Experts studying the evolutionary differentiation of organisms into male and female sexes have identified a very small sex chromosomal region called “OSU” in the first male-like green alga. They found OSU by comparing the whole genome sequences of two different types of algae from immediately before and after the evolution from male and female sexes. The results suggest that the first male might have had an extremely reduced OSU sex chromosomal region.
Modern female and male organisms have vastly different sex cells, or gametes: large female egg cells and small, fast-moving male sperm cells. These distinct male and female forms are thought to have evolved from ancestral organisms that reproduced using nearly identically shaped gametes.
Previous scientific models portrayed the so-called sex chromosomal regions within those nearly identically shaped gametes expanding over time by gaining genes that caused differentiation into sexes with female and male traits. However, the exact evolutionary process of how that expansion happened and which genes were acquired was previously unknown.
Using next-generation sequencing, a research team led by Associate Professor Hisayoshi Nozaki of the University of Tokyo’s Graduate School of Science and collaborators at the National Institute of Genetics, in Mishima, Japan, generated completely new nuclear genome assemblies of both sexes of two different types of volvocine algae: the similar-gametes genus Yamagishiella and the different-gametes genus Eudorina.
The two bookend either side of the evolutionary transition from isogamy to anisogamy, or from organisms that reproduce with similar gametes to those that reproduce with distinctly different male and female gametes. Researchers compared the four genomes and identified simple, reduced sex chromosomal regions in both algae types. Remarkably, the male sex chromosomal region of Eudorina is markedly reduced, measuring only 7 kilobase pairs (kbp) – 7,000 base pairs – long, with only a single male-specific gene, OTOKOGI (the region is called OSU for Otokogi-containing Small Unit).
The researchers conclude that the first step to unique male and female gametes in volvocine algae presumably occurred without the increase in size and complexity of the sex chromosomal regions that was predicted by earlier models. Their new analysis indicates that the intial male might have harbored a very small sex chromosomal region like OSU.
Since the functional evolution of the single male-limited gene OTOKOGI within the small OSU region can be considered to be the main basis of the intial transition to male, the scientific team plans to conduct additional research studies on the fuction of OTOKOGI and other genes downstream to better understand the evolution of anisogamy.
“I have spent my career trying to answer the question of how the male and female sexes evolved. Cutting-edge next-generation DNA sequencing methods and an excellent team of researchers have finally unveiled the answer,” said Nozaki.
“In 1992, I used morphological data to describe what was then the new genus of Yamagishiella. At the time, I could not have imagined DNA data would be available to enhance my phylogenetic and taxonomic studies. Modern molecular phylogenetic studies clarified the phylogenetic position of Yamagishiella, suggesting that Eudorina and Yamagishiella represent the key organisms for unveiling the transition from isogamy to anisogamy in evolutionary biology. In the present research, we analyzed this evolutionary process by using the whole nuclear genome data, and discovered a very reduced male sexual chromosomal region, OSU, in Eudorinathat corresponds to the first anisogamous organism.”
Source: University of Tokyo