A small blueprint of “Adam”
Next-generation genome sequencing resolves highly reduced sex chromosomal region “OSU” in initial transition to male
Overview of the press release
According to the Bible, the first male and female were “Adam” and “Eve”, respectively. However, how male and female genders initially appeared has remained a mystery in the biological sciences. Male and female genders differing in gamete size (anisogamy) are considered to have evolved from primitive sexual reproduction (isogamy) in which gametes of the two sexes (mating types plus and minus) are of equal size. Based on a previous theoretical study and a recent genome comparison between two volvocine green algae (a model lineage for gender evolution), isogamous, unicellular Chlamydomonas and oogamous, complex multicellular Volvox, the most probable basis for this evolutionary transition may have been the expansion of the sex chromosomal regions (genome sequences that differ in gene composition and arrangement between the two sexes) by acquiring further genes. However, how and which genes might have been acquired by the sex chromosomal regions and contributed to the initial transition to anisogamy has been unresolved because of the lack of genome information from the evolutionary intermediate volvocine algae.
Using next-generation genome sequencing, researchers from the Department of Biological Sciences, Graduate School of Science, the University of Tokyo, and the National Institute of Genetics have generated de novo nuclear genome assemblies of both sexes of two volvocine algae, isogamous Yamagishiella and anisogamous Eudorina, that bracket the evolutionary transition to anisogamy. Based on the prior genome comparison, simple, reduced sex chromosomal regions have been resolved in the two volvocine algae, with only two sex-limited genes—the mating type minus/male-limited “OTOKOGI”/MID gene and the mating type plus/female-limited FUS1 gene. Remarkably, the researchers found that Eudorina has a markedly reduced male sex chromosomal region called “Otokogi-containing Small Unit” (OSU), which measures only 7 kbp long with the “OTOKOGI” gene. Thus, the first step to anisogamy in volvocine algae presumably occurred without an increase in size and complexity, and the initial male that evolved might have harbored a very small sex chromosomal region like “OSU”. Since the functional evolution of the single male-limited “OTOKOGI” gene within the small “OSU” region can be considered to be the main basis of the initial transition to male, further comparative analyses will provide more insight into the functions and downstream genes of “OTOKOGI”/MID.
Figure: The model lineage for gender evolution “volvocine green algae” focusing two intermediate algae (Yamagishiella and Eudorina) that bracket the transition from isogamy to anisogamy. Both algae have simple sex chromosomal regions (MT) with only two sex-limited genes (FUS1 and “OTOKOGI”/MID). Since Eudorina has a very small MT called ”OSU”, the first transition to male might have occurred based on only the functional evolution of “OTOKOGI” within a small region like “OSU”.
Journal Communications Biology Title Anisogamy evolved with a reduced sex-determining region in volvocine green algae Authors Takashi Hamaji, Hiroko Kawai-Toyooka*, Haruka Uchimura, Masahiro Suzuki, Hideki Noguchi, Yohei Minakuchi, Atsushi Toyoda, Asao Fujiyama, Shin-ya Miyagishima, James G. Umen & Hisayoshi Nozaki* DOI 10.1038/s42003-018-0019-5 Paper link https://doi.org/10.1038/s42003-018-0019-5
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