Mitochondrial uniparental inheritance achieved after fertilization challenges the nuclear-cytoplasmic conflict hypothesis for anisogamy evolution

Abstract

In eukaryotes, a fundamental phenomenon underlying sexual selection is the evolution of gamete size dimorphism between the sexes (anisogamy) from an ancestral gametic system with gametes of the same size in both mating types (isogamy). The nuclear-cytoplasmic conflict hypothesis has been one of the major theoretical hypotheses for the evolution of anisogamy. It proposes that anisogamy evolved as an adaptation for preventing nuclear-cytoplasmic conflict by minimizing male gamete size to inherit organelles uniparentally. In ulvophycean green algae, biparental inheritance of organelles is observed in isogamous species, as the hypothesis assumes. So we tested the hypothesis by examining whether cytoplasmic inheritance is biparental in Monostroma angicava, a slightly anisogamous ulvophycean that produces large male gametes. We tracked the fates of mitochondria in intraspecific crosses with PCR-RFLP markers. We confirmed that mitochondria are maternally inherited. However, paternal mitochondria enter the zygote, where their DNA can be detected for over 14 days. This indicates that uniparental inheritance is enforced by eliminating paternal mitochondrial DNA in the zygote, rather than by decreasing male gamete size to the minimum. Thus, uniparental cytoplasmic inheritance is achieved by an entirely different mechanism, and is unlikely to drive the evolution of anisogamy in ulvophyceans.

Keywords: anisogamy; isogamy; organelle inheritance; ulvophycean green algae.

Figure 1.

Haplo-diplontic sexual life cycle of M. angicava with haploid multicellular male or female gametophyte generation and diploid unicellular sporophyte generation. n: Haploid stage; 2n: diploid stage.

Figure 2.

Tests of mitochondria PCR-RFLP markers, mitochondria PCR-RFLP band patterns of offspring zygotes/sporophytes/gametophytes and sex-specific molecular marker band patterns of offspring gametophytes. P: paternal RFLP band pattern. M: maternal RFLP band pattern. Bars on the left side indicate 500 bp. (a) Tests for the candidates of mitochondria PCR-RFLP markers. We designed each primer set to exhibit different band patterns between male (mg: MA060316B-mg) and female (fd: MA060316B-fd) strains. (b,c) Mitochondria DNA inheritance in offspring zygotes/sporophytes at early developmental stages with M8 marker. (b) The results from fertilization to the 21st day (ZCA strain). Paternal mitochondria DNA disappeared after the 21st day (highlighted by a dotted square). (c) The detailed results from fertilization to the 14th day (ZEB strain). Both paternal and maternal mitochondria DNAs persisted for at least 14 days. (d–f) Mitochondria PCR-RFLP and sex-specific molecular marker band patterns of offspring gametophytes. Offspring gametophytes were produced by a sporophyte of Z41 zygote strain. Numbers (1–10) indicate individual offspring gametophytes. (d) Mitochondria marker M5. (e) Male-specific marker. (f) Female-specific marker.