The paper in Communications Biology is here: http://go.nature.com/2G75unt
(Poster image: Eudorina anisogamy by Hiroko Kwai-Toyooka)
Female and male are thought to have evolved from ancestral isogamous organisms with conjugating gametes of identical form. The cause of this evolution was considered to be the expansion of the "sex chromosomal regions" (genome sequences that differ in gene aranagement and composition between the two sexes) by acquiring genes to contribute to the differentiation between femaleness and maleness (Charlesworth 1978; Ferris et al. 2010). However, how and which genes were acquired has been previously unresolved
In the present paper, the most advanced sequencing methodology “next-generation sequencing” and high technology and efforts provided by the co-authors have resolved this problem in the intial transition to anisogamy, and now begun to resolve my long-standing questions since 1992.
Figure 1. Yamagishiella Nozaki in Nozaki & Kuroiwa (1922), the most advanced memner of the isogamous volvocine green algae.
I described a new genus Yamagishiella as the most advanced member of the isogamous volvocine green algae in 1992, based on only morphological data (Figure 1). I then thought that the use of DNA sequencing would not be possible for my phylogenetic and taxonomic studies of the colonial volvocine green algae. However, since 2000, chloroplast multigene phylogenetic analyses of these algae became possible for us to resolve the phylogenetic position of Yamagishiella within the volvocine green algae (Nozaki et al. 2000). Our phylogenetic results demonstrated that Yamagishiella and Eudorina (another volvocine green alga with similar vegetative colonies but with anisogamous sexual reproduction; see Poster image) bracket evolutionary transition from isogamy to anisogamy, suggesting that these two volvocine green algae represent key organisms for unveiling the transition from isogamy to anisogamy in the evolutionary biology.
In the present paper, we analyzed this very attractive evolutionary issue by using the whole nuclear genome data from both sexes of Yamagishiella and Eudorina. We resolved a very reduced male sex chromosomal region (ca. 7 kbp) with only a single male-limited gene, MID, in Eudorina that corresponds to the first anisogamous organism. Thus, the “first male” might have emerged based on such a small sex chromosomal region during the 200 MY volvocine evolution (Herron et al. 2009). Our study suggested that the functional evolution of the single male-limited gene MID harboring in the highly reduced sex chromosomal region can be considered to be the main basis of the intial transition to male. Further comparative analyses are thus very fruitful regarding the functions and downstream genes of MID in order to unveil the “actual blueprint” in the autosomal regions behind the emergence of the first male within the volvocine green lineage.
Three of the authors from left to right: Takashi Hamaji, Hiroko Kawai-Toyooka and Hisayoshi Nozaki
Charlesworth, B. (1978) The population genetics of anisogamy. J. Theor. Biol. 73, 347–357. https://doi.org/10.1016/0022-5193(78)90195-9
Ferris, P. et al. (2010) Evolution of an expanded sex-determining locus in Volvox. Science 328, 351–354. http://science.sciencemag.org/content/328/5976/351
Herron, M. D., Hackett, J. D., Aylward, F. O. & Michod, R. E. (2009) Triassic origin and early radiation of multicellular volvocine algae. Proc. Natl. Acad. Sci. USA 106, 3254–3258. http://www.pnas.org/content/106/9/3254
Nozaki, H., Misawa, K., Kajita, T., Kato, M., Nohara, S. & Watanabe M. M. (2000) Origin and evolution of the colonial volvocales (Chlorophyceae) as inferred from multiple, chloroplast gene sequences. Mol. Phylogenet. Evol. 17, 256-68. https://doi.org/10.1006/mpev.2000.0831
Nozaki, H. & Kuroiwa, T. (1992) Ultrastructure of the extracellular matrix and taxonomy of Eudorina, Pleodorina and Yamagishiella gen. nov. (Volvocaceae, Chlorophyta). Phycologia 31, 529-541. https://doi.org/10.2216/i0031-8884-31-6-529.1