Asexual thalli originated from sporophytic thalli via apomeiosis in the green seaweed Ulva

Abstract

Apomixis is an asexual reproduction system without fertilization, which is an important proliferation strategy for plants and algae. Here, we report on the apomeiosis in the green seaweed Ulva prolifera, which has sexual and obligate asexual populations. Genomic PCR of mating type (MT)-locus genes revealed asexual thalli carrying both MT genomes. Observation of the chromosomes during the formation of each type of reproductive cell revealed that cells in asexual thalli performed apomeiosis without chromosome reduction. Moreover, genotyping revealed that laboratory-cultured sporophytic thalli produced not only each type of gametophyte but also diploid thalli carrying the mt− and mt+ genome (mt± thallus strains). The mt± thallus strain released diploid biflagellate zoids, with ultrastructure and behavior similar to mt+ gametes. Additionally, a transcriptomic analysis revealed that some meiosis-related genes (Mei2L and RAD1) were highly expressed in the quadriflagellate zoosporoids. Our results strongly suggest that asexual thalli originally evolved via apomeiosis in sporophytic thalli.

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

“…In contrast, mating type- or sex-specific positioning of the mating structure is established in ulvophycean green seaweeds examined to date, in which male and female gametes always belong to the type α and β gametes 42 , respectively, but the mating type- or sex-determining gene has not been identified. Nevertheless, this relationship between the mating type (sex) and MSP (type α and β) was confirmed in the slightly anisogamous species U. prolifera and U. partita , respectively, using the mt – (male) and mt + (female) specific genes in the mating type locus, such as PRA1m (mt – ), and PRA1p (mt + ) 21 , 22 , which were identified using genome sequencing for both mating types of U. partita 21 and confirmed as mating type-specific genes by polymerase chain reaction (PCR)-based genotyping in U. prolifera 22 . The presence or absence of a mating type-specific gene is correlated with the MSP (type α or β).…”

The underexplored potential of green macroalgae in aquaculture

“…In most Ulva species, asexual reproduction can occur from unfertilised gametes parthenogenetically; 158,159 or from vegetative propagation through regeneration of fragmented tissue 88 . However, some species or subpopulations reproduce exclusively through asexual propagation, via direct germination of biflagellate gametes or quadriflagellate zoospores 160,161 …”

“…88 However, some species or subpopulations reproduce exclusively through asexual propagation, via direct germination of biflagellate gametes or quadriflagellate zoospores. 160,161 Extensive research on Ulva has allowed the development of laboratorial propagation methods, including classic genetic crosses by gamete conjugation as well as via parthenogenesis. 157 In some species, for example U. mutabilis, U. lactuca and U. linza, gametogenesis can be induced by removal of 'sporulation inhibitors' by cutting vegetative thalli, washing the fragments and transfer to fresh culture media.…”

Exploring, harnessing and conserving marine genetic resources towards a sustainable seaweed aquaculture

“…eucheumatoids, see details in Box 1). For sexually reproducing species, an important research focus is to control life history transitions or exploit natural plasticity of life histories (Maggs, 1988) and to find individuals able to propagate asexually, to facilitate biobanking and germplasm amplification (Ichihara et al, 2019;Li et al, 2017;Takahashi & Mikami, 2017). Comparable to crops such as banana and potato, asexually reproducing seaweeds are far easier to propagate; however, the development of new cultivars still relies on the identification of agronomically valuable individuals in the wild, or on the ability to perform controlled crossings.…”