Gamete dimorphism of the isogamous green alga (Chlamydomonas reinhardtii), is regulated by the mating type-determining gene, MID

Abstract

The gametes of chlorophytes differ morphologically even in isogamy and are divided into two types (α and β) based on the mating type- or sex-specific asymmetric positioning of the mating structure (cell fusion apparatus) with respect to the flagellar beat plane and eyespot, irrespective of the difference in gamete size. However, the relationship between this morphological trait and the mating type or sex determination system is unclear. Using mating type-reversed strains of the isogamous alga Chlamydomonas reinhardtii, produced by deletion or introduction of the mating type-determining gene MID, we revealed that the positioning of the mating structure is associated with conversion of mating types (mt– and mt+), implying that this trait is regulated by MID. Moreover, the dominant mating type is associated with the type β phenotype, as in the chlorophyte species Ulva prolifera. Our findings may provide a genetic basis for mating type- or sex-specific asymmetric positioning of the chlorophyte mating structure.

Swimming zygote formation mediated by symmetrically arranged cell fusion sites in the gametes of Chlamydomonas eugametos (Chlorophyceae, Chlorophyta)

“…This trait is mating type‐specific and is regulated by the mating type‐determining gene MID (Innami et al . 2022). Innami et al .…”

“…Innami et al . (2022) designated the former and the latter arrangements as ‘mating structure position’ (MSP) types α and β, respectively, and defined gametes with MSP types α and β as type α and β gametes, respectively (Fig. 2a).…”

“…side of the flagellar beat plane (approximate plane of flagellar beating through the flagellar base as shown in Fig. 1) to the eyespot (type α), while that of the mt À gametes is present on the same side of the beat plane to the eyespot (type β) (Holmes & Dutcher 1989;Gaffal et al 1991;Miyamura et al 2009;Innami et al 2022) (Fig. 2a).…”

“…This trait is mating type-specific and is regulated by the mating type-determining gene MID (Innami et al 2022). Innami et al (2022) designated the former and the latter arrangements as 'mating structure position' (MSP) types α and β, respectively, and defined gametes with MSP types α and β as type α and β gametes, respectively (Fig. 2a).…”

Parthenogenesis, sexual conflict, and selection on fertilization rates in switching environments

“…While anisogamy is the most commonly observed mode of sexual reproduction in eukaryotic organisms [4], having evolved several times in evolutionary history [5], it is now widely accepted to be derived from isogamy (equal gamete sizes) [6]. While rarer, isogamous species include study organisms such as the yeast Saccharomyces cerevisiae [7] and the green algae Chlamydomonas rein-hardtii [8], where self-incompatible mating types play the role of ancestral versions of the true sexes [9]. Indeed, the volvocine algae (of which C. reinhardtii is a member), provide neat empirical examples of these transitions, with phylogenetic analysis indicating that numerous independent lineages have undergone the transition from isogamy, to anisogamy, and finally to oogamy [3, 10].…”