly in isogamous and anisogamous species in Ulvophyceae (Nakayama and Inouye 2000, Miyamura et al 2003, O Kelly et al 2004 in addition to isogamous species, C. reinhardtii (Chlorophyceae) (Holmes and Dutcher 1989, Gaffal et al 1991, Miyamura et al 2009) and Nephroselmis olivacea (Nephroselmidophyceae) (Suda et al 2004). In the Ulvophyceae, this phenomenon has been reported for several species in the Ulvales (Miyamura et al 2003, O Kelly et al 2004, the Ulotrichales (Nakayama and Inouye 2000) and the Bryopsidales (Miyamura et al 2005, Miyamura andNagumo 2007), but it has not been studied in other orders. Therefore, it remains uncertain whether this phenomenon is a universal feature in the Ulvophyceae, or not.…”

Visualizing the Spatial Arrangement of Flagella-Eyespots-Cell Fusion Sites in Gametes and Planozygotes of Chlamydomonas reinhardtii (Chlorophyceae, Chlorophyta) with High-Resolution FE-SEM

“…In the flagellate gametes of green algae, fusion generally occurs at specific site of the cell. Recently, it has been found that the cell fusion sites of the gametes occupy different positions between the opposite mating types in several green algae (see Miyamura and Nagumo 2007). This was first indicated in biflagellate isogametes of chlorophycean alga Chlamydomonas reinhardtii Dangeard (Holmes andDutcher 1989, Gaffal et al 1991), in which cell fusion site is differentiated to specialized cell membrane "mating structure" (Goodenough and Weiss 1975).…”