Diploid Apogamy in Red Algal Species of the Genus Pyropia
Faculty of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Hakodate 041-08611, Japan
Corresponding author: Koji Mikami, Faculty of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Japan, Tel/Fax: +81-138-40-8899; E-mail: email@example.com
Citation: Mikami K (2019) Diploid apogamy in red algal species of the genus Pyropia. J Aquat Res Mar Sci 2019: 206-208.
Received Date: 24 July, 2019; Accepted Date: 31 July, 2019; Published Date: 05 August, 2019
Bangiales is an order of red algae in the class Bangiophyceae of the division Rhodophyta [1,2] that has recently been subdivided into fifteen genera including Bangia, Pyropia, Porphyra, and Boreophyllum . Most seaweeds in the Bangiales feature a heteromorphic haploid-diploid life cycle wherein both the haploid gametophyte and the diploid sporophyte develop multicellular bodies that appear in temporally distinct periods of the year [4-6]. In most plants and seaweeds, transitions from gametophyte to sporophyte and from sporophyte to gametophyte are triggered by fertilization of male and female gametes and meiosis, respectively [4-8]. However, meiosis is not involved in the formation of gametophytes in Bangiales, even though the transition from gametophyte to sporophyte is mediated by fertilization [9,10]. In the Bangiales, meiotic cell division instead occurs early during the development of gametophytes [11-20].
The life cycle of the marine red alga Pyropia yezoensis, in particular, thus does not conform to the general concept of a requirement for meiosis in gametophyte production. In fact, we recently demonstrated that gametophyte identity in P. yezoensis is established without meiosis in the conchosporangia, which are parasitically produced on sporophytes [10,21]. Based on these findings, we proposed that P. yezoensis has a triphasic life cycle consisting of gametophyte, sporophyte, and conchosporophyte, which represents novel nomenclature denoting the conchosporangium as a life cycle generation .
The production of diploid gametophytes without meiosis, as found in conchosporophytes of P. yezoensis , is generally designated as apospory, whereas the production of haploid sporophytes from somatic cells in haploid gametophytes without fertilization of gametes is named apogamy [8,21]. Apospory and apogamy together are termed apomixis and represent an asexual strategy for reproduction from somatic cells without ploidy change [8,23], a phenomenon that has been observed primarily in ferns and vascular plants [22,24-26].
In seaweeds, production of sporophytes from somatic cells has been observed in thalli of P. yezoensis treated with hydrogen peroxide  and laboratory-cultured female gametophytes of P. haitanensis . Although these phenomena were initially described as apogamy or parthenogenesis [10,27], these definitions may be misnomers. In the case of P. yezoensis, although the production of sporophytes from somatic cells fits apogamy, the resultant sporophytes were proposed to be diploid, whereas apogamous sporophytes should be haploid (Figure 1). The diploidy of the sporophytes produced from somatic cells was confirmed in P. haitanensis by karyotype analysis, which indicated that the ploidy of red-colored cells pre-programmed to generate sporophytes is autonomously doubled before the production of sporophyte filaments . However, it is incorrect to describe this process as parthenogenesis, since parthenogenesis denotes the development of sporophytes from gametes (Figure 1). Therefore, neither apogamy nor parthenogenesis fully describes the aforementioned, unique phenomena in these two species of Pyropia. As far as we know, there is currently no nomenclature that denotes haploid somatic cell-derived generation of diploid sporophytes without fertilization.
Figure 1: Schematic representation of “diploid apogamy” in comparison to so-called apogamy and parthenogenesis.
Sporophyte production from somatic cells with chromosome duplication is designated “diploid apogamy” and so-called apogamy is renamed as “haploid apogamy”. Chromosome duplication occurs in diploid parthenogenesis (automixis), but not in haploid parthenogenesis.
In animals, fertilization-independent production of diploid zygotes through gamete duplication (chromosome duplication) is one of the strategies categorized as automixis (Figure 1), which refers to diploid parthenogenesis based on development of maternal oocytes and polar bodies produced by meiosis [28-30]. Since such a spontaneous chromosomal duplication is also observed in P. haitanensis , the production of sporophytes from somatic cells in Pyropia is partly analogous to gamete duplication in automixis. Given that homologous recombination occurs during gamete duplication, automixis is recognized as a form of sexual reproduction .
Taking all of these cases together, it seems that sporophyte production from somatic cells in Pyropia can be viewed as a hybrid process similar to apogamy in plants and gamete duplication in animals, wherein the former establishes sporophyte identity in haploid gametophytic cells and the latter is responsible for the production of normal diploid sporophyte filaments. This should clearly be categorized as a form of apomixis, since the phenomenon is independent of fertilization. Thus, as shown in Figure 1, we tentatively designate this unique strategy “diploid apogamy” to distinguish it from the general term “apogamy,” which could be more specifically termed “haploid apogamy” because of the absence of ploidy change. This nomenclature is analogous to that used for parthenogenesis, which is subdivided into haploid parthenogenesis and diploid parthenogenesis in animals (Figure 1) [32-34]. In fact, spontaneous chromosome duplication was also observed in one-third of parthenosporophytes during the first cell division of non-fertilized gametes in the brown alga Ectocarpus siliculosus , indicating the presence of both haploid and diploid parthenogenesis in seaweeds. Thus, to distinguish between apogamous phenomena with and without chromosome duplication, it is reasonable to use the terms haploid and diploid apogamy.
As mentioned above, P. yezoensis and P. haitanensis utilize characteristic reproductive strategies such as apospory for the establishment of gametophyte identity and diploid apogamy for the production of normal diploid sporophytes from haploid gametophytic somatic cells, respectively [10,27]. These findings suggest that there are unique regulatory mechanisms for reproduction in the genus Pyropia. Thus, further study on apomixis in Pyropia could provide new information about regulatory factors and genes involved in diploid apogamy and generation switches during the life cycle. By analogy, E. siliculosus life cycle mutants like OUROBOROS and SAMSARA have helped to elucidate the regulatory system of life cycle phase transitions [36,37]. However, no life cycle mutant has been reported in Pyropia, Bangia, and Porphyra. Therefore, future work should focus on isolating life cycle mutants to advance research on the regulatory mechanisms common to haploid-diploid life cycles, diploid apogamy, and apospory in the Bangiales.